In one of his books in the late 80s of the twentieth century, W6SAI, Bill Orr proposed a simple antenna - 1 element square, which was installed vertically on one mast. The W6SAI antenna was made with the addition of an RF choke. The square is made for a range of 20 meters (Fig. 1) and is installed vertically on one mast. In continuation of the last knee of a 10-meter army telescope, fifty centimeters piece of texto-textolite is inserted, the shape is no different from the upper knee of the telescope, with a hole at the top, which is the top insulator. The result is a square with an angle at the top, an angle at the bottom and two angles on the extensions on the sides. From the point of view of efficiency, this is the most advantageous option for placing the antenna, which is located low above the ground. The power point turned out to be about 2 meters from the underlying surface. The cable connection node is a piece of thick fiberglass 100x100 mm, which is attached to the mast and serves as an insulator. The perimeter of the square is equal to 1 wavelength and is calculated by the formula: Lm = 306.3 \ F MHz. For a frequency of 14.178 MHz. (Lm \u003d 306.3 \ 14.178) the perimeter will be equal to 21.6 m, i.e. side of the square = 5.4 m. 0.25 wavelength. This piece of cable is a quarter-wave transformer, transforming Rin. antennas of the order of 120 ohms, depending on the objects surrounding the antenna, the resistance is close to 50 ohms. (46.87 ohms). Most of the 75 ohm cable segment is located strictly vertically along the mast. Further, through the RF connector is the main transmission line cable 50 ohms with a length equal to an integer number of half-waves. In my case, this is a segment of 27.93 m, which is a half-wave repeater. This method of powering is well suited for 50 ohm equipment, which today in most cases corresponds to R out. Silos of transceivers and the nominal output impedance of power amplifiers (transceivers) with a P-loop at the output. When calculating the cable length, you should remember about the shortening factor of 0.66-0.68, depending on the type of plastic cable insulation. With the same 50 ohm cable, an RF choke is wound next to the mentioned RF connector. His data: 8-10 turns on a 150mm mandrel. Winding coil to coil. For antennas on the low bands - 10 turns on a mandrel 250 mm. The HF choke eliminates the curvature of the antenna pattern and is a Shut-off Choke for HF currents moving along the cable sheath in the direction of the transmitter. The antenna bandwidth is about 350-400 kHz. with SWR close to unity. Outside the passband, the SWR rises strongly. Antenna polarization is horizontal. Stretch marks are made of wire with a diameter of 1.8 mm. broken by insulators at least every 1-2 meters. If you change the feeding point of the square, feeding it from the side, as a result we get a vertical polarization, more preferable for DX. Use the same cable as for horizontal polarization, i.e. a quarter-wave length of a 75 ohm cable goes to the frame, (the central core of the cable is connected to the upper half of the square, and the braid to the bottom), and then a multiple of half a wave of a 50 ohm cable. The resonant frequency of the frame when changing the power point will go up by about 200 kHz. (at 14.4 MHz.), so the frame will have to be slightly lengthened. An extension wire, a cable of about 0.6-0.8 meters can be included in the lower corner of the frame (in the former power point of the antenna). To do this, you need to use a segment of a two-wire line of the order of 30-40 cm. Wave resistance does not play a big role here. A jumper is soldered on the loop at a minimum SWR. The radiation angle will be 18 degrees, not 42, as with horizontal polarization. It is highly desirable to ground the mast at the base.
Antenna horizontal frame
We make a frame active antenna for simple shortwave radios.
Is it possible to listen to the broadcast for people who do not have space to install large, full-size antennas? One of the outputs is a loop active antenna mounted directly on the table, near the radio.
The practical manufacture of such an antenna will be discussed in this article ...
So, a small-sized active loop antenna is an antenna consisting of one or more turns of copper wire (tube) or even a coaxial cable. There are plenty of examples of such antennas on the web.
I made my antenna in the form of a vertical structure, which is installed on a table near the radio. The loop active antenna is a kind of large inductor, made of copper wire with a diameter of 1.2 mm and contains four turns. The number of turns is chosen at random)). The diameter of the manufactured loop antenna is approximately 23 cm:
To reduce its own capacitance, the turns of the antenna are wound with a pitch of 10 mm. To maintain the constancy of the winding pitch, as well as to give the entire structure the necessary rigidity, intermediate spacers made of fiberglass 2 mm thick were used. The sketch of the spacers is shown below: 
This is what the intermediate spacer in the antenna looks like:
To give stability to all this design, support posts are used, also made of fiberglass, and which serve as antenna legs: 
The copper wire is threaded into the appropriate holes in the spacers and posts, and fixed in them with a drop of cyanoacrylate glue.
This is how the rack looks like in a manufactured copy of the antenna:
General view of the manufactured antenna:
For the sake of interest, I connected the manufactured loop antenna to the AA-54 antenna analyzer.
The antenna's own resonance was found at a frequency of 14.4 MHz.
In the photo below, the display of the AA-54 antenna analyzer at the time of measuring the parameters of the loop antenna at the resonance frequency:
As you can see, the antenna impedance at a frequency of 14.4 MHz is 13.5 ohms, the active resistance is 7.3 ohms, the reactance is relatively small - minus 11.4 ohms and is capacitive in nature.
The inductance of the loop antenna (and it, in fact, is an inductor) was 7.2 μH.
This is all that concerns the manufacture and parameters of the loop antenna itself.
But, since the antenna is active, it means that it also contains an antenna amplifier.
When choosing an antenna amplifier circuit, I was guided by the principle of choosing something not too abstruse and complex, and easy to manufacture.
Google, as always, dumped a mountain of schemes)) Without hesitation, I chose one of them, which seemed interesting to me.
The circuit of this antenna amplifier was published somewhere else in the early 2000s in one of the foreign magazines. This amplifier seemed interesting to me from the point of view that it has a balanced input - just right for my loop antenna.
Schematic diagram of the antenna amplifier:
In the original, transistors of the BF series were used in this amplifier - something like BF4 **.
These were not available, so I assembled an amplifier from what was at hand - 2N3904, 2N3906, S9013.
Actually, the amplifying stage is assembled on VT1VT2 transistors. An emitter follower is assembled on the VT3 transistor to match the high output impedance of the amplifier with the relatively low input impedance of radio receivers.
The amplifier is powered by a voltage of 6 V. The operating modes of the transistors are set by selecting the resistor R3. The voltages at the electrodes of the transistors are indicated in the diagram.
The amp worked almost immediately. I tried to install transistors KT315, Kt361 in this amplifier, but its efficiency immediately deteriorated noticeably, so I refused this option. I assembled the antenna amplifier on the circuit board, but I also prepared a printed circuit board for it:
As a receiver for field tests of an active loop antenna with an amplifier,
By connecting the output of the antenna amplifier to the input of the receiver and turning on the power, I immediately noticed an increase in the noise level. This is not surprising - the antenna amplifier contributes ...
The last stage of testing was to connect the actual loop antenna to the input of the antenna amplifier and try to receive any signals from the air..
And it succeeded! Many stations working with single-sideband modulation on the 40 m band are well audible. It is clear that the stations are not heard as loudly as on a full-size antenna. Yes, and you can not compare a normal antenna with a loop antenna located next to the receiver. Also, during the operation of an active loop antenna, a slightly increased noise level is observed. You need to put up with this - this is a fee for small size. It is also desirable to place such an antenna away from all kinds of interference sources - charging, energy-saving light bulbs, network equipment, etc.
conclusions: such an antenna quite has the right to life, it receives a lot of stations. For those who do not have the opportunity to hang a large, long antenna, this can be a way out of the situation.
Video demonstration of the operation of a loop active antenna on the 7 MHz band:
The HF band contains a number of radio frequencies (27 MHz, commonly used by drivers), broadcasting many stations. There are no TV shows here. Today we will consider the amateur series involved by various radio enthusiasts. Frequencies 3.7; 7; 14; 21, 28 MHz of the HF band, related as 1: 2: 4: 6: 8. It is important, as we will see later, it becomes possible to make an antenna that would catch all the ratings (the issue of coordination is the tenth thing). We believe that there will always be people who will use the information, catch radio broadcasts. Today's topic is a do-it-yourself HF antenna.
We will disappoint many, today we will again talk about vibrators. The objects of the Universe are formed by vibrations (the views of Nikola Tesla). Life attracts life, it is movement. To give life to a wave, vibrations are necessary. Changes in the electric field generate a response of the magnetic one, so the frequency that carries information to the ether crystallizes. The immobilized field is dead. A permanent magnet will not generate a wave. Figuratively speaking, electricity is masculine, exists only in motion. Magnetism is a rather feminine quality. However, the authors delved into philosophy.
It is considered preferable to use horizontal polarization for transmission. Firstly, the azimuth radiation pattern is not circular (it was said in passing), there will certainly be less interference. We know that various objects like ships, cars, tanks are being equipped for communication. You can not lose commands, orders, words. The object will turn in the wrong direction, but is the polarization horizontal? We disagree with well-known, respected authors who write: vertical polarization was chosen as a connection for an antenna of a simpler design. Touch the matter of amateurs, it is rather about the continuity of the legacy of previous generations.
We add: with horizontal polarization, the parameters of the Earth have less effect on wave propagation, in addition, with vertical polarization, the front suffers attenuation, the lobe rises to 5 - 15 degrees, which is undesirable when transmitting over long distances. For vertically polarized (single-ended) antennas, good grounding is essential. Directly depends on the efficiency of the antenna. It is better to bury wires with a length of the order of a quarter of a wave with earth, the more, the higher the efficiency. Example:
- 2 wires - 12%;
- 15 wires - 46%;
- 60 wires - 64%;
- ∞ wires - 100%.
An increase in the number of wires reduces the wave resistance, approaching the ideal (of the indicated type of vibrator) - 37 Ohm. Note that the quality should not be brought closer to the ideal, 50 ohms do not need to be coordinated with the cable (in connection, PK - 50 is used). Great deal. Let's supplement the package of information with a simple fact, with horizontal polarization, the signal is added to the reflected Earth, giving an increase of 6 dB. Vertical polarization shows so many minuses, they use it (it turned out interesting with the ground wires), they put up with it.
The device of HF antennas is reduced to a simple quarter-wave, half-wave vibrator. The second ones are smaller in size, they accept worse, the second ones are easier to agree on. The masts are placed vertically using spacers, extensions. They described a structure hung on a tree. Not everyone knows: at a distance of half a wave from the antenna there should be no interference. It concerns iron, reinforced concrete structures. Wait a minute to rejoice, at a frequency of 3.7 MHz the distance is ... 40 meters. The antenna is eight stories high. Building a quarter-wave vibrator is not easy.
It is convenient to build a tower to listen to the radio, we decided to recall the old way of catching long waves. You will find internal ferromagnetic antennas in Soviet-era receivers. Let's see if the designs are suitable for their intended purpose (catching the broadcast).
HF Magnetic Antenna
Suppose there is a need to accept frequencies of 3.7 - 7 MHz. Let's see if it is possible to design a magnetic antenna. Formed by a core of round, square, rectangular section. The dimensions are calculated by the formula:
do = 2 √ rs / π;
do is the diameter of the round rod; h, c - height, width of a rectangular section.
Winding is not carried out for the entire length, in fact, you need to calculate how much to wind, choose the type of wire. Let's take an example of an old design textbook, let's try to calculate a 3.7 - 7 MHz HF antenna. Let us take the resistance of the input stage of the receiver 1000 Ohm (in practice, readers measure the input resistance of the receiver on their own), the parameter of the equivalent attenuation of the input circuit, at which the specified selectivity is achieved, der equal to 0.04.
The antenna, which we are designing, is part of the resonant circuit. It turns out a cascade endowed with a certain selectivity. How to solder, think for yourself, just follow the formulas. Those conducting the calculation will need to find the maximum, minimum capacitance of the trimmer capacitor, using the formula: Cmax \u003d K 2 Cmin + Co (K 2 - 1).
K is the subband coefficient, determined by the ratio of the maximum resonant frequency to the minimum. In our case, 7 / 3.7 = 1.9. It is selected from incomprehensible (according to the textbook) considerations, according to the example given by the text, we take equal to 30 pF. Let's not go wrong. Let Cmin = 10 pF, we find the upper limit of the adjustment:
Cmax \u003d 3.58 x 10 + 30 (3.58 - 1) \u003d 35.8 + 77.4 \u003d 110 pF.
Rounded off, of course, you can take a larger range variable capacitor. The example gives 10-365 pF. We calculate the required inductance of the circuit using the formula:
L \u003d 2.53 x 10 4 (K 2 - 1) / (110 - 10) 7 2 \u003d 13.47 μH.
The meaning of the formula is clear, let's add, 7 is the upper limit of the range, expressed in MHz. Selecting the core of the coil. At the frequencies of the range, the magnetic permeability of the core is M = 100, we select the ferrite grade 100NN. We take a standard core 80 mm long, 8 mm in diameter. The ratio l / d \u003d 80 / 8 \u003d 10. From the directories we extract the effective value of the magnetic permeability md. It turns out 41.
We find the winding diameter D = 1.1 d = 8.8, the number of winding turns is determined by the formula:
W = √(L / L1) D md mL pL qL;
formula coefficients are read visually using the graphs below. The figures will show the reference figures used above. Look for the brand of ferrite, a person does not live by bread alone. D is expressed in centimeters. The authors obtained: L1 = 0.001, mL = 0.38, pL = 0.9. qL can be calculated using the formula:
qL = (d / D) 2 = (8 / 8.8) 2 = 0.826.
We substitute the numbers in the final expression for calculating the number of turns of a ferrite HF antenna, it turns out:
W \u003d √ (13.47 / 0.001) x 0.88 x 41 x 0.38 x 0.9 x 0.826 \u003d 373 turns.
The cascade must be connected to the first amplifier of the receiver, bypassing the input circuit. Let's say more, now we have calculated the means of selectivity in the range of 3.7-7 MHz. In addition to the antenna, it turns on the input circuit of the receiver at the same time. Therefore, it will be necessary to calculate the inductance of coupling with the amplifier, fulfilling the conditions for ensuring selectivity (we take typical values).
Lsv \u003d (der - d) Rin / 2 π fmin K 2 \u003d (0.04 - 0.01) 1000 / 2 x 3.14 x 3.7 x 3.61 \u003d 0.35 μH.
The transformation ratio will be m = √ 0.35 / 13.47 = 0.16. We find the number of turns of the communication coil: 373 x 0.16 = 60 turns. We wind the antenna with a PEV-1 wire with a diameter of 0.1 mm, we wind the coil with a PELSHO with a diameter of 0.12 mm.
Many people are probably interested in several questions. For example, the appointment of formulas for calculating a variable capacitor. The author bashfully avoids the question, supposedly the initial capacity of the circuit. Diligent readers will calculate the resonant frequencies of a parallel circuit in which an initial capacitance of 30 pF is soldered. We will make a slight mistake in recommending that a tuning capacitance of 30 pF be placed next to the variable capacitor. The chain is being worked on. Beginners are interested in the electrical circuit, which will include a home-made HF antenna ... The parallel circuit, the signal from which is taken by the transformer, is formed by wound coils. The core is common.
An independent HF antenna is ready. You will find this in a tourist receiver (models with a dynamo are popular today). HF antennas (and even more so MW) would be great if the design was made in the form of a typical vibrator. Such designs are not used by portable equipment. The simplest HF antennas take up a lot of space. Better reception. The purpose of the HF antenna is to improve signal quality. In the apartment, balconies. They told how to make a miniature HF antenna. Use vibrators in the country, in the field, forest, in open areas. The material is provided by the design guide. The book is full of errors, and the result seems to be tolerable.
Even old textbooks sin with misprints missed by editors. It concerns more than one branch of radio electronics.
Today, when most of the old housing stock is privatized, and the new one is certainly private property, it is becoming increasingly difficult for a radio amateur to install full-sized antennas on the roof of his house. The roof of a residential building is part of the property of every resident of the house where they live, and they will never allow you to walk on it again, much less install some kind of antenna and spoil the facade of the building. Nevertheless, today there are such cases when a radio amateur enters into an agreement with the housing department to rent part of the roof with his antenna, but this requires additional financial resources and this is a completely different topic. Therefore, many novice radio amateurs can afford only those antennas that can be installed on a balcony or loggia, at the risk of being reprimanded by the house manager for damaging the facade of the building with an absurd protruding structure.
Pray to God that some “know-it-all activist” doesn’t hint at the harmful radiation of the antenna, as from cellular antennas. Unfortunately, it must be admitted that a new era has come for radio amateurs of the secrecy of their hobby and their HF antennas, despite the paradox of their legality in legal terms of this issue. That is, the state allows broadcasting on the basis of the “Law on Communications of the Russian Federation”, and the levels of permitted power comply with the standards for HF radiation SanPiN 2.2.4 / 2.1.8.055-96, but they have to be invisible in order to avoid pointless evidence of the legality of their activities.
The proposed material will help a radio amateur understand antennas with a large shortening that can be placed on the space of a balcony, loggia, on the wall of a residential building or on a limited antenna field. The material “HF Balcony Antennas for Beginners” reviews antenna options by different authors, previously published both in paper and electronic form, and selected for the conditions of their installation in a limited space.
Explanatory comments will help the beginner understand how the antenna works. The presented materials are aimed at beginner radio amateurs to gain skills in building and choosing mini-antennas.
- Hertz dipole.
- Shortened Hertz dipole.
- Spiral antennas.
- magnetic antennas.
- capacitive antennas.
1. Hertz dipole
The most classic type of antenna is undeniably the Hertzian dipole. This is a long wire, most often with a half-wave antenna web size. The antenna wire has its own capacitance and inductance, which are distributed over the entire antenna web, they are called distributed antenna parameters. The capacitance of the antenna creates the electric component of the field (E), and the inductive component of the antenna, the magnetic field (H).
The classical Hertzian dipole by its nature has impressive dimensions and is half a long wave. Judge for yourself, at a frequency of 7 MHz, the wavelength is 300/7 = 42.86 meters, and half a wave will be 21.43 meters! Important parameters of any antenna are its characteristics from the side of space, these are its aperture, radiation resistance, effective height of the antenna, radiation pattern, etc. A half-wave dipole is a linear radiator widely used in the practice of antenna technology. However, every antenna has its advantages and disadvantages.
We note right away that for the good operation of any antenna, at least two conditions are required, this is the presence of an optimal bias current and effective formation of an electromagnetic wave. HF antennas can be either vertical or horizontal. By setting a half-wave dipole vertically, and reducing its height by turning the fourth part into counterweights, we get the so-called quarter-wave vertical. Vertical quarter-wave antennas, for their effective operation, require a good "electronic ground", because. the soil of the planet "Earth" has poor conductivity. The radio earth is replaced by connecting counterweights. Practice shows that the minimum required number of counterweights should be about 12, but it is better if their number exceeds 20 ... 30, and ideally it is necessary to have 100-120 counterweights.
It should never be forgotten that an ideal vertical antenna with a hundred counterweights has an efficiency of 47%, and an antenna with three counterweights has an efficiency of less than 5%, which is clearly shown in the graph. The power supplied to the antenna with a small number of counterweights is absorbed by the earth's surface and surrounding objects, heating them. Exactly the same low efficiency expects a low-lying horizontal vibrator. Simply put, the earth reflects poorly and absorbs the emitted radio wave well, especially when the wave has not yet formed in the near zone from the antenna, like a clouded mirror. Better reflects the sea water surface and does not reflect the sandy desert at all. According to the theory of reciprocity, the parameters and characteristics of the antenna are the same for both reception and transmission. This means that in the reception mode near the vertical with a small number of counterweights, large losses of the useful signal occur and, as a result, an increase in the noise component of the received signal.
The counterweights of the classical vertical must be at least the length of the main pin, i.e. the displacement currents flowing between the pin and counterweights occupy a certain amount of space, which is involved not only in the formation of the directivity diagram, but also in the formation of the field strength. With a high approximation, we can say that each point on the pin corresponds to its own mirror point on the counterweight, between which bias currents flow. The fact is that displacement currents, like all conventional currents, flow along the path of least resistance, which in this case is concentrated in a volume limited by the radius of the pin. The resulting radiation pattern will be a superposition (superposition) of these currents. Returning to what was said above, this means that the efficiency of a classical antenna depends on the number of counterweights, i.e. the more counterweights, the greater the bias current, the more efficient the antenna, THIS IS THE FIRST CONDITION for good antenna performance.
The ideal case is considered to be a half-wave vibrator located in an open space in the absence of absorbing soil, or a vertical located on an all-metal surface with a radius of 2-3 wavelengths. This is necessary so that the soil of the earth or objects surrounding the antenna do not interfere with the effective formation of an electromagnetic wave. The fact is that the formation of a wave and the coincidence in phase of the magnetic (H) and electric (E) components of the electromagnetic field does not occur in the near zone of the Hertz dipole, but in the middle and far zones at a distance of 2-3 wavelengths, THIS SECOND CONDITION for good work antennas. This is the main drawback of the classical Hertzian dipole.
The generated electromagnetic wave in the far zone is less affected by the earth's surface, bends around it, is reflected and propagates in the medium. All of the above very brief concepts are needed in order to understand the further essence of building amateur balcony antennas, to look for such an antenna design in which the wave is formed inside the antenna itself.
Now it is clear that the placement of full-size antennas, a quarter-wave pin with counterweights or a half-wave dipole of the Hertzian HF band is almost impossible to place within a balcony or loggia. And if a radio amateur managed to find an accessible antenna attachment point on the building opposite from the balcony or window, then today this is considered great luck.
2. Shortened Hertz dipole.
With limited space at their disposal, the radio amateur has to compromise and reduce the size of the antennas. Antennas are considered electrically small if their dimensions do not exceed 10 ... 20% of the wavelength λ. In such cases, a shortened dipole is often used. When the antenna is shortened, its distributed capacitance and inductance decrease, respectively, its resonance changes towards higher frequencies. To compensate for this deficiency, additional inductors L and capacitive loads C are introduced into the antenna as lumped elements (Fig. 1).
The maximum efficiency of the antenna is achievable by placing extension coils at the ends of the dipole, because the current at the ends of the dipole is maximum and more uniformly distributed, which ensures the maximum effective height of the antenna hd = h. The inclusion of inductors closer to the center of the dipole will reduce its own inductance, in this case the current to the ends of the dipole drops, the effective height decreases, and after it the efficiency of the antenna.
Why do we need a capacitive load in a shortened dipole? The fact is that with a large shortening, the quality factor of the antenna greatly increases, and the bandwidth of the antenna becomes narrower than the amateur radio range. The introduction of capacitive loads increases the capacitance of the antenna, reduces the quality factor of the formed LC circuit and expands its bandwidth to an acceptable one. A shortened dipole is tuned to the operating frequency in resonance either by inductors or by the length of conductors and capacitive loads. This provides compensation for their reactances at the resonant frequency, which is necessary according to the conditions of coordination with the power feeder.
Note: Thus, we compensate for the necessary characteristics of a shortened antenna to match it with the feeder and space, but reducing its geometric dimensions ALWAYS leads to a decrease in its efficiency (COP).
One of the examples of calculating an extension inductor was described in the Journal of Radio, number 5, 1999, where the calculation is carried out from the existing emitter. The inductors L1 and L2 are placed here at the feed point of the quarter-wave dipole A and the counterweight D (Fig. 2.). This is a single band antenna.
You can also calculate the inductance of a shortened dipole on the RN6LLV radio amateur website - it gives a link to download a calculator that can help in calculating the lengthening inductance.
There are also proprietary shortened antennas (Diamond HFV5), which have a multi-band version, see Fig. 3, in the same place its electrical diagram.
The operation of the antenna is based on the parallel connection of resonant elements tuned to different frequencies. When moving from one range to another, they practically do not affect each other. Inductors L1-L5 are extension coils, each designed for its own frequency range, just like capacitive loads (antenna extension). The latter have a telescopic design, and by changing their length they are able to adjust the antenna in a small frequency range. The antenna is very narrowband.
* Mini antenna for 27MHz band, the author of which is S. Zaugolny. Let's take a closer look at her work. The author's antenna is located on the 4th floor of a 9-storey panel building in the window opening and is essentially a room antenna, although this version of the antenna will work better outside the window (balcony, loggia) perimeter. As can be seen from the figure, the antenna consists of an oscillatory circuit L1C1 tuned to resonance at the frequency of the communication channel, and the communication coil L2 acts as a matching element with the feeder, fig. 4.a. The main emitter here are capacitive loads in the form of wire frames with dimensions of 300 * 300 mm and a shortened symmetrical dipole consisting of two pieces of wire 750 mm each. If we take into account that a vertically located half-wave dipole would take a height of 5.5 m, then an antenna with a height of only 1.5 m is a very convenient option for placement in a window opening.
If we exclude the resonant circuit from the circuit and connect the coaxial cable directly to the dipole, then the resonant frequency will be in the range of 55-60 MHz. Based on this scheme, it is clear that the frequency-setting element in this design is an oscillatory circuit, and the antenna is shortened by 3.7 times and has not greatly reduced its efficiency. If in this design an oscillatory circuit tuned to other lower frequencies of the HF band is used, of course the antenna will work, but with much lower efficiency. For example, if such an antenna is tuned to the 7 MHz amateur band, then the antenna shortening factor from half the wave of this band will be 14.3, and the antenna efficiency will drop even more (by the square root of 14), i.e. more than 200 times. But nothing can be done about it, you have to choose such an antenna construct that would be as effective as possible. This design clearly shows that the radiating elements here are capacitive loads in the form of wire squares, and they would perform their functions better if they were all-metal. The weak link here is the oscillatory circuit L1C1, which must have a high quality factor-Q, and part of the useful energy in this design is uselessly spent inside the plates of the capacitor C1. Therefore, an increase in the capacitance of the capacitor, although it reduces the resonance frequency, but it also reduces the overall efficiency of this design. When designing this antenna for lower frequencies of the HF range, attention should be paid to what would be the maximum at the resonant frequency L1, and the minimum C1, while not forgetting that capacitive radiators are part of the resonant system as a whole. The maximum overlap in frequency is desirable to design no more than 2, and the emitters were located as far as possible from the walls of the building. The balcony version of this antenna with camouflage from prying eyes is shown in fig. 4.b. It was a similar antenna that was used for some time in the middle of the 20th century on military vehicles in the HF band with a tuning frequency of 2-12 MHz.
* Single-band variant "Undying Fuchs Antenna"(21MHz) is shown in Fig.5.a. A pin 6.3 meters long (almost half a wave) is fed from the end by a parallel oscillatory circuit with the same high resistance. Mr. Fuchs decided that this is how the parallel oscillatory circuit L1C1 and the half-wave dipole are coordinated with each other, and so it is ... As you know, the half-wave dipole is self-sufficient and works for itself, it does not need counterweights like a quarter-wave vibrator. The emitter (copper wire) can be placed in a plastic rod. Such a fishing rod can be pulled out of the balcony railing and put back for the duration of work on the air, but in winter this creates a number of inconveniences. As a "ground" for the oscillatory circuit, a piece of wire of only 0.8 m is used, which is very convenient when placing such an antenna on a balcony. At the same time, this is an exceptional case when a flower pot can be used as a ground (joke). The inductance of the resonant coil L2 is 1.4 μH, it is made on a frame with a diameter of 48 mm and contains 5 turns of 2.4 mm wire with a 2.4 mm pitch. As a resonant capacitor with a capacity of 40 pF, two pieces of RG-6 coaxial cable are used in the circuit. The segment (C2 according to the diagram) is an invariable part of the resonant capacitor with a length of no more than 55-60cm, and a shorter segment (C1 according to the diagram) is used for fine tuning to resonance (15-20cm). The communication coil L1 in the form of one turn over the L2 coil is made with an RG-6 cable with a 2-3 cm gap in its braid, and the SWR adjustment is carried out by moving this turn from the middle towards the counterweight.
Note: The Fuchs antenna works well only in the half-wave version of the emitter, which can also be shortened by the type of helical antennas (read below).
* Multi-band balcony antenna option shown in fig. 5 B. It was tested back in the 50s of the last century. Here, the inductance plays the role of an extension coil in autotransformer mode. A capacitor C1 at 14 MHz tunes the antenna into resonance. Such a pin needs good grounding, which is difficult to find on the balcony, although for this option you can use an extensive network of heating pipes for your apartment, but it is not recommended to supply more than 50 watts of power. The inductor L1 has 34 turns of a copper tube with a diameter of 6 mm, wound on a frame with a diameter of 70 mm. Branches from 2,3 and 4 turns. In the range of 21 MHz, the switch P1 is closed, P2 is open, In the range of 14 MHz, P1 and P2 are closed. At 7 MHz, the position of the switches is the same as at 21 MHz. In the range of 3.5 MHz, P1 and P2 are open. Switch P3 determines the coordination with the feeder. In both cases, it is possible to use a rod about 5m, then the rest of the emitter will hang down to the ground. It is clear that the use of such antenna options should be above the 2nd floor of the building.
Not all examples of shortening of dipole antennas are presented in this section; other examples of shortening of a linear dipole will be presented below.
3. Spiral antennas.
Continuing the discussion of the topic of shortened balcony antennas, one cannot ignore the helical antennas of the HF band. And of course, it is necessary to recall their properties, which have almost all the properties of the Hertzian dipole.
Any shortened antenna, the dimensions of which do not exceed 10-20% of the wavelength, refers to electrically small antennas.
Features of small antennas:
- The smaller the antenna, the less ohmic losses should be in it. Small antennas made from thin wires cannot work effectively, as they experience increased currents, and the skin effect requires low surface resistances. This is especially true for antennas with radiator sizes much less than a quarter of a wavelength.
- Since the field strength is inversely proportional to the size of the antenna, a decrease in the size of the antenna leads to an increase in very large field strengths near it, and with an increase in the input power, it leads to the appearance of the "St. Elmo's fires" effect.
- The lines of force of the electric field of shortened antennas have some effective volume in which this field is concentrated. It has a shape close to an ellipsoid of revolution. In fact, this is the volume of the near quasi-static field of the antenna.
- A small antenna with dimensions of λ/10 or less has a quality factor of about 40-50 and a relative bandwidth of no more than 2%. Therefore, it is necessary to introduce a tuning element into such antennas within one amateur band. Such an example is easy to observe in magnetic antennas with small dimensions. Increasing the bandwidth reduces the efficiency of the antenna, therefore, one should always strive to increase the efficiency of ultra-small antennas in different ways.
* Reducing the size of a symmetrical half-wave dipole first led to the appearance of extending inductors (Fig. 6.a), and a decrease in its interturn capacitance and a maximum increase in efficiency led to the appearance of an inductor to the design of helical antennas with transverse radiation. A spiral antenna (Fig. 6.b.) is a shortened classical half-wave (quarter-wave) dipole coiled into a spiral with distributed inductances and capacitances along its entire length. For such a dipole, the quality factor has increased, and the bandwidth has become narrower.
To expand the bandwidth, a shortened helical dipole, like a shortened linear dipole, is sometimes equipped with a capacitive load, Fig.6.b.
Since in the calculations of single-vibrator antennas, the concept of effective antenna area (A eff.) is practiced quite widely, we will consider the possibilities of increasing the efficiency of helical antennas using end disks (capacitive load) and turn to a graphical example of the distribution of currents in Fig. 7. Due to the fact that in a classical helical antenna the inductor (folded antenna sheet) is distributed along the entire length, the current distribution along the antenna is linear, and the current area increases slightly. Where, Iap is the antinode current of the helical antenna, Fig. 7.a. And the effective area of the antenna Aeff. determines that part of the area of the front of a plane wave from which the antenna removes energy.
To expand the bandwidth and increase the area of effective radiation, it is practiced to install end disks, which increases the efficiency of the antenna as a whole, Fig. 7.b.
When it comes to unbalanced (quarter-wave) helical antennas, you should always remember that Aeff. largely depends on the quality of the land. Therefore, you should know that the same efficiency of a quarter-wave vertical is provided by four counterweights with a length of λ / 4, six counterweights with a length of λ / 8 and eight counterweights with a length of λ / 16. Moreover, twenty counterweights with a length of λ /16 provide the same efficiency as eight counterweights with a length of λ /4. It becomes clear why balcony radio amateurs came to a half-wave dipole. It works for itself (see Fig. 7.c.), the lines of force are closed to their elements and the "earth", as in the designs in Fig. 7.a;b. he doesn't need. In addition, helical antennas can also be provided with lumped elements of extension-L (or shortening-C) of the electrical length of the helical radiator, and their helix length may differ from the full-length helix. An example of this is a variable capacitor (will be discussed below), which can be considered not only as an element for tuning a series oscillatory circuit, but also as a shortening element. Also a spiral antenna for portable stations on the 27 MHz band (Fig. 8). There is an extension inductor for a short spiral.
* compromise solution can be seen in the design of Valery Prodanov (UR5WCA), - a balcony spiral antenna 40-20m with a coefficient of shortening K = 14, is quite worthy of the attention of radio amateurs without a roof, see Fig.9.
Firstly, it is multi-band (7/10/14MHz), and secondly, to increase its efficiency, the author doubled the number of helical antennas and connected them in phase. The absence of capacitive loads in this antenna is due to the fact that the expansion of the bandwidth and Aeff. The antenna is achieved by in-phase inclusion in parallel of two identical radiation elements. Each antenna is wound with a copper wire on a PVC pipe with a diameter of 5 cm, the length of the wire of each antenna is half a wave for the 7 MHz band. Unlike the Fuchs antenna, this antenna is matched to the feeder by means of a broadband transformer. The output of transformers 1 and 2 has a common mode voltage. Vibrators in the author's version stand at a distance of only 1 m from each other, this is the width of the balcony. With the expansion of this distance within the balcony, the gain will increase slightly, but the antenna bandwidth will expand significantly.
* Radio amateur Harry Elington(WA0WHE, source "QST", 1972, January. Fig. 8.) built an 80m helical antenna with a velocity coefficient of about K=6.7, which in his garden can be disguised as a support for a night lamp or a flagpole. As can be seen from his comment, foreign radio amateurs also care about their relative calm, although the antenna is installed in a private courtyard. According to the author, a helical antenna with a capacitive load on a pipe with a diameter of 102 mm, a height of about 6 meters and a counterweight of four wires, easily reaches an SWR of 1.2-1.3, and with SWR = 2 it works in a bandwidth up to 100 kHz. The electrical length of the wire in the spiral was also half a wave. The half-wave antenna is powered from the end of the antenna via a coaxial cable with a wave impedance of 50 ohms through a KPI of -150pF, which turned the antenna into a series oscillatory circuit (L1C1) with a radiating inductance of the spiral.
Of course, in terms of transmission efficiency, the vertical spiral is inferior to the classical dipole, but according to the author, this antenna is much better for reception.
* Coiled Antennas
To reduce the size of a linear half-wave dipole, it is not necessary to twist it into a spiral.
In principle, the helix can be replaced by other forms of half-wave dipole folding, for example, according to Minkowski, Fig. 11. A dipole with a fixed frequency of 28.5 MHz can be placed on a substrate with dimensions of 175mm x 175mm. But fractal antennas are very narrow-band, and for radio amateurs they are only of cognitive interest in transforming their designs.
Using another method of shortening the size of the antennas, the half-wave vibrator or vertical can be shortened by compressing it into a meander shape, fig.12. At the same time, the parameters of a vertical or dipole type antenna change insignificantly when they are compressed by no more than twice. If the horizontal and vertical parts of the meander are equal, the gain of the meander antenna is reduced by about 1 dB, and the input impedance is close to 50 ohms, which makes it possible to feed such an antenna directly with a 50-ohm cable. Further size reduction (NOT wire length) results in a reduction in gain and input impedance of the antenna. However, the performance of a square wave antenna for the shortwave range is characterized by increased radiation resistance relative to linear antennas with the same shortening of the wire. Experimental studies have shown that with a meander height of 44 cm and with 21 elements at a resonant frequency of 21.1 MHz, the antenna impedance was 22 ohms, while a linear vertical of the same length has an impedance 10-15 times less. Due to the presence of horizontal and vertical sections of the meander, the antenna receives and radiates electromagnetic waves of both horizontal and vertical polarization.
By squeezing or stretching it, you can achieve antenna resonance at the desired frequency. The meander step can be 0.015λ, but this parameter is not critical. Instead of a meander, you can use a conductor with triangular bends or a spiral. The required length of the vibrators can be determined experimentally. As a starting point, it can be assumed that the length of the "straightened" conductor should be about a quarter of a wavelength for each arm of a split vibrator.
* "Tesla Spiral" in the balcony antenna. Following the cherished goal of reducing the size of the balcony antenna and minimizing losses in Aeff, instead of end disks, radio amateurs began to use a more technologically advanced flat “Tesla spiral” than the meander, using it as an extension inductance of a shortened dipole and end capacitance at the same time (Fig. 6. A.). The distribution of magnetic and electric fields in a flat Tesla inductor is shown in fig. 13. This corresponds to the theory of radio wave propagation, where field-E and field-H are mutually perpendicular.
There is also nothing supernatural in antennas with two flat Tesla spirals, and therefore the rules for constructing a Tesla spiral antenna remain classic:
- the electrical length of the helix can be an unbalanced antenna as a quarter-wave vertical or a folded half-wave dipole.
- The larger the winding pitch and the larger its diameter, the higher its efficiency and vice versa.
- The greater the distance between the ends of a coiled half-wave vibrator, the higher its efficiency and vice versa.
In a word, we got a folded half-wave dipole in the form of flat inductors at its ends, see Fig.14. To what extent to reduce or increase this or that design, the radio amateur decides after going out to his balcony with a tape measure (after agreeing with the last resort, with his mother or wife).
The use of a flat inductor with large gaps between the turns at the ends of the dipole solves two problems at once. This is a compensation of the electrical length of the shortened vibrator by distributed inductance and capacitance, as well as an increase in the effective area of the shortened antenna Aeff, expanding its bandwidth at the same time, as in Fig. 7.b.c. This solution simplifies the design of a shortened antenna and allows all dispersed LC - antenna elements to work with maximum efficiency. There are no non-working elements of the antenna, for example, as a capacitance in magnetic ML-antennas, and inductance in EN-antennas. It should be remembered that the skin effect of the latter requires thick and highly conductive surfaces, but considering the antenna with a Tesla inductor, we see that the coiled antenna repeats the electrical parameters of a conventional half-wave vibrator. In this case, the distribution of currents and voltages along its entire length of the antenna web is subject to the laws of a linear dipole and remains unchanged with some exceptions. Therefore, the need for thickening of the antenna elements (Tesla spiral) is completely eliminated. In addition, power is not consumed to heat the antenna elements. The facts listed above make us think about the high budget of this design. And the simplicity of its manufacture is from the hand of someone who at least once in his life held a hammer in his hands and bandaged his finger.
Such an antenna with some interference can be called an inductive capacitive one, in which there are LC radiation elements, or a Tesla coil antenna. In addition, taking into account the near field (quasi-static) can theoretically give even higher field strengths, which is confirmed by field tests of this design. The EH-field is created in the body of the antenna and, accordingly, this antenna is less dependent on the quality of the earth and surrounding objects, which in fact is a godsend for the family of balcony antennas. It is no secret that such antennas have long existed among radio amateurs, and this publication provides material on the transformation of a linear dipole into a helical antenna with transverse radiation, then into a shortened antenna with the conditional name "Tesla spiral". A flat spiral can be wound with a wire of 1.0-1.5 mm, because. high voltage is present at the end of the antenna, and the current is minimal. A wire with a diameter of 2-3mm will slightly improve the efficiency of the antenna, but will significantly deplete your wallet.
Note: The design and manufacture of shortened antennas such as "spiral" and "Tesla coil" with an electrical length of λ/2 compares favorably with a spiral with an electrical length of λ/4 due to the lack of a good "ground" on the balcony.
Antenna power.
We consider an antenna with Tesla spirals as a symmetrical half-wave dipole folded into two parallel spirals at its ends. Their planes are parallel to each other, although they can be in the same plane, Fig. 14. Its input impedance is only slightly different from the classic version, so the classic matching options are applicable here.
Windom linear antenna see Fig.15. refers to vibrators with asymmetric power supply, it is distinguished by its “unpretentiousness” in terms of matching with the transceiver. The uniqueness of the Windom antenna lies in its application on several bands and ease of manufacture. Transforming this antenna into "Tesla spirals", in space a symmetrical antenna will look like in Fig. 16.a, - with Gamma matching, and the asymmetric Windom dipole, fig.16.b.
To decide which antenna option to choose for the implementation of your plans to turn your balcony into an “antenna field”, it is better to read this article to the end. The design of balcony antennas compares favorably with full-sized antennas in that their parameters and other combinations can be made without going to the roof of your house and not injuring the house manager once again. In addition, this antenna is a practical guide for beginner radio amateurs, when you can practically “on your knees” learn all the basics of building elementary antennas.
Antenna Assembly
Based on practice, it is better to take the length of the wire that makes up the antenna web with a small margin, a little more by 5-10% of its estimated length, it should be an insulated single-core copper wire for wiring with a diameter of 1.0-1.5 mm. The supporting structure of the future antenna is assembled (by soldering) from PVC heating pipes. Of course, in no case should pipes with reinforced aluminum pipes be used. Dry wooden sticks are also suitable for the experiment, see Fig. 17.
There is no need for a Russian radio amateur to tell the step-by-step assembly of the supporting structure, it is enough for him to look at the original product from afar. Nevertheless, when assembling a Windom antenna or a symmetrical dipole, it is worth first marking the calculated feed point on the canvas of the future antenna and fixing it in the middle of the traverse, where the antenna will be powered. Naturally, the length of the traverse is included in the overall electrical size of the future antenna, and the longer it is, the higher the antenna efficiency.
Transformer
The antenna impedance of a symmetrical dipole will be slightly less than 50 ohms, therefore, see the connection diagram in Fig. 18.a. can be arranged by simply turning on the magnetic latch or using gamma matching.
The resistance of the folded Windom antenna has a little less than 300 ohms, so you can use the data in Table 1, which captivates with its versatility using just one magnetic latch.
The ferrite core (latch) must be tested before being installed on the antenna. To do this, the secondary winding L2 is connected to the transmitter, and the primary winding L1 to the antenna equivalent. They check SWR, heating of the core, as well as power losses in the transformer. If the core heats up at a given power, then the number of ferrite latches must be doubled. If there are unacceptable power losses, then it is necessary to select a ferrite. See Table 2 for the ratio of power loss to dB.
No matter how convenient ferrite is, I still believe that for the radiated radio wave of any mini-antenna, where a huge EH field is concentrated, it is a “black hole”. The close location of the ferrite reduces the efficiency of the mini-antenna by µ/100 times, and all attempts to make the antenna as efficient as possible become in vain. Therefore, in mini-antennas, air-core transformers are most preferred, Fig. 18.b. Such a transformer, operating in the range of 160-10m, is wound with a double wire 1.5mm on a frame with a diameter of 25 and a length of 140mm, 16 turns with a winding length of 100mm.
It is also worth remembering that the feeder of such an antenna experiences a large intensity of the radiated field on its braid and creates a voltage in it that adversely affects the operation of the transceiver in the transmission mode. It is better to eliminate the antenna effect by blocking the feeder-choke without using ferrite rings, see Fig.19. These are 5-20 turns of coaxial cable wound on a frame with a diameter of 10 - 20 centimeters.
Such feeder chokes can be installed in the immediate vicinity of the antenna web (body), but it is better to go beyond the high field concentration and install at a distance of about 1.5-2 m from the antenna web. The second such choke, installed at a distance of λ / 4 from the first, will not interfere.
Antenna tuning
Setting up the antenna brings great pleasure, and moreover, such a construct is recommended for laboratory work in specialized colleges and universities, without leaving the laboratory, on the topic "Antennas".
You can start tuning by finding the resonance frequency and adjusting the SWR of the antenna. It consists in moving the feed point of the antenna in one direction or another. There is no need to move the transformer or power cable along the traverse and cut the wires mercilessly to specify the power point. Here everything is close and simple.
It is enough to make sliders in the form of “crocodiles” at the inner ends of the flat spirals on one side and on the other, as shown in Fig. 20. Having previously provided for a slight increase in the length of the spiral, taking into account the settings, we move the sliders from different sides of the dipole to the same length, but in opposite directions, thereby we move the feed point. The result of the tuning will be the expected SWR of no more than 1.1-1.2 at the found frequency. Reactive components should be minimal. Of course, like any antenna, it must be located in a place as close as possible to the conditions of the installation site.
The second step will be to tune the antenna exactly to resonance, this is achieved by shortening or lengthening the vibrators on both sides by equal pieces of wire using the same sliders. That is, you can increase the tuning frequency by shortening both turns of the spiral by the same size, and, on the contrary, reduce the frequency by lengthening. At the end of the setup at the future installation site, it is necessary to securely connect, isolate and fix all antenna elements.
Antenna Gain, Bandwidth and Beam Angle
According to radio amateurs, this antenna has a lower beam angle of about 15 degrees than a full-sized dipole and is more suitable for DX communications. The Tesla coil dipole has a -2.5 dB attenuation compared to a full size dipole installed at the same height from the ground (λ/4). The bandwidth of the antenna at the level of -3 dB is 120-150 kHz! When placed horizontally, the described antenna has a figure-of-eight radiation pattern like a full-size half-wave dipole, and the radiation pattern minima provide attenuation up to -25 dB. It is possible to improve the efficiency of the antenna, as in the classical version, by increasing the placement height. But when placing antennas in the same conditions at heights of λ / 8 and below, the Tesla spiral antenna will be more effective than a half-wave dipole.
Note: All Tesla coil antenna data looks perfect, but even if this antenna arrangement is worse than a dipole by 6dB, i.e. by one point on the S-meter scale, then this is already wonderful.
Other antenna designs.
With a dipole for a range of 40 meters and with other designs of dipoles up to a range of 10 meters, everything is now clear, but let's return to a spiral vertical for a range of 80 meters (Fig. 10.). Here, preference is given to a half-wave helical antenna, and therefore the “ground” is only nominally needed here.
The power supply of such antennas can be carried out as in Fig. 9 by means of a summing transformer or in Fig. 10. variable capacitor. Of course, in the second case, the antenna bandwidth will be much narrower, but the antenna has the ability to tune in range, and yet, according to the author's information, at least some kind of grounding is necessary. Our task, being on the balcony, is to get rid of it. Since the antenna is powered from the end (in the "antinode" of the voltage), the input impedance of a shortened half-wave helical antenna can be about 800-1000 ohms. This value depends on the height of the vertical part of the antenna, on the diameter of the "Tesla spiral" and on the location of the antenna relative to surrounding objects. To match the high input impedance of the antenna with a low feeder resistance (50Ω), you can use a high-frequency autotransformer in the form of an inductor with a tap (Fig. 21.a), which is widely practiced in half-wave, vertically located linear antennas at 27 MHz by SIRIO, ENERGY, etc.
Data of a matching autotransformer for a half-wave CB antenna in the range of 10-11m:
D = 30mm; L1=2 turns; L2 = 5 turns; d=1.0mm; h=12-13 mm. Distance between L1 and L2 = 5mm. Coils are wound on one plastic frame turn to turn. The cable is connected to the central core to the outlet of 2 turns. The web (end) of the half-wave vibrator is connected to the "hot" output of the coil L2. The power for which the autotransformer is designed is up to 100 watts. Choice of coil tap is possible.
Data of the matching autotransformer for a half-wave antenna of the spiral type with a range of 40m:
D = 32mm; L1=4.6 μH; h=20 mm; d=1.5mm; n=12 turns. L2=7.5 μH; ; h=27 mm; d=1.5mm; n=17 turns. The coil is wound on one plastic frame. The cable is connected to the central core to the outlet. The antenna web (the end of the helix) is connected to the "hot" output of the L2 coil. The power for which the autotransformer is designed is 150-200W. Choice of coil tap is possible.
The dimensions of the antenna "Tesla spiral" range 40m:the total length of the wire is 21m; The outer diameter of the spiral will be 0.9m
Data of the matching autotransformer for the 80m range helix antenna: D = 32mm; L1=10.8 μH; h=37 mm; d=1.5mm; n=22 turns. L2=17.6 μH; ; h=58 mm; d=1.5mm; n=34 turns. The coil is wound on one plastic frame. The cable is connected to the central core to the outlet. The antenna web (the end of the helix) is connected to the "hot" output of the L2 coil. Choice of coil tap is possible.
The dimensions of the antenna "Tesla spiral" range 80m:the total length of the wire is 43m; The outer diameter of the spiral will be 1.2m
Coordination with a half-wave spiral dipole when fed from the end, can be carried out not only by means of an autotransformer, but also by Fuchs, a parallel oscillatory circuit, see Fig.5.a.
Note:
- When feeding a half-wave antenna from one end, tuning to resonance can be done from either end of the antenna.
- In the absence of at least some kind of grounding, it is necessary to install a locking feeder-choke on the feeder.
Vertical Directional Antenna Option
Given a pair of Tesla coil antennas and some territory to place them, you can create a directional antenna. Let me remind you that all operations with this antenna are completely identical with antennas of linear dimensions, and the need to curtail them is not due to the fashion for mini-antennas, but to the lack of locations for linear antennas. The use of two-element directional antennas with a distance between them of 0.09-0.1λ makes it possible to design and build a directional Tesla spiral antenna.
This idea is taken from "KB JOURNAL" N 6 for 1998. This antenna is well described by Vladimir Polyakov (RA3AAE), which can be found on the Internet. The essence of the antenna is that two vertical antennas located at a distance of 0.09λ are fed out of phase by one feeder (one with a braid, the other with a central core). Power is supplied by the type of the same Windom antenna, only with single-wire power, Fig. 22 .. Phase shift between opposite antennas is created by tuning them lower and higher in frequency, as in classic directional Yagi antennas. And coordination with the feeder is carried out by simply moving the feed point along the web of both antennas, moving away from the zero feed point (the middle of the vibrator). By moving the feed point from the middle for some distance X, you can achieve resistance from 0 to 600 ohms, as in the Windom antenna. We will need only about 25 ohms of resistance, so the displacement of the feed point from the middle of the vibrators will be very small.
The electrical circuit of the proposed antenna with approximate dimensions given in wavelengths is shown in Fig.22. And the practical tuning of the Tesla coil antenna to the desired load resistance is quite feasible using the technology of Fig. 20. The antenna is powered at points XX directly by a feeder with a wave impedance of 50 Ohm, and its braid must be isolated with a locking feeder-choke, see Fig.19.
30m RA3AAE vertical directional helical antenna option
If for some reason the radio amateur is not satisfied with the version of the Tesla spiral antenna, then the version of the antenna with spiral radiators is quite feasible, Fig. 23. Let's take a look at her calculation.
We use the length of the half-wave helix wire:
λ=300/MHz =300/10.1; λ/2 -29.7/2=14.85. Accept 15m
Let's calculate the step for coils on a pipe with a diameter of 7.5 cm, the length of the spiral winding = 135 cm:
Circumference L \u003d D * π \u003d -7.5 cm * 3.14 \u003d 23.55 cm. \u003d 0.2355 m;
number of turns of a half-wave dipole -15m/ 0.2355=63.69= 64 turns;
winding step on a rube 135 cm long. - 135cm/64=2.1cm..
Answer: on a pipe with a diameter of 75 mm we wind 15 meters of copper wire with a diameter of 1-1.5 mm in the amount of 64 turns with a winding pitch = 2 cm.
The distance between identical vibrators will be 30*0.1=3m.
Note: Antenna calculations were carried out with rounding for the possibility of shortening the winding wire during tuning.
To increase the bias current and ease of adjustment, it is necessary to make small adjustable capacitive loads at the ends of the vibrators, and on the feeder, at the connection point, it is necessary to put on a locking feeder-choke. The displaced feed points correspond to the dimensions in fig. 22. It should be remembered that unidirectionality in this design is achieved by a phase shift between opposite spirals by tuning them with a difference of 5-8% in frequency, as in classic directional Uda-Yaga antennas.
Rolled up "Bazooka"
As you know, the noise situation in any city leaves much to be desired. This also applies to the radio frequency spectrum due to the widespread use of switching power converters for household appliances. Therefore, I made an attempt to use the well-proven Bazooka type antenna in the Tesla spiral antenna. In principle, this is the same half-wave vibrator with a closed system, like all loop antennas. It was not difficult to place it on the traverse presented above. The experiment was carried out at a frequency of 10.1 MHz. A television cable with a diameter of 7 mm was used as the antenna web. (fig.24). The main thing is that the cable braid is not aluminum like its sheath, but copper.
Even experienced radio amateurs “pierce” on this, taking the gray cable braid for tinned copper when buying. Since we are talking about QRP - an antenna for a balcony, and the input power is up to 100 W, then such a cable will be quite suitable. The shortening coefficient of such a cable with polyethylene foam is about 0.82. Therefore, the length L1 (Fig. 25.) For a frequency of 10.1 MHz. It was 7.42cm each, and the length of the L2 extension conductors with this antenna layout was 1.83cm. The input impedance of the folded "Bazooka" after installation in an open area was about 22-25 ohms and is not regulated by anything. Therefore, a 1: 2 transformer was required here. In a trial version, it was made on a ferrite latch with simple wires from sound speakers with a ratio of turns according to Table 1. Another version of the 1:2 transformer is shown in fig. 26.
Aperiodic broadband antenna "Bazooka"
Not a single radio amateur who has at his disposal even an antenna field on the roof of his house or in the yard of a cottage will refuse a survey broadband antenna based on a Tesla coiled feeder. The classic version of an aperiodic antenna with a load resistor is known to many, here the Bazooka antenna acts as a broadband vibrator, and its bandwidth, as in the classical versions, has a large overlap towards higher frequencies.
The antenna circuit is shown in fig. 27, and the power of the resistor is about 30% of the input power to the antenna. If the antenna is used only as a receiving antenna, the power of the 0.125W resistor is sufficient. It should be noted that the antenna "Tesla spiral", installed horizontally, has a figure-of-eight radiation pattern and is capable of conducting spatial selection of radio signals. Installed vertically, it has a circular radiation pattern.
4. Magnetic antennas.
The second, no less popular type of antenna is an inductive radiator with shortened dimensions, this is a magnetic frame. The magnetic frame was discovered in 1916 by K. Brown and was used until 1942 as a receiver in radio receivers and direction finders. This is also an open oscillatory circuit with a frame perimeter less than ≤ 0.25 wavelength, it is called “magnetic loop” (magnetic loop), and the abbreviated name has acquired an abbreviation - ML. The active element of the magnetic loop is the inductance. In 1942, a radio amateur with the radio call sign W9LZX used such an antenna for the first time at the mission broadcast station HCJB, located in the mountains of Ecuador. Thanks to this, the magnetic antenna immediately conquered the amateur radio world and has since been widely used in amateur and professional communications. Magnetic loop antennas are one of the most interesting types of small-sized antennas that are conveniently placed both on balconies and on window sills.
It takes the form of a loop of conductor that is connected to a variable capacitor to achieve resonance, where the loop is the radiating inductance of an oscillating LC circuit. The emitter here is only an inductance in the form of a loop. The dimensions of such an antenna are very small, and the perimeter of the frame is usually 0.03-0.25 λ. The maximum efficiency of the magnetic loop can reach 90% with respect to the Hertzian dipole, see Fig.29.a. The capacitance C in this antenna does not participate in the radiation process and carries a purely resonant character, as in any oscillatory circuit, fig. 29.b..
The antenna efficiency strongly depends on the active resistance of the antenna web, on its dimensions, on its placement in space, but to a greater extent on the materials used for the construction of the antenna. The bandwidth of a loop antenna usually ranges from units to tens of kilohertz, which is associated with a high quality factor of the formed LC circuit. Therefore, the efficiency of an ML antenna is highly dependent on its quality factor, the higher the quality factor, the higher its efficiency. This antenna is also used as a transmitting antenna. With small frame sizes, the amplitude and phase of the current flowing in the frame are practically constant along the entire perimeter. The maximum radiation intensity corresponds to the plane of the frame. In the perpendicular plane of the frame, the radiation pattern has a sharp minimum, and the overall pattern of the loop antenna has the shape of a "figure eight".
Electric field strength E electromagnetic wave (V/m) at a distance d from transmitting loop antenna, is calculated by the formula:
EMF E , induced in reception loop antenna, is calculated by the formula:
The figure-of-eight directional pattern of the frame allows using its minimums of the diagram in order to tune it in space from closely spaced interference or unwanted radiation in a certain direction in the near zones up to 100 km.
In the manufacture of the antenna, it is required to observe the ratio of the diameters of the radiating ring and the coupling coil D / d as 5/1. The connection coil is made of a coaxial cable, located in close proximity to the radiating ring on the opposite side of the capacitor, and looks like in Fig.30.
Since a large current flows in the radiating frame, reaching tens of amperes, the frame in the frequency ranges of 1.8-30 MHz is made of a copper tube with a diameter of about 40-20 mm, and the resonance tuning capacitor should not have rubbing contacts. Its breakdown voltage should be at least 10 kV with input power up to 100 W. The diameter of the radiating element depends on the range of frequencies used and is calculated from the wavelength of the high-frequency part of the range, where the perimeter of the frame is P = 0.25λ, counting from the upper frequency.
Probably one of the first W9LZX, German shortwave DP9IV with an ML antenna installed on the window, with a transmitter power of only 5 W, in the 14 MHz band, he made a QSO with many European countries, and with a power of 50 W - with other continents. It was this antenna that became the starting point for the experiments of Russian radio amateurs, see Fig.31.
The desire to create an experimental compact indoor antenna, which can just as safely be called an EH antenna, in close cooperation with Alexander Grachev ( UA6AGW), Sergey Tetyukhin (R3PIN) designed the next masterpiece, see Fig.32.
It is this low-budget design of the indoor version of the EH-antenna that can please a newcomer radio amateur or a summer resident. The antenna circuit includes both a magnetic emitter L1; L2, and a capacitive one in the form of telescopic "whiskers".
Special attention in this design (R3PIN) deserves the resonant system for matching the feeder with the antenna Lsv; C1, which once again increases the quality factor of the entire antenna system and allows you to slightly increase the gain of the antenna as a whole. As the primary circuit, together with the "whiskers" as in the design of Yakov Moiseevich, here the braid of the cable of the antenna web acts. The length of these "whiskers" and their position in space, it is easy to achieve resonance and the most efficient operation of the antenna as a whole according to the current indicator in the frame. And providing the antenna with an indicator device allows us to consider this version of the antenna as a completely finished construct. But whatever the design of magnetic antennas, you always want to increase its efficiency.
Double-loop magnetic antennas in the form of a figure eight relatively recently began to appear among radio amateurs, see Fig.33. Its aperture is twice as large compared to the classical one. Capacitor C1 can change the resonance of the antenna with a frequency overlap of 2-3 times, and the total perimeter of the circle of two loops ≤ 0.5λ. This is commensurate with a half-wave antenna, and its small radiation aperture is compensated by an increased quality factor. Coordination of the feeder with such an antenna is best done by inductive coupling.
Theoretical digression: The double loop can be considered as a mixed LL and LC oscillatory system. Here, for normal operation, both arms are loaded on the radiation medium synchronously and in phase. If a positive half-wave is applied to the left shoulder, then exactly the same is applied to the right shoulder. The self-induction EMF that originated in each arm will, according to the Lenz rule, be opposite to the induction EMF, but since the induction EMF of each arm is opposite in direction, the self-induction EMF will always coincide with the direction of the induction of the opposite arm. Then the induction in the coil L1 will be summed up with the self-induction from the coil L2, and the induction of the coil L2 - with the self-induction L1. Just as in the LC circuit, the total radiation power can be several times greater than the input power. Energy can be supplied to any of the inductors and in any way.
The double frame is shown in Fig.33.a.
The design of a two-loop antenna, where L1 and L2 are connected to each other in the form of a figure of eight. So there was a two-frame ML. Let's call it conditionally ML-8.
ML-8, unlike ML, has its own peculiarity - it can have two resonances, the oscillatory circuit L1; C1 has its own resonant frequency, and L2; C1 has its own. The task of the designer is to achieve the unity of resonances and, accordingly, the maximum efficiency of the antenna, therefore, the dimensions of the loops L1; L2 and their inductances must be the same. In practice, an instrumental error of a couple of centimeters changes one or another inductance, the resonance tuning frequencies diverge somewhat, and the antenna receives a certain frequency delta. In addition, doubling the inclusion of identical antennas expands the bandwidth of the antenna as a whole. Sometimes designers do this on purpose. In practice, ML-8 is actively used by radio amateurs with radio call signs RV3YE; US0KF; LZ1AQ; K8NDS and others unequivocally stating that such an antenna works much better than a single-loop one, and changing its position in space can be easily controlled by spatial selection. Preliminary calculations show that for the ML-8 for a range of 40 meters, the diameter of each loop at maximum efficiency will be slightly less than 3 meters. It is clear that such an antenna can only be installed outdoors. And we dream of an efficient ML-8 antenna for a balcony or even a window sill. Of course, you can reduce the diameter of each loop to 1 meter and adjust the resonance of the antenna with capacitor C1 to the required frequency, but the efficiency of such an antenna will drop by more than 5 times. You can go the other way, keep the calculated inductance of each loop, using not one, but two turns in it, leaving the resonant capacitor with the same rating, respectively, and the quality factor of the antenna as a whole. Undoubtedly, the antenna aperture will decrease, but the number of turns "N" will partially compensate for this loss, according to the formula below:
From the above formula it can be seen that the number of turns N is one of the multipliers of the numerator and is in the same row, both with the area of the turn-S, and with its quality factor-Q.
For example, a radio amateur OK2ER(See Fig.34.) considered it possible to use a 4-turn ML with a diameter of only 0.8m in the range of 160-40m.
The author of the antenna reports that at 160 meters the antenna works nominally and is used more for radio surveillance. In the range of 40m. it is enough to use a jumper that reduces the working number of turns by half. Let's pay attention to the materials used - the copper pipe of the loop is taken from water heating, the clips connecting them into a common monolith are used for installing plastic water pipes, and a sealed plastic box was purchased at an electrician's store. Coordination of the antenna with the feeder is capacitive, and is performed according to any of the presented schemes, see Fig.35.
In addition to the above, we need to understand that the following antenna elements have a negative effect on the quality factor-Q of the antenna as a whole:
From the above formula, we see that the active resistance of the inductance Rk and the capacitance of the oscillatory system Sk, standing in the denominator, should be minimal. It is for this reason that all MLs are made from a copper pipe of as large a diameter as possible, but there is a case when the hinge web is made of aluminum. The quality factor of such an antenna and its efficiency drops by 1.1-1.4 times. As for the capacitance of the oscillatory system, everything is more complicated here. With a constant loop size L, for example, at a resonant frequency of 14 MHz, the capacitance C will be only 28 pF, and the efficiency = 79%. At a frequency of 7 MHz, efficiency = 25%. Whereas at a frequency of 3.5 MHz with a capacitance of 610 pF, its efficiency = 3%. Therefore, ML is most often used for two ranges, and the third (lowest) is considered an overview. Therefore, it is necessary to make calculations based on the highest range with a minimum capacity of C1.
Double magnetic antenna for a range of 20m.
The parameters of each loop will be as follows: With a web (copper pipe) diameter of 22 mm, a double loop diameter of 0.7 m, a distance between turns of 0.21 m, the loop inductance will be 4.01 μH. The required design parameters of the antenna for other frequencies are summarized in Table 3.
Table 3
|
Tuning frequency (MHz) |
Capacitor C1 (pF) |
Bandwidth (kHz) |
|
In height, such an antenna will be only 1.50-1.60 m. Which is quite acceptable for an antenna of the type - ML-8 of a balcony version and even an antenna hung outside the window of a residential high-rise building. And its wiring diagram will look like in Fig. 36.a.
Antenna Power may be capacitive or inductive. The capacitive coupling options shown in Fig. 35 can be selected at the request of the radio amateur.
The most budget option is an inductive coupling, but its diameter will be different.
Calculation of the diameter (d) of the ML-8 connection loop made from the calculated diameter of two loops.
The circumference of two loops is after recalculation 4.4 * 2 = 8.8 meters.
Calculate the imaginary diameter of two loops D = 8.8m / 3.14 = 2.8 meters.
Calculate the diameter of the communication loop-d= D/5. = 2.8/5 = 0.56 meters.
Since in this design we use a two-turn system, the communication loop must also have two loops. We twist it in half and get a two-turn communication loop with a diameter of about 28cm. The selection of communication with the antenna is carried out at the time of clarifying the SWR in the priority frequency range. The communication loop can have a galvanic connection with the zero voltage point (Fig. 36.a.) and be located closer to it.
Electric emitter, this is another additional element of radiation. If the magnetic antenna emits an electromagnetic wave with the priority of the magnetic field, then the electric emitter will perform the function of an additional emitter of the electric field-E. In fact, it should replace the initial capacitance C1, and the drain current, which previously passed uselessly between the closed plates of the capacitor C1, now works for additional radiation. In this case, the share of the input power will additionally be emitted by electric emitters, Fig. 36.b. The bandwidth will increase to the limits of the amateur band as in EH antennas. The capacitance of such emitters is low (12-16pF, no more than 20), and therefore their efficiency in low-frequency ranges will be low. You can get acquainted with the work of EH antennas at the links:
For resonating a magnetic antenna, it is best to use vacuum capacitors with high breakdown voltage and high quality factor. Moreover, using a gearbox and an electric drive, antenna tuning can be carried out remotely.
We are designing a budget balcony antenna, which can be approached at any time, changed its position in space, rebuilt or switched to another frequency. If at points “a” and “b” (see Fig. 36.a.) instead of a scarce and expensive variable capacitor with large gaps, connect a capacitance made of RG-213 cable segments with a linear capacitance of 100pF / m, then you can instantly change the frequency settings, and the tuning capacitor C1 to refine the tuning resonance. The "capacitor cable" can be rolled up and sealed in any of the ways. Such a set of containers can be available for each range separately, and included in the circuit through a conventional electrical outlet (points a and b) paired with an electrical plug. Approximate C1 capacities by ranges are shown in Table 1.
Antenna tuning indication it is better to do it directly on the antenna itself (it's clearer). To do this, it is enough to wind tightly 25-30 turns of MGTF wire not far from the communication coil on the L1 canvas (zero voltage point), and seal the tuning indicator with all its elements from precipitation. The simplest circuit is shown in Fig.37. The maximum readings of the device P will indicate a successful antenna tuning.
To the detriment of the efficiency of the antenna As the material of the loops L1; L2, you can use cheaper materials, for example, a PVC pipe with an aluminum layer inside for laying a water pipe with a diameter of 10-12 mm.
DDRR Antenna
Despite the fact that the classic DDRR antenna is inferior to the quarter-wave vibrator by 2.5 dB in terms of its efficiency, its geometry turned out to be so attractive that the DDRR was patented by Northrop and put into mass production.
As in the case of the Groundplane, the main factor in the decent efficiency of the DDRR antenna is a good counterbalance. It is a flat metal disk with high surface conductivity. Its diameter must be at least 25% greater than the diameter of the ring conductor. The elevation angle of the main beam is the smaller, the higher the ratio of the diameters of the counterweight disk and increases if as many radial counterweights with a length of 0.25λ are fixed around the circumference of the disk, ensuring their reliable contact with the counterweight disk.
The DDRR antenna considered here (Fig. 38) uses two identical rings (hence the name "double-ring-circular"). At the bottom, instead of a metal surface, a closed ring with dimensions similar to the top one is used. All grounding points are connected to it according to the classical scheme. Despite a slight decrease in the efficiency of the antenna, this design is very attractive for placing it on a balcony, in addition, with this solution, it is also of interest to connoisseurs of the 40-meter range. Using square structures instead of rings, the antenna on the balcony resembles a clothes dryer and does not cause unnecessary questions from the neighbors.
All its dimensions and capacitor ratings are presented in Table 4. In the budget version, an expensive vacuum capacitor can be replaced with feeder segments according to the range, and fine tuning can be done with a 1-15pF trimmer with an air dielectric, remembering that the linear capacity of the cable is RG213 = (97pF / m).
Table 4
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Amateur bands, (m) |
||||||||
|
Frame perimeter (m) |
||||||||
Practical experience with a dual ring DDRR antenna was described by DJ2RE. The 10-meter band antenna under test was made of a copper tube with an outer diameter of 7 mm. To fine-tune the antenna, two copper rotary plates 60x60 mm in size were used between the upper "hot" end of the conductor and the lower ring.
The comparison antenna was a rotary three-element Yagi, located 12 m from the ground. The DDRR antenna was at a height of 9 m. Its lower ring was grounded only through the screen of the coaxial cable. During the test reception, the qualities of the DDRR antenna as a circular radiator immediately appeared. According to the test author, the received signal was two points lower on the S-meter of the Yagi signal with a gain of about 8 dB. When transmitting with a power of up to 150 W, 125 communication sessions were performed.
Note: According to the test author, it turns out that the DDRR antenna at the time of testing had a gain of about 6 dB. This phenomenon is often misleading due to the proximity of different antennas of the same range, and the properties of EMW re-radiation by them loses the purity of the experiment.
5. Capacitive antennas.
Before starting this topic, I would like to recall the history. In the 60s of the 19th century, while formulating a system of equations for describing electromagnetic phenomena, J.K. Maxwell was faced with the fact that the equation for the DC magnetic field and the equation for the conservation of electric charges of alternating fields (the continuity equation) are incompatible. To eliminate the contradiction, Maxwell, without any experimental data, postulated that the magnetic field is generated not only by the movement of charges, but also by a change in the electric field, just as the electric field is generated not only by charges, but also by a change in the magnetic field. The value where is the electric induction, which he added to the conduction current density, Maxwell called bias current. Electromagnetic induction has a magnetoelectric analogue, and the field equations have acquired a remarkable symmetry. Thus, one of the most fundamental laws of nature was speculatively discovered, the consequence of which is the existence of electromagnetic waves. Subsequently, G. Hertz, relying on this theory, proved that the electromagnetic field radiated by an electric vibrator is equal to the field radiated by a capacitive radiator!
If so, let's make sure once again what happens when a closed oscillatory circuit turns into an open one and how can the electric field E be detected? To do this, next to the oscillatory circuit, we will place an indicator of the electric field, this is a vibrator, in the gap of which an incandescent lamp is included, it is not yet lit, see Fig. 39.a. We gradually open the circuit, and we observe that the electric field indicator lamp lights up, fig. 39.b. The electric field is no longer concentrated between the plates of the capacitor, its lines of force go from one plate to another through open space. Thus, we have experimental confirmation of J.K. Maxwell's statement that a capacitive radiator generates an electromagnetic wave. In this experiment, a strong high-frequency electric field is formed around the plates, the change of which in time induces eddy displacement currents in the surrounding space (Eikhenvald A.A. Electricity, fifth ed., M.-L.: State Publishing House, 1928, Maxwell's first equation), forming a high-frequency electromagnetic field!
Nikola Tesla drew attention to this fact, that with the help of very small emitters in the HF range, it is possible to create a fairly effective device for emitting an electromagnetic wave. This is how the N. Tesla resonant transformer was born.
* The design of the EH antenna by T. Hard and the transformer (dipole) by N. Tesla.
Is it worth it to say once again that the EH antenna designed by T. Hard (W5QJR), see Fig.40, is a copy of the original Tesla antenna, see Fig.1. Antennas differ only in size, where Nikola Tesla used frequencies calculated in kilohertz, and T. Hard created a design for operation in the HF range.
The same resonant circuit, the same capacitive radiator with an inductor and a coupling coil. The Ted Hard antenna is the closest analogue to the Nikola Tesla antenna and was patented as, "Coaxial inductor and dipole EH antenna" (US Patent US 6956535 B2 dated 10/18/2005) for operation in the HF band.
Ted Hard's capacitive HF antenna is inductively coupled to the feeder, although a number of capacitive, direct-coupled, and transformer-coupled capacitive antennas have long existed.
The basis of the supporting structure of the engineer and radio amateur T. Hard is an inexpensive plastic pipe with good insulating characteristics. The foil in the form of cylinders tightly fits it, thereby forming antenna emitters with a small capacitance. The inductance L1 of the formed series oscillatory circuit is located behind the emitter aperture. The inductor L2, located in the center of the emitter, compensates for the antiphase radiation of the coil L1. The antenna power connector (from the generator) W1 is located at the bottom, which is convenient for connecting a power feeder that goes down.
In this design, the antenna is tuned by two elements, L1 and L3. By selecting the turns of the L1 coil, the antenna is tuned to the serial resonance mode according to the maximum radiation, where the antenna acquires a capacitive character. The tap from the inductor determines the input impedance of the antenna and whether the radio amateur has a feeder with a characteristic impedance of 50 or 75 ohms. By selecting a tap from the L1 coil, you can achieve SWR \u003d 1.1-1.2. The inductor L3 achieves compensation from a capacitive nature, and the antenna takes on an active character, in terms of input resistance close to SWR = 1.0-1.1.
Note: Coils L1 and L2 are wound in opposite directions, and coils L1 and L3 are perpendicular to each other to reduce mutual influence.
This antenna construct undoubtedly deserves the attention of radio amateurs who have at their disposal only a balcony or loggia.
Meanwhile, developments do not stand still and radio amateurs, having appreciated the invention of N. Tesla and the design of Ted Hart, began to offer other options for capacitive antennas.
* Antenna family "Isotron" is a simple example of flat curved capacitive radiators, it is produced by the industry for operation by its radio amateurs, see Fig.42. Antenna "Isotron" has no fundamental difference with the antenna T. Hord. All the same series oscillatory circuit, all the same capacitive emitters.
Namely, the radiation element here is the radiating capacitance (Sizl.) in the form of two plates bent at an angle of about 90-100 degrees, the resonance is adjusted by decreasing or increasing the bend angle, i.e. their capacities. According to one version, communication with the antenna is carried out by direct connection of the feeder and a series oscillatory circuit, in this case, the SWR determines the L / C ratio of the formed circuit. According to another version, which radio amateurs began to use, communication is carried out according to the classical scheme, through the communication coil Lsv. The SWR in this case is tuned by changing the connection between the series resonance coil L1 and the coupling coil Lb. The antenna is efficient and to some extent effective, but it has a main drawback, the inductor, when it is located in the factory version, is located in the center of the capacitive radiator, works in antiphase with it, which reduces the efficiency of the antenna by about 5-8 dB. It is enough to turn the plane of this coil by 90 degrees and the antenna efficiency will increase significantly.
The optimal dimensions of the antenna are summarized in Table 5.
* Multi-range option.
All Isotron antennas are single-band, which causes a number of inconveniences when switching from band to band and placing them. When two (three, four) such antennas are connected in parallel, mounted on a common bus, operating at frequencies f1; f2 and fn, their interaction is excluded due to the high resistance of the series oscillatory circuit of the antenna not participating in resonance. When manufacturing two single-resonant antennas connected in parallel on a common bus, the efficiency (efficiency) and bandwidth of such an antenna will be higher. Using the last option of in-phase connection of two single-band antennas, it must be remembered that the total input impedance of the antennas will be half as much and it is necessary to take appropriate measures by referring to (Table 1). Modification of the antenna on a common substrate is shown in fig. 42 (bottom). Needless to say, the locking feeder choke is an integral part of any mini-antenna.
Studying the simplest "Isotron", we came to the conclusion that the gain of this antenna is not enough due to the placement of a resonant inductor between the radiating plates. As a result, this design was improved by radio amateurs in France, and the inductor was moved outside the working environment of the capacitive radiator, see Fig.43. The antenna circuit is directly connected to the feeder, which simplifies the design, but still complicates full coordination with it.
As can be seen from the presented figures and photos, this antenna is quite simple in design, especially in tuning it to resonance, where it is enough to slightly change the distance between the emitters. If the plates are interchanged, the upper one is made “hot” and the lower one is connected to the feeder braid, a common bus is made for a number of other similar antennas, then a multi-band antenna system can be obtained, or a number of identical antennas connected in phase can increase the overall gain.
Radio amateur with radio call sign F1RFM, kindly provided for a general review of his antenna design with calculations for 4 amateur radio bands, the diagram of which is shown in Fig. 44.
* Antenna "Biplane"
The "Biplane" antenna is named for its similarity to the placement of the twin wings of early 20th century aircraft designed by the "Biplane", and its invention belongs to a group of radio amateurs (Fig. 45). The "Biplane" antenna consists of two series oscillatory circuits L1;C1 and L2;C2 connected in anti-parallel. Feed emitters, symmetrical with direct connection. The planes of capacitors C1 and C2 are used as radiating elements. Each emitter is made of two duralumin plates and located on both sides of the inductors.
Inductors are wound oppositely or perpendicular to each other to avoid interference. The area of each plate, according to the authors, will be 64.5 cm2 for the 20-meter band, 129 cm2 for the 40-meter band, 258 cm2 for the 80-meter band, and 516 cm2 for the 160-meter band, respectively.
The adjustment is carried out in two stages and can be carried out by elements C1 and C2 by changing the distance between the plates. The minimum SWR is achieved by changing the capacitances C1 and C2 by tuning the transmitter to the frequency. The antenna is very difficult to set up and requires a complex sealing design from the influence of external precipitation. It has no development prospects and is unprofitable.
On the topic of capacitive antennas, it is worth noting that they have occupied a special niche among radio amateurs who do not have the opportunity to install full-fledged antennas, which only have a balcony or loggia. Radio amateurs who have the opportunity to install a low mast on a small antenna field also use such antennas. All shortened antennas have the common name QRP antennas. In addition, radio amateurs have a number of errors when installing and operating antennas of a shortened type, this is the absence of a blocking "feeder-choke" or a very close location of the latter on a ferrite base to the canvas of a shortened antenna. In the first case, the antenna feeder begins to radiate, and in the second, the ferrite of such a choke is a “black hole” and reduces its effectiveness.
* EH antenna of the USSR SA troops of the 40s - 50s of the last century.
The antenna was welded from duralumin pipes with a diameter of 10 and 20 mm. Flat, broadband symmetrical split dipole about 2 meters long and 0.75 m wide. Operating frequency range 2-12MHz. Why not a balcony antenna? It was mounted on the roof of a mobile radio room in a horizontal position at a height of about 1m.
Back in the 90s, the author of this article reproduced this design on the balcony of the second floor, and the emitters were made under the clothes dryer on wooden bars outside the balcony. Instead of ropes, copper insulated wires were stretched, see Fig. 46.a. The antenna was tuned using an oscillatory circuit L1C1, a capacitor C2 for coupling with the antenna and a coupling coil Lsv. with transceiver, see fig. 46.b. All capacitors with air insulation with a capacity of 2 * 12-495pF were used from tube radios of the 60s.
Inductor L1 diameter 50 mm; 20 turns; wire 1.2 mm; pitch 3.5 mm. On top of this coil, a plastic pipe sawn along along the length (50 mm) was tightly worn. A communication coil Ls was wound over it. - 5 turns with taps from 3; 4 and 5 turns of 2.2 mm wire. For all capacitors, only stator contacts were used, and the axes (rotors) on capacitors C2 and C3 were connected by an insulating jumper for rotation synchronism. The two-wire line should be no more than 2.0-2.5 meters, this is just the distance from the antenna (dryer) to the matching device standing on the windowsill. The antenna was built in the range of 1.8-14.5 MHz, but when changing the resonant circuit to other parameters, such an antenna could work up to 30 MHz. In the original, in series with the transmission line in this design, current indicators were provided that were adjusted to the maximum readings, but in a simplified version, between the two wires of the two-wire line, a fluorescent lamp hung perpendicular to it, which glowed only in the middle at the minimum power output, and at maximum power ( at resonance) the glow reached the edges of the lamp. Coordination with the radio station was carried out by switch P1 and monitored by an SWR meter. The bandwidth of such an antenna was more than sufficient for operation on each of the amateur bands. With an input power of 40-50W. The antenna did not interfere with television neighbors. Others now, when everyone has switched to digital and cable television, you can bring up to 100W.
This type of antenna belongs to the capacitive ones and differs from the EH antennas only in the emitter switching circuit. It differs in their shape and size, but at the same time, it has the ability to change over the HF range and be used for its intended purpose - drying clothes ...
* Combining E-emitter and H-emitter.
Using a capacitive radiator outside the balcony (loggia), this construct can be combined with a magnetic antenna, as Alexander Grachev did ( UA6AGW) by combining a magnetic frame with a half-wave shortened dipole. In the amateur radio world, it is quite well known and practiced by the author at their summer cottage. The electrical circuit of the antenna is quite simple and is shown in Fig. 47.
Capacitor C1 is trimmer within the range, and the required range can be set by connecting an additional capacitor to the contacts of K1. Antenna and feeder matching is subject to the same laws, i.e. connection loop at the zero voltage point, see Fig.30. Fig.31. Such a modification has the advantage that its installation can be made really invisible to prying eyes, and besides, it will work quite effectively in two or three amateur frequency bands.
A shortened dipole in the form of a plastic-based spiral fits perfectly inside the loggia with wooden frames, but the owner of this antenna did not dare to put it outside the loggia. It does not seem that the owner of this apartment is delighted with this beauty.
Balcony antenna - dipole 14/21/28 MHz fit well outside the balcony. She is unobtrusive and draws no attention to herself. You can build such an antenna by contacting the link
Afterword:
In conclusion of the material on balcony HF antennas, I would like to say to those who do not have and are not expected to have access to the roof of their house - it is better to have a bad antenna than none at all. Everyone can work with a three-element Uda-Yaga antenna or a double square, but not everyone can choose the best option, design and build a balcony antenna, and work on the air at the same level. Do not change your hobby, it will always come in handy for you to relax your soul and train your brain, while on vacation or at retirement age. Communication over the air is much more useful than communication over the Internet. Men who do not have a hobby, do not have a goal in life, live less.
73! Sushko S.A. (ex. UA9LBG)