Magnetic antenna. Magnetic antennas for ultra-long-range radio communications. Properties of magnetic antennas

Smaller magnetic loop antennas are relatively rarely used by Ham-radio amateurs. However, despite their disadvantages, such as low efficiency and narrow bandwidth, they have a number of advantages. This is the possibility of spatial and frequency selection of a radio signal, i.e. orienting the antenna to maximize the useful signal or minimize the interference signal. Isolation of a useful signal by frequency detuning, as well as its small geometric dimensions relative to the wavelength. Therefore, frame antennas are most widely used as receiving antennas for direction finders and broadcast receivers operating in the long, medium and short wave bands.

Such antennas are most often used in camping conditions, and can be tuned in range with a threefold change in frequency. The efficiency of the antenna depends on its geometric dimensions relative to the wavelength, see fig. 1.

This antenna is also used as a transmitting antenna. With small frame sizes, the amplitude and phase of oscillations 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 diagram of the loop antenna has a figure-of-eight shape.

Electric field strength E electromagnetic wave (V/m) at a distance d (??3) from transmitting loop antenna, calculated by the formula:

Where:
I — current in the frame (A); n — number of turns; d — distance (km);
S — frame area (m2); ? — operating wavelength (m);
Cos? — the angle between the plane of the frame and the direction to the point in question.

EMF E induced in reception loop antenna, calculated by the formula:

Where:
n — number of turns;
S - frame area;
E — electric field strength at the observed point;
Cos? — the angle between the plane of the frame and the direction of arrival of the wave.

The figure-of-eight radiation pattern of the frame allows you to use its minimums in the diagram in order to detune it in space from nearby interference or unwanted radiation in a certain direction in near zones up to 100 km.

The antenna design is classic and is shown in Fig. 2, it consists of an open oscillatory circuit in the form of an unfolded inductance, tuned by capacitor C to resonance. According to DK5CZ, the bandwidth also increases threefold with increasing tuning frequency and, at a level of 0.707, has a bandwidth from 3 to 30 kHz. When manufacturing an antenna, it is required to maintain the ratio of the diameters of the radiating ring and the coupling turn D/d as 5/1, it is made of coaxial cable, is located in close proximity to the radiating ring on the opposite side of the capacitor, and looks like in Fig. 3.

Since a large current flows in the radiating frame, reaching tens of amperes, the frame in the frequency range 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 10 kV with an input power of 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 λв, where the frame perimeter P = 0.25λв.

We expand the frame bandwidth and increase efficiency

The only problem that arises with all shortened loop antennas is narrowband. In the range of 180-160 m with an antenna quality factor of 200...250, the bandwidth at the level of 0.707 will be about 6 kHz, which is a big drawback when changing the frequency of a radio station. The antenna can be adjusted within the range discretely, using a relay and a set of constant capacitors.

You can expand the bandwidth of the loop antenna and increase its operating efficiency by using several similar antennas, which are located in such a way relative to each other that there is a magnetic coupling between them. This means that the frames must be parallel to each other. In this case, it is enough to power only one antenna, and the rest will expand the bandwidth of the entire system and increase the signal level by about 3 dB. In Fig. 4a shows the frequency response of a single loop antenna, in Fig. 4b - frequency response of two (or several) such antennas.

The frames must have the same geometric and electrical parameters and are installed parallel to each other at a distance of no more than the diameter of the frame. The distance is determined by the required bandwidth without sacrificing additional gain. The communication loop is installed on any of the frames, so that the second one works independently. A loop antenna works even better if there are three of them installed, i.e. one in the middle, and two additional ones are placed at a distance of half the diameter of the frame on both sides in the same plane.

If a radio amateur has difficulty rotating such a structure, then you can use the goniometer principle and place the frames perpendicularly. Then only the communication loop needs to be rotated. The result is almost a direction finder.

73! UA9LBG & Radio-Vector-Tyumen

Article 2. Magnetic antennas (magnetic loop):

Antenna is a device for emitting and/or receiving electromagnetic waves by directly converting electric current into radiation (during transmission) or radiation into electric current (during reception).

Magnetic antenna(magnetic loop) is an antenna in which the emission and reception of electromagnetic waves is carried out due to the magnetic component; the electrical component is negligible and is usually neglected.

(On the ODLR.ru forum in November 2010, there was a discussion of one antenna - a broom, for a tube receiver, using a balcony version. I inserted my piece, and the result was an article.)

And so I’ll try to write it in the style of a true story.

But we are talking about antennas. I lived then in the military town of Kalininets, in the common people’s name “Alabino post office”. Every day in the morning, I took the bus to Golitsino, took the train to the Fili platform, then took the metro to Nogina Square (now Kitay-Gorod). then walk to Pokrovsky Boulevard, within the walls of his native alma mater. In the evening, the same route, but in reverse. And only on Fridays there was an exception to the rule; there was a stop in the Fili area.

My friend RA3AHQ lived not far from the platform; in the world he is Alexander Bolgarinov (now lives in Maryino). I took a couple of “fire extinguishers” and went to visit. Alexander had an imported Kenwood “TS-450” transceiver, which was very cool at that time. Such exceptions to the rules happened almost every week, and only on Fridays. One day we were sitting, sipping some red wine and turning the vernier knob, listening to the conversations of radio amateurs. My attention was attracted by an unusual structure on the windowsill, I ask if you are from Das, and Sasha says that this antenna is called a magnetic loop and shows an article in the magazine Radio No. 7 for 1989, page 90, in the section for abroad. In a word, this is the article that Sergey Kashekhlebov cited in the discussion on the forum. I arrived home, begged a halo hoop from a neighbor, and within two hours, I made my first radio communication on 40 m with Peter, my antenna was mounted on a board, the KPI was screwed to the halo hoop (the duralumin is not soldered). This was my first experience, after which there were other experiences, but more on that later.

In 2000, I was hired by a company that dealt professionally with radio communication systems. There was one project in the Arctic, we went for testing. We took with us several types of antennas, these are traditional triangles, made of antenna rope, and spiral-pin, at the base of which there were automatic antenna tuners (Icom AT-130) and one ML (Magnetic loop) design, made of coaxial cable, corrugated braid 30 mm thick. The diameter of the emitter was 4 m, the antenna was fixed on an ordinary wooden pole with a cross, and attached to an iron trailer. After a certain time, we get in touch, test the passage, and draw up a daily schedule for the passage. And suddenly everything disappeared, there was only “white noise” on the air, and nothing more. They told me on the phone from the base that there was a magnetic storm and a break for an indefinite period of time. Out of boredom, I started clicking and switching antennas on the amateur bands. Imagine my surprise when I heard radio amateurs working on 40 meters. I'm for the microphone and let's go. I asked all the correspondents to listen to two more antennas, switched to “delta” and helical pin, and then ML, I didn’t hear anything on those antennas and they didn’t hear me either.

Later, I persuaded the commercial director to buy a couple of antennas in Germany; I wanted different sizes, but they bought the same type. At that time, production was established there and Christian DK5CZ was in charge of this (heaven rest in peace, the key was silent). But people are still continuing his work. So let's go back here. The German design was not practical, the emitter diameter was 1.7 m, solid, inconvenient for transportation. In general, we made our own antenna, the emitter consisted of three segments, the material was AD-30 (I took a piece of the German one for chemical analysis), the KPI was made in the form of a butterfly and had a capacity from 170 to 200 peaks, this made it possible to cover 3 amateur bands for transmission (160 m, 80 m and 40 m), with a radiator diameter of 4 m. But this is not the main thing, the main thing is how this antenna worked.

Everyone who visited our team probably noticed that in the immediate vicinity of the radio station (300-500 m) there are three power lines running in a semicircle, one of them is 500 kV. So our chatter is always 8-9 points according to the S-meter. And when I placed the ML horizontally on the roof (on pegs 1 m high), using it as a receiving antenna, then.... There was ZERO noise, and only a useful signal. Stations began to be heard that were at a level of 2-3 points, and which I would never have heard. This was on the 20m band.

Second. Our guests, approaching the school, saw amateur antennas on the neighboring house, this is a radio amateur, Alexander, he likes to participate in HF competitions in the single-band competition, on the 17th floor there are 2 Cushcraft 40_2CD elements, i.e. he sits at 40 meters and that’s it, but we are completely shut up. At 40 m the S-meter rests on the opposite wall, and at other higher bends it is no better. This went on for several years. And what do you think. When we installed ML for reception, it works at the beginning of the SSB section, 7.045 MHz, and we are at the end, 7.087 MHz, we don’t feel it, as if it’s not there.

There were also tests on the Northern Dvina River. An ML antenna was mounted on the ship (with a radiator diameter of 1.7 m - the same one - German). It was at the end of May, we were going downstream near the city of Kotlas, at about 3.00 on 40 m I heard ER4DX working for Latin America, Vasily. He has an antenna with several elements and a “kind” assistant. I asked to join the group, and using the S-meter I received signals from Latin American stations at 7 points, and the report from them received 7 points.

Yes, by the way, here is a link to the site: the DK5CZ site has everything there. And there is also the MagLoop4 program, which allows you to calculate magnetic frames, which can be made in the form of a circle, triangle, square, but here is the link, test it yourself: Magloop4 modeling program If you have any questions about using the program, I can conduct a master class, so to speak, or an open lesson . P.S. As a receiving antenna, a design made of a 10 mm copper tube (water pipe) was used and the capacitor was a variable one from a tube radio (tuned once to the middle of the range). And at the end of the article I will post a scan of the ML instructions.

Answer from one of the ODLR users. Inspired by Pavel’s unprecedented academic material, I remembered a sports apparatus (a gymnastic metal hoop), made by the famous Khrunichev rocket and space company and unnecessarily resting behind the sofa... I decided to experiment in a hurry... Within an hour of craft work, I made it from it antenna shown in the attached photos... The shunt capacitor (0.01 uF) was selected for maximum and purity of the weak useful signal... The result is wonderful! The reception is great! And if you take the structure outside the balcony, then you don’t need anything better! The concept is right! Very satisfied. Thanks Pavel! The topic has rapidly moved towards the exchange of specific practical results....

My answer. Alexander. All this is good that you did, but it seems to me that it will have the same effect if you place the container in an ordinary triangle or square made of ordinary wire. It looks like the capacitor plays the role of a shunt or filter plug (it seems so to me). The link to the DK5CZ website provides a schematic design of the MLoop antenna. It consists of an emitter and an excitation loop, their dimensions are respectively 5:1, look at the figure. The loop is made of coaxial cable, and it is not electrically connected to the emitter (in my designs), and I made my first halohoop in exactly the same way. But in other experiments, gamma matching was done instead of a loop. In other cases, the role of a capacitor was played by the air gap at the place where the emitter was cut, then the perimeter of the emitter was equal to half the wavelength, by the way, this is confirmed by the program.

P.S. A friend of mine experimented with these antennas on the 145 MHz band and made a double antenna, i.e. two emitters located on one traverse (When viewed from above, the design looks like two wheels on the same axis). Khashnik was controlled. The result is very interesting, I mean the radiation pattern. And in comparison with a multi-element antenna, this design did not lose. Returning to the design of the antenna itself, it is my personal opinion that it is the antenna power system, be it a loop or another type, that gives the effect that the electrical component in the signal is negligible and is neglected, i.e. There is mainly a magnetic component present. Hence the name of the antenna - Magnetic frame. Please note that the excitation loop is made specifically with cuts.

User responses. Pavel, I visited you more than once, but I wasn’t interested in antenna management, but in vain... Enlighten the people, take a photo to the studio, please.

Since there was no digital camera in those days, I used a point-and-shoot camera. By the way, I forgot. There was another experience of using it. I defended my diploma at the All-Russian Academy of Sciences using antennas of this type, the diploma was classified as “secret”, but I think that after many years it can be said about this, especially since there is one photo, this is a fragment of an explanatory note during the defense. This was in May 1990.

Then preparation for the field competition "Radio Expedition Pobeda". April 2000, roof of a school (which later became a testing site). And this is a trip to Volokolamsk, to the monument to sapper soldiers (May 8-9, 2000), we worked as RP3AIW. This is just an antenna made from a cable “on a cross”.

In September 2000, I was already in the Arctic. In the first photo there is an installation of a spiral-whip antenna with a tuner (9 m high, homemade) and a typo on the photo inscription, not 2001, but 2000. In the distance a lighting mast is visible; between two of these a delta (triangle) with a perimeter of 90 m was mounted. The second photo is a magnetic frame, located horizontally at a distance of 80 cm from the iron roof of the oil workers' trailer.

February 2001, tests again. Roof of the school. Antenna with a radiator diameter of 4 m. The first antenna ordered in production. I conducted experiments on the air, both in distance and in comparison with other types of antennas, so I was “popular” on the air and many radio amateurs gladly came to watch and take part in this process. By the way, on the main site, in the guest book there is a review from one of the radio amateurs.

June 2001, tests of the receiving antenna, I wrote about it, made of a copper tube and upside down (conder at the bottom, vacuum).

July 2001, at one of the objects (there is also a typo on the photo caption, not 2000, but 2001).

August 2001. Received antenna AMA-5, from DK5CZ. Nearby, it was made in Russia with a diameter of 1.7 m (you can see the bolts on the emitter, at the junction of the segments) and “horizontally” located with a diameter of 4 m (an improved, or rather improved, model).

June 2002. Lake Pleshcheyevo, a meeting of radio amateurs in the central part of Russia. They brought an antenna with a radiator diameter of 4 m, installed it near the tent and compared it with all the ones the members of the meeting had (and there were dipoles and J-antennas, and triangles).

July 2002. Northern Dvina River. Initially, they brought an antenna with a radiator diameter of 4 m, but later replaced it with an antenna with a radiator diameter of 1.7 m. The reason was that they did not pass in height under the bridges.

In September, tests were carried out with an antenna with a radiator diameter of 1.7 m on the tugboat "Limenda Komsomolets" (Limenda is a river flowing into the Northern Dvina) near the city of Kotlas.

Variable capacitors. The first photo is from the AMA-5 antenna, the rest are ours.

Automatic tuners were made - more precisely, a program was written for a single-chip processor, the commands of which control the electric motor - turning the capacitor.

A book by engineer S.I. appeared. Shaposhnikov “Radio reception and radio receivers” from the series Radio Amateur Library, published by the Nizhny Novgorod Radio Laboratory named after. IN AND. Lenin, 1924.

This book has a section on antennas, I will reprint it and post a scan of the drawing.

"Reception without antennas"

Section "Reception without antennas"

Reception for frames. If on the wooden frame shown in Fig. 27a, wind a certain number of turns of insulated wire, to the ends of which attach a variable capacitor C, you will get a closed oscillatory circuit that can oscillate in a wave, the length of which depends on the capacitance C and the self-inductance L of the frame. Such a contour, located in a vertical plane and called a receiving frame, has the following properties:

1. The magnetic lines of the electromagnetic wave, crossing the vertical parts of the turns, induce forced oscillations in the frame, to which the frame’s own wave can be tuned with capacitor C. If a detector circuit is connected to capacitor C, then the operation of transmitters can be received on such a frame.
2. The frame has a guiding effect, i.e. being installed as shown in Fig. 27, and tuned to the incoming wave, it best receives signals in the directions indicated by arrows 1 and 2, i.e. wave arriving in the plane of the frame, and does not receive waves arriving in directions 3 and 4 at all, i.e. waves arriving perpendicular to the plane of the frame. Thus, by placing the frame in a certain direction in which the loudest sound is obtained, we can determine in which direction from it the transmitting station is located.

Frames have their own advantages and disadvantages. The first include their lightweight design, small size, allowing them to be installed at home, directing their action, etc. Their main disadvantage is that they perceive too little energy, so the detector can only receive them over short distances. However, when working with a good amplifier, powerful transmitters are received through frames over thousands of miles.

Here are some frame sizes that are considered the most advantageous. The frame is square, with a side = 70 cm. For a wave of 300 m, 4 turns are placed; 600 m - 7 turns; 800 m - 10 turns; 1200 m - 14 turns; 1600 m - 20 turns; 2500 m - 40 turns, etc. The coil from the coil is laid at a distance of one centimeter. The capacitance of capacitor C should be about 1000 pF.

Frames can be of various sizes and shapes. The most practical is considered to be a diamond-shaped frame placed on a corner, Fig. 27th century

(Links to information from the Internet)

• Magnetic Loop Antennas - by PY1AHD (a superb loop site!) Brazil.
• Stealth ST-940B Mobile HF NVIS Magnetic Loop Antenna - by Stealth Telecom. United Arab Emirates.
• HF LOOP AND HALF-LOOP ANTENNAS - by STAREC. France.
• PA3CQR Magnetic loop antenna page - by PA3CQR. Netherlands.
• 80m Frame Antenna - by SM0VPO. Sweden.

This publication is intended for beginners
radio amateurs and for those who do not have access
on the roof of your house. Sushko S.A. (ex. UA9LBG)

Due to their small size, ML-type magnetic antennas (Magnetic Loop) are becoming increasingly popular. All of them can be placed on balconies and window sills. It is undeniable that single-turn magnetic antennas with a vacuum capacitor and a communication loop have gained classic popularity, with the help of which radio communications can be carried out even with other continents.

Double-frame antennas in the shape of a figure of eight relatively recently began to appear among radio amateurs, although at the dawn of the emergence of CB communications in Russia, such antennas were practiced with some success in automobile radio security systems in the 27 MHz range, see Fig. 1.a. The car antenna consisted of two identical frames (loops) L1; L2 and a common resonant capacitor C1, located at the voltage antinode. With an antenna perimeter of about 5 meters, radio amateur Sterlikov A. ( RA9SUS) made connections with 36 countries with power up to 30 W. The antenna was powered directly from the coaxial cable. And such antennas have been in practice since the late 60s and early 70s of the last century. The equivalent circuit of such an antenna is shown in Fig. 1.b.

Although single-turnM.L.Currently widely used among radio amateurs, the peculiarity of the two-turn one is that its aperture is twice as large as the classical one. Capacitor C1 can change the resonance of the antenna with a frequency overlap of 2-3 times, and the total circumference of the two loops is ≤ 0.5λ. This is comparable to a half-wave antenna, and its small radiation aperture is compensated by an increased quality factor. It is better to match the feeder with such an antenna through inductive or capacitive coupling.

Theoretical digression: The double loop can be considered as a mixed oscillatory systemLL andLC systems. Here, for normal operation, both arms are loaded onto the radiation medium synchronously and in phase. If a positive half-wave is applied to the left shoulder, then exactly the same one is applied to the right shoulder. The self-induction emf generated in each arm will, according to Lenz’s 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 induction of the opposite arm. Then the induction in coil L1 will be summed with the self-induction from coil L2, and the induction of coil L2 will be summed with the self-induction of 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.

By transforming the antenna from a rectangular shape to a round one (Fig. 1.a), we get the antenna shown in Fig. 2.a. It is rightly believed that a round shape of a magnetic antenna is more effective than a rectangular one.

The design of the frames L1 and L2 was gradually simplified; they began to be included in the form of a figure eight, in Figure 2.a. and 2.b. This is how the two-frame ML in the form of a figure eight appeared. Let's call it 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 designer’s task is to achieve unity of resonances and maximum efficiency of the antenna, therefore, the manufacture of loops L1 and L2 should be the same. In practice, an instrumental error of several centimeters changes one or the other inductance, the resonance tuning frequencies diverge, and the antenna receives a certain frequency delta. Sometimes the designer does this intentionally. This is especially convenient for multi-turn loops. In practice, ML-8s actively use LZ1AQ; K8NDS and others unequivocally assert that such an antenna works much better than a single-frame antenna, and changing its position in space can be easily controlled by spatial selection, which is confirmed by the photo below of the antenna at 145 MHz.

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 effective ML-8 antenna for a balcony or even a windowsill. 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, maintaining the calculated inductance of the loop, using not one, but two turns in it, leaving the resonant capacitor with the same rating. There is no doubt that 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 is clear that the number of turns N is one of the factors of the numerator and is on a par with both the area of ​​the turn-S and its quality factor-Q.

For example, a radio amateur OK2ER(see Fig. 3) considered it possible to use a 4-turn ML with a diameter of only 0.8 m in the range of 160-40 m.

The author of the antenna reports that the antenna works nominally at 160 meters and is mainly used by him for radio surveillance. In the 40m range. It is enough to use a jumper, which 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 the sealed plastic box was purchased at an electrical store. The matching of the antenna with the feeder is capacitive, and most likely according to one of the presented schemes, see Fig. 4.

In addition to the above, we need to understand what negatively affects the quality factor-Q of the antenna as a whole:

From the above formula, we see that the active inductance resistance Rk and the capacitance of the oscillatory system C should be minimal. It is for this reason that all MLs are made from a copper pipe of the largest possible diameter, but there are cases when the loop fabric is made from aluminum, and the quality factor of such an antenna and its efficiency drops from 1.1 to 1.4 times.

As for the capacitance of the oscillatory system, everything is more complicated. With a constant loop size L, for example at a resonant frequency of 14 MHz, capacitance C will be only 28 pF, and 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%. For this reason, ML is most often used for two ranges, and the third (lowest) is considered simply overview. Consequently, when making calculations we will “dance from the stove”, i.e. from the highest range selected by the radio amateur with a minimum capacity of C1.

ML-8 radiation pattern remains exactly the same as the ML version. For both antenna options, the eight-point radiation pattern and the corresponding polarization are completely preserved. In the photo, using a gas-discharge lamp, the radiation levels of the antenna from different sides are clearly shown.

Designing an antenna for the 20m range.

Now that we're armed with some basic knowledge of ML-8 design, we'll try to manually calculate our antenna.

The wavelength for a frequency of 14.5 MHz is (300/14.5) - 20.68 m.

The circumference of each quarter-wave loop is L1; L2 will be 5.17m. Let's take -5m.

The frame diameter will be: 5/3.14 - 1.6 m.

Conclusion: A single ML hinge may fit into the interior of a balcony, but ML-8 is unlikely...

Let's fold each loop in half, but its diameter, while maintaining the given inductance (4 μH), will differ slightly downward. Let's resort to a fairly popular amateur radio calculator and determine the geometric dimensions of a two-turn loop with the same inductance.

In accordance with the calculations, the parameters of each loop will be as follows: With a blade (copper pipe) diameter of 22 mm, the diameter of the double loop will be 0.7 m, the distance between the turns will be 0.21 m, and the loop inductance will be 4.01 μH. The necessary design parameters of the loop for other frequencies are summarized in Table 1.

Table 1.

Note: The ML-8 antenna has not only an expanded bandwidth, but also increased gain.

The height of such an antenna will be only 1.50-1.60 m. Which is quite acceptable for an antenna of the ML-8 type for a balcony version and even for an antenna hung outside the window of a residential multi-storey building. And its wiring diagram will look like in Fig. 6.a.

Antenna power can be capacitively or inductively coupled. Options for capacitive coupling are shown in Fig. 4 and can be selected at the request of the radio amateur.

The most budget option is inductive coupling. There is no need to repeat the schematic representation of the communication loop; it is completely identical to that of ML-type antennas, with the exception of calculating its perimeter.

Calculation of the diameter (d) of the communication loop ML-8 is made from the calculated diameter of two loops.

The circumference of the two loops after recalculation is 4.4*2 = 8.8 meters.

Let's calculate the imaginary diameter of two loops D = 8.8 m / 3.14 = 2.8 meters.

Let's 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 28 cm. 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. 6.a.) and be located closer to it.

Antenna configuration and display elements

1. To tune a magnetic antenna into resonance, it is best to use vacuum capacitors with a high breakdown voltage and high quality factor. Moreover, using a gearbox and an electric drive, its adjustment can be done remotely.

We are designing a budget balcony antenna that you can approach at any time, change its position in space, rearrange or switch to another frequency. If at points “a” and “b” (see Fig. 6.a.), instead of a scarce and expensive variable capacitor with large gaps, you connect a capacitance made from sections of RG-213 cable with a linear capacitance of 100 pF/m, then you can instantly change the frequency settings, and use tuning capacitor C1 to clarify the tuning resonance. The capacitor cable can be rolled into a roll and sealed in any of the following ways. Such a set of containers can be had for each range separately, and connected to the circuit using a regular electrical outlet paired with an electrical plug. Approximate C1 capacities by range are shown in Table 1.

2. It is better to indicate that the antenna is tuned to resonance directly on the antenna itself (it’s more clear this way). To do this, it is enough not far from the communication coil on canvas 1 (zero voltage point) to tightly wind 25-30 turns of MGTF wire, and seal the setting indicator with all its elements from precipitation. The simplest diagram is shown in Fig. 7.

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 serve as an additional electric field emitter-E. In fact, it should replace the initial capacitance C1, and the drain current, which previously passed uselessly between the closed plates of C1, now works for additional radiation. Now a portion of the supplied power will additionally be emitted by electric emitters, Fig. 6.b. The bandwidth will increase to the limits of the amateur radio band as in EH antennas. The capacity of such emitters is low (12-16 pF, 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 using the following links:

Antenna type ML-8 radio observer significantly simplifies the design as a whole. Cheaper materials can be used as the material for loops L1; L2, for example, a PVC pipe with an aluminum layer inside for laying a water pipe with a diameter of 10-12 mm. Instead of high-voltage capacitors, you can use ordinary ones with a small TKE, and for smooth tuning to frequency, use dual varicaps controlled from the radio observation site.

Conclusion

All mini-antennas, no matter what they are, require a lot of labor and metalworking skills in relation to simple tension and classic antennas. But without the ability to install external antennas, radio amateurs are forced to use both EH and ML antennas. The design of the two-turn Magnetic Loop is convenient in that all adjustment, matching and indication elements can be placed in one sealed housing. The antenna itself can always be hidden from picky neighbors using one of the available methods, an excellent example is in the photo below.

The good results obtained with the Magnetic Loop antenna prompted I1ARZ to try to build an antenna for the low frequency bands. He initially intended to build a circular loop antenna (Fig. 1) with a perimeter of about 10.5 m, which is a quarter of the wavelength at 7 MHz. For this purpose, a loop was made from a copper tube with a diameter of 40 mm with thin walls. However, during the work it turned out that bending and unbending tubes of this size is quite difficult, and the shape of the antenna was changed from round to square. Some reduction in efficiency is compensated by a significant simplification of manufacturing.

For the range of 1.8...7.2 MHz, you can use a copper tube with a diameter of 25...40 mm. You can also use duralumin tubes, but not everyone has the ability to weld in argon. After assembly, the entire antenna frame is covered with several layers of protective varnish.

The tuning capacitor is very important for proper operation of the antenna. It must be of good quality, with a large gap between the plates. A vacuum capacitor with a capacity of 7...1000 pF with a permissible voltage of 7 kV is used. It can withstand power in the antenna of more than 100 W, which is quite enough. In the case where the 160 m range is used, the capacitance should reach 1600 pF.

A square-shaped loop is assembled from four copper tubes 2.5 m long and 40 mm in diameter. The tubes are connected together using four copper water pipes. The tubes are welded to the elbows. Opposite sides of the frame should be parallel to each other. A piece 100 mm long is cut out in the middle of the upper tube, a Teflon spindle is inserted into the cutout and secured on both sides with clamps and screws. The diagonal of the loop is 3.4 m, the total length is 10.67 m (together with copper plates 50 mm wide, to which the ends of the tube are attached, providing connection to the tuning capacitor). To ensure reliable contact, the plates must be welded to the ends of the tube after they are attached.

Figure 2 shows the design of the frame along with the base and supporting mast. The mast must be dielectric, for example made from a fiberglass rod. You can also use a plastic tube. At the bottom, the frame is fixed to the supporting mast with steel clamps (Fig. 3).

To strengthen the lower horizontal piece of the frame, a heated copper tube of slightly larger diameter is stretched over it over a length of approximately 300 mm. The motor that rotates the capacitor is mounted on a steel pipe at a height above the roof of about 2 m. To give rigidity to the entire structure, at least three guy wires are installed below the motor.

The easiest way to match the antenna frame and power line is with a coil of coaxial cable type RG8 or RG213. The diameter of the coil is determined empirically (about 0.5 m). Connection of the internal core and cable sheath is carried out in accordance with Fig. 4

After the matching coil is set to the lowest SWR, a corrugated plastic tube is pulled over the connection point to protect it from precipitation. A coaxial connector must be installed at the end of the matching coil. In the place of the lower fastening of the matching turn, a piece of copper tape is threaded under the duralumin mounting clamp, which, after bending, is soldered to the shielding sheath of the cable. It is needed for good electrical contact with a grounded duralumin tube (Fig. 5). In the upper part, the matching coil is attached to the dielectric mast with rubber clamps.

If the antenna is located on the roof, a DC motor drive unit is required to remotely control the tuning capacitor. For this purpose, any small tape motor with a small gearbox is suitable. The motor is connected to the capacitor axis by an insulating clutch or a plastic gear. The capacitor axis must also be mechanically connected to a 22 kOhm potentiometer of group A. Using this potentiometer at the bottom, the position of the tuning capacitor is determined. The complete diagram of the control unit is shown in Fig. 6.

Naturally, the potentiometer must be located on the same side as the motor, connecting them with two plastic gears or a friction gear. The entire tuning unit is housed in a hermetically sealed plastic case (or tube). The cable to the motor and the wires from the potentiometer are laid along the fiberglass support mast. If the antenna is located close to the radio station (for example, on a balcony), tuning can be done directly using a long roller on an insulated handle.

Tuning capacitor placement

As already mentioned, the fixed and movable parts of the tuning capacitor are connected to the upper, cut part of the frame using two copper plates about 0.5 mm thick, 50 mm wide and 300 mm long each. The tuning capacitor is placed in a plastic tube, which is attached to a vertical fiberglass support mast (Fig. 7). The top of the frame is connected with a Teflon spindle and secured to the supporting fiberglass post using U-bolts.

Settings

Set the TRX to the equivalent load, switch the TRX output to the antenna. Do not use the antenna tuner in this experiment. With reduced output power, start rotating the capacitor until you obtain a minimum SWR. If you cannot achieve a low SWR in this way, try to slightly deform the matching coil. If the SWR does not improve, the turn must be either lengthened or shortened. With a little patience, you can achieve an SWR of 1... 1.5 in the ranges of 1.8...7 MHz. The following SWR values ​​have been achieved: 1.5 at 40 m, 1.2 at 80 m and 1.1 at 160 m.

results

The antenna tuning is very “sharp”. In the range of 160 m, the antenna bandwidth is a few kilohertz. The radiation pattern (DP) is almost circular. Figure 8 shows patterns in the horizontal plane for various vertical radiation angles.

The antenna gives the best results in the range of 40 m. With a power of 50 W, the author established many connections with the east coast of the USA with a report of 59. At distances of up to 500 km during the day, the reports were 59+20...25 dB. The antenna is also very good at reception, since a fairly “sharp” setting reduces the noise and signals of strong stations operating nearby. The antenna works surprisingly well in the range of 160 m. From the first attempts, communication was established at a distance of over 500 km with a report of 59+20 dB. From a fundamental point of view, in this range the antenna efficiency is much lower than in the 40 m range (see table).

Concluding remarks

• The antenna should be placed as far as possible from large metal objects such as fences, metal poles, drainpipes, etc.
• It is not recommended to place the antenna indoors, since the antenna frame emits a strong magnetic field during transmission, which is harmful to health.
• When working with powers above 100 W, the frame heats up under the influence of high current.
• At the highest range, the polarization of the antenna is horizontal.

The table above shows the main electrical parameters of the antenna in the indicated ranges. A similar antenna can be built for higher frequency ranges, correspondingly reducing the size of the frame and the capacitance of the tuning capacitor.

Experiments with magnetic loop antennas

Alexander Grachev UA6AGW

Last year I came across a 6-meter piece of coaxial cable. Its exact name: “Coaxial cable 1″ flexible LCFS 114-50 JA, RFS (15239211).” It has a very light weight, instead of an outer braid there is a solid corrugated pipe made of oxygen-free copper with a diameter of about 25 mm, the central conductor is a copper tube
about 9 mm in diameter (see photo). This prompted me to start building a loop antenna. This is what I want to talk about.

The first antenna was built according to the DF9IV design. With a diameter of about 2 m and the same length of the power loop, made of coaxial cable, it worked very well for reception, but frankly poorly for transmission, the SWR reached 5-6.
The reception operating band (at a level of –6 dB) is about 10 kHz. At the same time, it perfectly suppressed electrical interference; with a certain orientation in space, the suppression of the interfering station was easily more than 20 dB.

After some thought, I came to the conclusion that the reason for the high SWR is the use of an internal conductor with its relatively small diameter by the exciting element. It was decided not to use the internal conductor at all, leaving it in the form of an open loop.

The tuning capacitor was soldered to the external screen. The receiving characteristics changed slightly, the minimum in the diagram became less pronounced, and the influence of surrounding objects became noticeable. But little has changed for the transmission. Then, after reading Grigorov’s article once again, it was decided to remove the outer braid from the frame cable and coat the copper in two layers with “HB” varnish (no more suitable one was found, however, it protects the copper well from
oxidation). And then, finally, the first positive results appeared. The SWR dropped to 1.5 and about 20 local connections were made. The antenna was at a height of 1.5 m and could rotate in a vertical plane.

For comparison, we used a dipole with a total length of 42.5 m, made of a field wire with a symmetrical power line from a telephone “noodle” about 20 m long (a sort of antenna of a “beggar radio amateur”), located on the roof of a 5-story building at a height of about 3- x meters. It worked on 40 and 80 meters, powered through a symmetrical matching device - SWR on both bands = 1.0. Unfortunately, the antennas were in different QTHs and there was no
opportunities to make direct comparisons. But the experience of using the dipole for a year made it possible to judge the effectiveness of the frame to a first approximation.

Now about the results: 1) SWR is about 1.5. 2) All correspondents noted a decrease (from 1 to 2 points) in the level of my signal, compared to the level with which they usually hear me on a dipole.

The rains that had begun by this time (as they say: “every other day, every day”) made further antenna experiments impossible. The main reason for the impossibility of further testing was the constant breakdown of the tuning
condenser due to increased air humidity.

I tried, perhaps, all the options available to me, I used connecting only stator plates, connecting two KPIs in series, I used capacitors from a coaxial cable, high-voltage capacitors
- it all ended in one thing - a breakdown. The only thing I didn’t try was vacuum capacitors, which was stopped by their prohibitive cost.

And here the idea came to use a capacitance in relation to the outer shield of the unused inner conductor. An attempt to calculate the required cable length based on the known linear capacity of the cable did not lead to reliable results, so the method of gradual approximation was used.

It was a shame to cut such a wonderful cable, but “hunting is worse than bondage.” Connection diagram in the figure. For power supply, a loop of coaxial cable 2 m long was used, according to the DF9IV circuit; the supply 50-ohm cable itself was 15 m long. It could be assumed that the total capacitance would be obtained in accordance with the formula of series-connected capacitors, but the tuning capacitor is, as it were, a continuation of its own cable capacity.
For tuning, a butterfly capacitor from VHF equipment was used.

The breakdowns completely stopped, the antenna retained all the basic parameters of the classic magnetic loop antenna, but became single-band.

The main results are as follows: 1) SWR of the order of 1.5 (depending on the length and shape of the supply loop). 2) The magnetic antenna is noticeably inferior to the dipole (described above) with a comparable suspension height. The experiments were carried out in the 80 m range.

I was prompted to engage in further experiments with magnetic antennas by an article by K. Rothhammel in the second volume of his book, dedicated to magnetic frames, and an article by Vladimir Timofeevich Polyakov on a frame-beam or real EH antenna, and for understanding the processes occurring in antennas and around them, it turned out to be very useful article about the near field of antennas.

After reading the article about the frame-beam antenna, I came up with several promising projects, but currently only one has been tested, and this is what we will talk about. The antenna diagram is shown in the figure, the appearance is in the photo:

All the experiments listed below were carried out in the 40m range. In the first experiments, the antenna was at a height of 1.5 m from the ground. Various methods of connecting the “dipole” (capacitive) part of the antenna to the frame were tried, but the one shown in the figure seemed optimal to me. Here an attempt has been made to retrofit a magnetic frame, which emits predominantly a magnetic component, with elements that emit mainly an electrical component.

You can look at the same antenna differently: a coil connected to the middle of the dipole, as it were, extends it to the required dimensions, and at the same time, the beams connected in parallel to the tuning capacitor have their own capacitance (with the indicated dimensions of the order of 30 - 40 pF) and enter into total capacitance of the tuning capacitor.

The circuit formed by the internal conductor and capacitor, in addition to increasing the signal level at reception approximately twice, apparently shifts the phase of the current of the frame itself, and provides the necessary phase matching (an attempt to turn it off leads to an increase in the SWR to 10 or more). Perhaps my theoretical reasoning is not entirely correct, but as further experiments have shown, the antenna works in this configuration.

Even during the very first experiments, an interesting effect was noticed - if, with the dipole part stationary, you turn
frame by 90 degrees - the reception signal level drops by approximately 10 - 15 dB, and by 180 degrees - reception drops almost to zero. Although it would be logical to assume that when rotated 90 degrees, the radiation patterns of the “dipole” part and the frame will coincide, but apparently not everything is so simple.

An intermediate version of the antenna was made, capable of rotating around its axis, in order to determine the radiation pattern; it turned out to be the same as that of the classic frame. The antenna was powered by the same communication loop as in the first experiments. Currently, the antenna is raised to a height of 3 meters, the rays run parallel to the ground.

About the results:

1) SWR = 1.0 at a frequency of 7050 kHz, 1.5 at 7000 kHz, 1.1 at 7100 kHz.
2) The antenna does not require range tuning. Using the capacitors of the transceiver's P-circuit, some adjustment of the antenna is possible if necessary.
3) The antenna is very compact.

At a distance of up to 1000 km, the frame and dipole have approximately the same efficiency, and at a distance of more than 1000 km, the frame works noticeably better than the wave dipole at the same suspension height, while the frame is four times
less than a dipole. The radiation pattern is close to circular, the minima are barely noticeable. About a hundred connections were made with 1;2;3;4;5;6;7;9 regions of the former USSR.

An interesting effect was noted - the estimate of the signal strength in most cases remained approximately the same, and at a distance to the correspondent of 300 km and 3000 km, this was not observed on the dipole. The reaction of the operators is interesting,
When I told you what I was working on, I was amazed that it was possible to work on this! All experiments were carried out on a homemade SDR transceiver with an output power of 100 W.

Material taken from the magazine CQ-QRP#27