Power supply methods and performance of magnetic loop antennas. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, setup and matching

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COAXIAL CABLE COMMUNICATION WITH TRANSMITTING MAGNETIC LOOP ANTENNA

When such antennas operate in transmission mode, two types of antenna connection with the feeder line are used - through a magnetic loop (Fig. 3.11, a) and through a gamma matching circuit (Fig. 3.11, b). Note that both the link loop and the gamma-matched cable shield connection point are exactly opposite the trimmer capacitor. This is necessary to maintain frame symmetry.

Rice. 3.11. Power supply of the transmitting magnetic frame

Typically, the diameter of the link loop is 0,2 of the diameter of the main frame. With the help of this loop, it is possible to provide satisfactory matching over the entire operating frequency range of the magnetic loop. We must try to ensure that the wire for the loop is not thinner than the one from which the magnetic frame is made.

The second type of matching is gamma matching. The thickness of the wire used in his circuit is about 2-5 times thinner than the frame wire. Its radius is 0,85-0,95 of the radius of the main frame. The length L in the scheme should not exceed 0,2 from the perimeter of the frame and most often choose a value of 0,1. Gamma matching requires more careful tuning for different bands than a communication loop, but at the same time it has a higher efficiency. When the frame works in two or three ranges for gamma matching, you can always find the optimal dimensions. If there is free access to the frame, then it is convenient to use closing jumpers for configuration. In any case, when you have to deal with magnetic frames, it is recommended to install a matching device.

If the frame serves only as a receiver, then there are usually no problems with matching. It is carried out using a transistor amplifier located directly near the frame, from the output of which the filtered and amplified RF signal is fed through a coaxial cable to the input of the receiver.

DIMENSIONS AND VERSIONS OF MAGNETIC LOOP ANTENNAS

The characteristic dimensions of the transmitting loop antenna are given in Table. 3.2.

Table 3.2.

Frame perimeter, cm	50	80	100	200	500	400
Highest operating frequency, MHz	29	21	14	7	3,5	1,9

With such dimensions, the frame effectively operates on three adjacent wavelength ranges, for example, 10, 15 and 20 or 40, 80 and 160 m. Its efficiency is maximum at the upper frequency, and decreases at lower frequencies. The data in this table is based on a magnetic frame without a screen. If there is an electrostatic shield, then the capacitance between it and the inner wire should be taken into account, which reduces the resonant frequency of the loop. For satisfactory operation, the frame perimeter must be at least 0,08 of the operating wavelength.

With the help of a capacitor, the frame can be tuned to even lower frequencies, however, in the transmission mode, such a design will become very ineffective.

As shown above, the input resistance of magnetic frames is low. This makes it difficult to match antenna systems in which the magnetic loop works for transmission, without tuning it into resonance with the operating frequency.

The loop antenna has its own inductance. It can be calculated using a known formula or measured using appropriate instruments. By attaching a variable capacitor to the open ends of the frame, we get a conventional oscillatory circuit that can be tuned over a wide frequency range. On fig. 3.11 shows two schemes for connecting a frame with a cable: through a communication loop (a) and using gamma matching (b); below them, the corresponding analogues on lumped elements are shown in the form of inductive and transformer coupling with the circuit.

In an oscillatory circuit formed by a frame and a capacitor, the electric field is concentrated inside the capacitor, and the magnetic field is concentrated around the frame. The results of solving the problem of finding the optimal dimensions of the frame and capacitance of the capacitor were given above. It follows from them that the length of the frame should be approximately 0,08 of the wavelength, and the capacitance of the capacitor should be about 30-50 pF in the range of 2-30 MHz.

A frame of shorter length will radiate less efficiently due to the low quality factor. The latter, as you know, is determined by the expression: Q=(L/C)/Rp, where L is the loop inductance, H; C - capacitance at the end of the frame, Ф; Rp - loss resistance in the frame, Ohm.

A single-turn frame, unlike multi-turn ones, has a maximum L / C ratio and a minimum loss resistance. A loop longer than 0,08 of the operating wavelength may not be able to be tuned to resonance, making it difficult to match.

Therefore, for operation in the transmission mode, it is advisable to use a single-turn frame. When tuned to resonance, when significant power is supplied from the transmitter and the loop is well matched, RF currents of hundreds of amperes can flow through it. Therefore, it is desirable that the transmitting magnetic loop antenna be made of large diameter copper pipe. You can polish its surface to a mirror finish. A variable capacitor must be of high quality, preferably without rubbing contacts. In an extreme case, you can get by with an ordinary paired capacitor of variable capacity, connected to the frame only by stator (fixed) sections (Fig. 3.12). Solid capacitors should not be used due to their low quality factor.

Rice. 3.12. Conventional variable capacitor in a magnetic frame

Note that sometimes there are reports of radio amateurs using non-tunable magnetic loop antennas to operate in the transmission mode.

The task of effectively matching such a frame with a transmitter is even theoretically very complex and goes beyond the usual amateur radio practice, so this type of antenna is not considered here. We do not recommend radio amateurs who do not have the appropriate theoretical and practical training to use such designs, as the result will be disappointing.

When magnetic loops serve as receiving antennas, the problem of efficiency is not so acute. Therefore, a capacitor with a solid dielectric or air ones with rubbing contacts are suitable for them. The frame is made multi-turn, which allows to reduce its size. For the frame, you can use a thin wire. A coaxial cable is often used, the inner core of which forms a frame, and the braid acts as its screen. Source: Grigorov I.N. Practical antenna designs.

COORDINATION OF THE FRAME AND SUPPLY CABLE

Inductive coupling and matching are also widespread due to their ease of implementation. The most commonly used option is shown in Fig. 20.7. A small inductive loop with a diameter ratio of 5:1 is placed inside the large loop. Thanks to the symmetrical connection, a 1 ohm coaxial cable can be connected via a 1:50 ring-core balun.

Rice. 20.7. Loop antennas with inductive coupling: a - symmetrical connection with a balancing transformer on a ring core 1:1; b - asymmetric connection; c - inductive coupling with shielding (detailed sketch).

With an unbalanced connection (Fig. 20.7b), the coaxial cable is connected directly. An electrically expedient method of inductive coupling is shown in fig. 20.7, c. Shown here is only a connecting coil of coaxial cable with a break in its screen in the middle of the coil. The screen of the part of the right half of the loop is soldered to the base of the large ring (see figure), and the antenna is grounded in this place. By slightly deforming the coaxial cable cable, the antenna is fine-tuned to the minimum SWR. It is believed that the diameter d should be the smaller, the higher the working quality factor of the antenna.

Literature:

1. K. Rothammel. Antennas. Volume 2. Edition 11, 2001

Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Antennas. Measurements, setup and matching.

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