How to power a shortened loop antenna. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / VHF antennas

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Recently, interest in loop antennas has increased. If earlier such antennas were used relatively rarely, now they are used as antennas for mobile communication systems, burglar alarm systems, etc.

The main advantage of such antennas is the significantly lower influence of the environment on the parameters of the loop antenna, which in some cases is decisive when choosing an antenna. However, it is very difficult to use such antennas with dimensions commensurate with the wavelength L in the KB band. Therefore, it is of particular interest to use loop antennas with a perimeter S smaller than the wavelength L. Such antennas can also be used as additional ones, reconciled with their one-way directivity, and installed in windows, loggias, balconies, as well as as part of complex directional antennas in low-frequency HF bands. The main element of such antennas is a frame with a perimeter S smaller than the wavelength. For placement on windows, balconies, the most convenient frame shape is rectangular. Consider such a frame with a perimeter S equal to the wavelength L, located in the vertical plane [I].

When such an antenna is powered from the side of vertical elements, both of these elements are excited in phase, and current antinodes and voltage nodes are located on them. Horizontal elements with voltage antinodes, in turn, are excited out of phase. Vertical elements can be represented as two parallel vibrators with curved ends, placed at a distance of L/4 and excited in the same phase. Due to the addition of the fields of these vibrators, excited in phase, the maximum field strength in the horizontal plane is in the directions of the axis of the frame, located perpendicular to the plane of the loop antenna.

Such a distribution pattern of currents and voltages along the loop, considered for the case S=L, is preserved even with a slight decrease in S compared to L. With a further decrease in the size of the loop antenna, the current distribution along the perimeter of the loop changes, and with a significant reduction in size compared to L ( S/L<0,25), instead of current nodes and antinodes, a uniform current distribution appears (the current almost does not change along the frame). The current in this case flows in one direction at each moment of time, therefore, it is in-phase, and therefore the radiation of any oppositely located elements of the frame is added in space in antiphase, leading, unlike a full-sized frame, to a minimum intensity in the direction of the frame axis. Thus, in terms of its radiating properties, such a frame turns out to be similar to an ordinary inductor, which can be made to radiate only by significantly increasing its quality factor Q and increasing the current.

However, the efficiency of such a radiating antenna will be very low due to the low radiation resistance R-radiation, and, consequently, the power radiated by the antenna Rizl is also low [2]. Therefore, it is more expedient to use antennas with a velocity factor of 0,25<K<1 (K=S/L), which, despite the decrease in efficiency compared to a full-size frame, radiate well and have a maximum radiation in the direction of the frame axis. One way to reduce the resonant frequency of a loop antenna is to include capacitance at the points of the antenna that have the maximum antiphase voltage [4]. In this case, a significant reduction in the resonant frequency is possible. At the same time, such a decrease in the frequency of the loop, which makes it possible to use it at lower frequencies, leads to a decrease in the ratio S to L, and, consequently, to a significant decrease in the radiation resistance Rizl, determined [197] by the ratio Kizl=4(S/L)1,3 . In this case, directly plugging the cable into the frame to power it, as is often done when using full-size frames, is not possible. To match the frame with the cable at a small Kizl, y- or O-matching is used [1]. The diagram of a loop antenna with a shortening capacitance and y-matching is shown in Fig. XNUMX.

In the considered variant of excitation of the vertical elements, the points at the midpoints of the horizontal elements A and B have a minimum antiphase voltage. This also means that the resistance between these points is very significant (on the order of several kilo-ohms).

The antenna can be powered by including a resonant circuit at these points, which also has a large resistance at the resonant frequency. In this case, the matching of the antenna with the feeder is carried out by selecting the transformation ratio when connecting the cable to part of the turns of the resonant circuit. In addition to autotransformer, transformer connection of the cable and the circuit is possible with the help of a coupling coil. Along with the possibility of excitation and matching, the inclusion of the circuit at points A and B also makes it possible to reduce the natural resonant frequency of the loop antenna due to the capacitance that is part of the parallel resonant circuit. In this case, the value of the capacitance of the resonant circuit in the tuned antenna turns out to be somewhat larger than in the case of a single circuit tuned to the same frequency. An antenna circuit with a resonant circuit is shown in Fig. 2.

To test the effectiveness of matching and shortening of the antennas using a resonant circuit, two rectangular loop antennas with perimeters S=5,6 m and S=12,8 m were made. Both antennas were made of copper wire with a diameter of 2 mm and installed in the window opening and on the balcony nine story building. The antennas were tuned and matched to the 50 ohm cable in two ways: with a shortening capacitor with a y-match and with the help of a resonant circuit. The calculated resonant frequencies of these frames are 53 and 23 MHz, and the experimental ones are 38 and 21,2 MHz, respectively. The shift of the resonant frequency in comparison with the calculated value is explained by the significant capacitance between the elements of the frame and metal elements: fittings, drains, balcony railings, etc.

The experimental determination of the resonant frequency of the loops was carried out by a G4-18 generator and a field indicator (for operation at frequencies above 35 MHz, a diode is switched on in parallel with the generator output of 0,1 ... 1 V, and the antenna is tuned using the 2nd harmonic of the signal frequency). The resonant circuit of the 1st antenna consists of an inductor with a diameter of 35 mm, containing 5 turns of wire with d=2 mm (winding length -20 mm), and a variable capacitor 12...495 pF. Transformer connection was carried out by a coupling coil consisting of 1 turn, and at a frequency of 14 MHz - from 2 turns located on the surface of the resonant circuit coil. The inductance of the coupling coil is compensated by the capacitance C2. The resonant circuit, included in the second antenna, consisted of an inductor with a diameter of 35 mm, containing 29 turns of wire d=l mm (winding length - 65 mm) and a capacitor. The communication coil had 3 turns of wire d=l mm. The resonant frequencies of the antennas, the dimensions and parameters of the matching elements are given in the table.

It has been found that when using both tuning and matching systems, a relatively low SWR value is achieved (approximately the same for different matching methods), but the process of matching and tuning is very different. When using a shortening capacitance and y-matching, this process looks rather complicated and consists of several stages: tuning the frame to the required resonant frequency, and then sequentially changing the length of the loop, the distance at which it is located, and the capacitance, compensating the loop inductance, accompanied by tuning resonant frequency and SWR control. Such a process of coordination and adjustment causes significant difficulties, especially in the absence of sufficient experience.

Matching with a resonant circuit is much simpler: the antenna is tuned by changing the capacitance of the resonant circuit, and then by changing the transformation ratio, the minimum SWR value is set (sometimes it is required to turn on the capacitance C2, which compensates for the inductance L2.) lower SWR, the efficiency of the antenna as a radiating system is determined primarily by the efficiency.

If most full-size antennas have this parameter, which determines

R izl Rizl
n=--------- - -----------------
Rsub Rred + Rloss

is close to 1, then for shortened antennas with a radiation resistance Rred comparable to Rpot, the efficiency turns out to be significantly reduced. Therefore, one must always remember that strongly shortened antennas, instead of radiation, convert the input energy into thermal energy. Regardless of the method of matching and tuning, shortened antennas turn out to be narrow-band and require adjustment when the frequency changes. And if for an antenna with y-matching and a shortening capacitance, the tuning process requires repeating almost all of the above steps with a change in frequency, then for an antenna with a resonant circuit, the tuning process is reduced to a small change in the capacitance of the resonant circuit. This makes such antennas very convenient, especially when a tuning element is available.

Literature

1. Rothammel K. Antennas. - M.: Energy, 1969
2. Grechikhin A. Electrically small antennas: possibilities and delusions / / Radio. - 1992. - No. 11. - P.8 -10.
3. Benkovsky 3., Lipinsky E. Amateur antennas of short and ultrashort waves. - M.: Radio and communication, 1983.
4. Meinke X., Gundlach F. Radio engineering reference book. T.1. M-L, GEI, 1960.

Authors: M. Anisimov (UA3POC), M. Anisimov (UA3PML), Tula; Publication: N. Bolshakov, rf.atnn.ru

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