Device for tuning antennas. Encyclopedia of radio electronics and electrical engineering

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

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This article proposes a device for measuring the resonant frequency of antennas with cable feeders. It does not allow to obtain any fundamentally new results, but is easier to manufacture and use. For example, a reflectometer from K. Rothammel's book "Antennas" requires several tens of watts of power to be supplied to the measuring line, and even more at low frequencies, otherwise the reflected wave in the measuring line will be very small in amplitude and insufficient for its linear detection by the diode. As a result, the device will show excellent SWR even with a decent mismatch. Isn't this where the frequent statements on the air come from, that either one or the other tuned their antennas very well at 1,8 MHz and the SWR is equal to one? If you do not increase the length of the measuring line three times against the one indicated in the book by K. Rothammel, then at 1,8 MHz even a power of half a kilowatt is barely enough for the incident wave to deflect the instrument needle to the end of the scale. A linear measurement of the reflected wave is out of the question. Her signal simply will not open the diode. The measurement of SWR at 1,8 MHz at permitted powers of 5 and 10 W with simple reflectometers seems to be generally unrealistic.

The proposed method is not associated with the registration of the reflected wave and it does not need any power, which, in addition to the obvious convenience during tuning, will reduce the load on the band. The method is based on the influence of the antenna on the oscillatory circuit to which the antenna is connected. It is known that the input impedance of the feeder is purely active and is equal to the wave impedance of the cable only in the case of perfect matching, i.e. if it is loaded with an active resistance equal to the wave resistance, and there is no reactive component. With a frequency mismatch, either an inductive or capacitive component appears in the input resistance.

If the feeder is connected in parallel to an oscillatory circuit, the inductive component will cause the frequency to go up, and the capacitive part to go down. Moreover, it is necessary to compare the deviation in relation to the position that exists when connected to the active resistance circuit in the form of a resistor, equal in magnitude to the wave resistance of the cable. To measure the resonant frequency of the circuit, it is convenient to include it in a tunable self-oscillator, the frequency of which is recorded by an external frequency meter (Fig. 1).

The connection of the antenna with the circuit must be weak, otherwise the generation will fail or be very unstable. Much attention must be paid to the switch S1, which must have a minimum parasitic inductance and capacitance; the length of the mounting wires from S1 to the equivalent resistor and the antenna socket should be kept to a minimum. When choosing a power source, it must be borne in mind that the amplitude of the generated voltage on the circuit must be large enough. Otherwise, during measurements, external powerful signals received by the antenna will cause oscillator frequency delays and measurements will either not work at all or will be inaccurate.

So, in one position of the switch S1, a non-inductive resistor "Equivalent" is connected to the circuit, equal to the wave resistance of the cable, and in the other position, the antenna feeder is connected.

Working with the device. Set the switch to the "Equivalent" position. Using the generator tuning knob, we set the frequency at which the antenna should operate using the frequency meter. Switch S1 to the "Antenna" position. The oscillator frequency will change. Note where the frequency has changed - up or down. Having made several measurements after several tens of kHz, one can find the frequency where its deviation has the opposite sign. Between two frequencies at which the deviation has opposite signs, one can find the frequency where the deviation is zero - the resonant frequency. I will give the protocol for the first, test switching on of the device when measuring the INV VEE antenna at 1,8 MHz. Due to the small height of the mast (15,5 m), the ends of the vibrators lay almost on the roof. Their lengths were measured with some margin.

The device showed a resonant frequency below the operating one. To calculate the shortening, a proportion was drawn up between the existing resonant frequency and the required one (1850 kHz) and it was determined which part of the vibrators (in percent) should be removed. Similar measurements on dipole-type antennas were made by the author at 3,5 and 7 MHz. The nature of the frequency deviation is the same everywhere: when measuring at a frequency above the resonant one, connecting the antenna instead of the equivalent causes the oscillator frequency to go up. When measuring at a frequency below the resonant, the departure is correspondingly down. That is, after making one trial measurement, you can see in which direction to rebuild in order to come to resonance (Ed. Note This is only true if the length of the feeder lies within 0 - 0,25; 0,5 - 0,75; 1,0 - 1,25, etc. from the wavelength). The device can also be used to measure the resonant frequency of the input impedance, amplifiers and other devices. It is only necessary that the device overlaps the studied range in frequency. If a PA, for example, is to have an input impedance of 50 ohms, we can compare its input impedance with an equivalent resistor.

After manufacturing, the device must be checked. To do this, you need to take 5 - 10m of cable of the same type as your antenna feeder. At the opposite end, load it with a resonance with a resistance equal to the wave resistance, and measure it with a device. If the device shows correctly, there will be no frequency deviation in the "Equivalent" and "Antenna" positions. By making such measurements at higher frequencies, one can estimate to what frequencies the device is suitable. But here it must be borne in mind that the wave impedance of the cable according to GOST can have deviations of up to ± 4% ("Electric cables, wires and cords. Reference book", Energoatomizdat, 1988). So for those who have the ability to measure the characteristic impedance of their cable, it is advisable to do so.

In the author's version, the device is made exactly in the likeness of the GPA ("RL", N 7,1992), with the difference that individual generators are not combined by output, but are used independently. This made it possible to do without KPI and vernier, as well as switching circuits. SB12A cores were taken for low-frequency bands. When using varicaps KB 105, the number of turns was: at 1,8 MHz - 40 turns dia. 0,35 mm; at 3,5 MHz - 20 turns of the same wire. For higher frequencies, coils can be made on polystyrene frames.

Author: G. Gonchar (UC2LB); Publication: N. Bolshakov, rf.atnn.ru

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