Active transmitting antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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The problem of creating transmitting antennas is of interest to many radio amateurs. As is known, full-sized antennas radiate well; such antennas, the dimensions of which are commensurate with the wavelength. However, it is often difficult to create a full-size antenna. Therefore, many are turning to shortened antennas.
When using such antennas, a paradoxical situation arises in which the low output impedance of the transistor stage Rout (for example, at Ek = 12,6 V, Pout = 10 W, Rout = 8 Ohm) is transformed into a high cable impedance of 50 or 75 Ohm, and then again is lowered to match the low impedance of the shortened antenna. It is easier to match the low output impedance (in our example, 8 ohms) of the transistor stage with the low resistance of a shortened loop antenna (with a shortening factor of 0,45 Rred=8 ohms). To do this, it is necessary to directly combine the output stage with a shortened antenna.
A schematic diagram of an active MW-band antenna is shown in the figure.
The amplifier stage operates in class "C" mode and can be used to amplify FM and CW signals. The output power of the cascade is 10 watts. It is achieved with an input power of 0,1 ... 1,0 W - depending on the type of transistor used. The highest gain is provided by transistors KT965A, KT966A, somewhat lower - KT958A, KT920V. To match the input resistance with the cable, a transformer is used, which is made on a ferrite ring with a diameter of 10 mm with a permeability of 400. The winding consists of 10 turns of two twisted PEV-2 wires with a diameter of 0,31 mm.
At the output of the amplifying stage, a shortened loop antenna is connected, having dimensions of 1,3x1,3 m and made of a copper bus with a cross section of 3x5 mm. The amplifier is included in the gap in the middle of one of the vertical sides, ensuring the operation of the antenna with vertical polarization. To reduce the natural resonant frequency, a shortening capacitance C3 is included at points 4, 4, which must have sufficient electrical strength (for example, a capacitor with an air dielectric).
The conductors connecting the frame with the transistor must have a minimum length (1 ... 2 cm).
To operate AM and SSB, a 500 ... 600 Ohm resistor is connected between points A and B, with which the quiescent current is set to 50 mA. It should be noted that in this case the temperature stability of the cascade will be low, and it can only be used indoors.
The transistor must be equipped with a heatsink with an area of at least 500 cm2. Throttle Dr1 - type DPM-2,4.
You can use a separate wire to power the active antenna, but it is especially convenient to power it through the input coaxial cable using coupling capacitors and chokes. The allowable current density for the central core of the cable is 3 A/mm2, which allows the use of any cables, except for the thinnest ones.
The active antenna can be fixed in a window opening or on a balcony.
The initial tuning of the antenna can be done with the GSS loosely coupled to the antenna (eg with a small loop) and a field strength indicator. Then the supply voltage is set (50% of the nominal) and the excitation voltage is applied to the transistor. By changing the value of the shortening capacitance, resonance is achieved, which manifests itself in an increase in the collector current and an increase in the readings of the field strength meter. After that, by moving point 2 on the side of the frame, a further increase in the readings of the tension indicator is achieved, periodically adjusting C4. Then the supply voltage is brought to the nominal.
Tuning can also be carried out using a frequency response meter by connecting the active antenna input to the FFR output, and connecting the FFC input to a small loop, loosely connected to the active loop antenna. It is advisable to check the SWR of the input circuit, which should be close to 1.
Literature
- Anisimov M. (UA3POC), Anisimov M. (UA3PML). How to power a shortened loop antenna. - Radio amateur KB and VHF, 1997, N12, c.30,31.
Author: M. Animiov (RU3PF, ex UA3POC); Publication: N. Bolshakov, rf.atnn.ru
See other articles Section VHF antennas.
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