active antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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On fig. 1 shows a diagram of an active antenna. In broadband amplifiers used in such antennas, the bandwidth is usually limited on the low-frequency side by the presence of coupling capacitors between the stages, and on the higher side, by the input capacitances of the active elements.
In most high-frequency, low-power bipolar transistors, the capacitance of the emitter junction is rarely less than 20 pF, and only in microwave ones it usually lies in the range of 6 ... 10 pF. Field-effect transistors have less input capacitance, but its effect on frequency characteristics is still noticeable.
The operating range of the amplifier used in this antenna is from 10 kHz to 30 MHz with a gain over the entire band of about 4,5 dB (Fig. 2, c). This was achieved by eliminating the isolation capacitors between the stages of the amplifier and compensating for the influence of the input capacitance of the transistor VT1 by combined feedback - negative at frequencies up to 10 MHz (through resistor R7) and positive at frequencies above 10 MHz (through capacitor C6). The optimal capacitance of this capacitor is selected during regulation to obtain the required frequency response. At the maximum value of the capacitance of the capacitor C6, even quasi-resonant phenomena are possible in the frequency range above 100 MHz (Fig. 2, b). If the antenna is supposed to be used up to 10 MHz, then the gain of the amplifier can be increased to 30 dB (Fig. 2, a). This is achieved by connecting the capacitor C7 in parallel with the source resistor R6.
At the input of the amplifier there is a limiting neon lamp HL1. A resistor is connected in parallel with it, the value of which depends on the operating frequency range of the active antenna. If it is above 3 MHz, it should be about 10 kΩ, up to 3 MHz - 100 kΩ, up to 0,5 MHz - 3,3 MΩ.
The output impedance of the amplifier is 50 ohms.
The following replacement of transistors is possible in the amplifier: VT1 - KP303G or KP307G; VT2 - KT3109V; VT3 - KT391A-2. Any neon lamp with an ignition voltage of about 70 V, for example, MN-3, TN-0,95. Capacitors, except for oxide, ceramic types KM-3, KM-4, KM-6. Oxide tantalum capacitors K53-1.
Author: Z.Najman
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