In Fig. 1 is a diagram of an active antenna. In broadband amplifiers, used in such antennas, the bandwidth is usually limited to the lower frequencies by the presence of the dividing capacitors between the stages, and higher input capacitances of the active elements.
Most high-frequency bipolar transistors low power capacity the emitter junction is rarely less than 20 pF, and only have a microwave it usually lies in the range 6...10 pF. FETs have lower the input capacitance, but its influence on frequency characteristics still noticeable.
The working range of the amplifier used in this antenna, ranges from 10 kHz to 30 MHz when the transmission ratio in the entire frequency bandwidth of the order of 4.5 dB (Fig. 2). This was achieved with the exception of the dividing capacitors between the stages amplifier and compensation of input capacitance of the transistor VT1 combined feedback - negative at frequencies up to 10 MHz (through resistor R7) and positive at frequencies above 10 MHz (via capacitor C6). Optimal the capacity of this capacitor is selected for regulation for obtaining the required amplitude-frequency characteristics. At the maximum value of the capacitance capacitor C6 is possible even quasi-resonant phenomena in the frequency domain over 100 MHz (Fig. 2, b). If the antenna you intend to use up to 10 MHz, the transmission coefficient of the amplifier can be increased to 30 dB (Fig. 2, a). This achieved by connecting a capacitor C7 in parallel with the source resistor R6.
The input amplifier has a restrictive neon lamp HL1. In parallel with it connected a resistor whose value depends on the operating frequency range active antenna. If it is higher than 3 MHz, it should be about 10 ohms, up to 3 MHz 100 ohms, up to 0.5 MHz And 3.3 MW.
The output impedance of the amplifier is 50 Ohms.
In the amplifier there is the following replacement transistors: VT1 - KG or KG; VT2 - CTV; VT3 - CTA-2. Neon lamp with any the ignition voltage of about 70 V, for example, MN-3, TN-0,95. Capacitors, except oxide, ceramic types km-3, km-4, km-6. Oxide tantalum capacitors K53-1.
Author: Z. Najman