Circuit diagram is designed to strengthen the input of a telescopic whip antenna. The Preamplifier is designed to cover the medium waveband from about 550Khz to 1650Khz. Tuning voltage is supplied via RV2, a 10k potentiometer connected to the 12 Volt power supply. RV1 is controlled so that the amplified signal is weak or strong signal will be attenuated. Control voltage applied to gate 2 of TR1, a dual-gate MOSFET, a voltage signal via gate 1; signal into multiple inputs are set through a 330uH coil and two MOSFETs KV1235 varicap diodes on inputs and by the same component in the BF981 MOSFET’s drain terminal. Both tuned circuits provide high selectivity in the entire tuning range. To help the stability of the MOSFET stage is fed from a 6.2V zener supply stable. The following is a schematic drawing:
To drive low impedance (50 ohms) receiver, the output impedance of the medium stage is enhanced by the BF981 is made of a composite amplifier Q2 BC548 and Q3 BC548. Q2 operating in Common emitter voltages increase by more than just 2, operates in emitter follower Q3 is to provide a circuit with low output impedance. Finally, this active antenna can be used in other bands by changing the values of the 330uH coil. To perform in some bands or relay switch can be used to change the value of the coil.
Source : zen22142.zen.co.uk