This circuit is designed to amplify the input from a telescopic whip antenna. The preamplifier is designed to cover the medium waveband from about 550Khz to 1650Khz. The tuning voltage is supplied via RV2, a 10k potentiometer connected to the 12 Volt power supply.
To drive low impedance (50 ohm) receivers, the medium output impedance of the BF981 stage is enhanced by the composite amplifier made from Q2 and Q3. Q2 is operating in common emitter boosting voltage levels by just over 2, Q3 is operating in emitter follower providing the circuit with low output impedance.
Finally this active antenna can be used on other bands by changing the values of the 330uH coils. To perform on multiple bands switches or relays can be used to change the value of the coils.
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