This circuit was developed by Luigi Falcone, I1FLC. He uses seven plug in coils covering 3.0 to 30Mhz in a Colpitts oscillator circuit.
The coaxial socket J2 permits connection to a frequency meter when required. The coils are wound on Teflon formers with two female sockets which plug-in to two male sockets on the instrument. TR2 forms a DC amplifier permitting the use of a 1mA FSD meter with sensitivity controlled by R7. With TR1 switched off (via S2) the instument forms a field strength meter/RF sniffer.
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