For checking resonances, tuned circuits, antennas, etc., this circuit covers the 2-to 20-MHz range. Ql serves a
s an oscillator tunable over this range via CI and bandswitched coils LI through L8. When the probe is coupled to a circuit resonant at the oscillation frequency, some RF power will be absorbed and the oscillator output will drop. Q2, D2, D3, and Q3 form an RF detector and dc amplifier to drive meter Ml, which will show the drop in Rf level, indicating resonance. R2 is a sensitivity control.