Antenna amplifier
I will post here an improvement suggested by a member of the forum.
I tried this change and I found that it really works. In practice, the phenomenon is amplified before reaching the base of TR2. This allows to have two main advantages, the first is to be able to use a shorter antenna, but the main advantage is to make the tuning of the sensor stage less difficult. In fact, it is no longer necessary to obtain the maximum gain with the risk of self-oscillations. Even the pcb is less critical. The missing gain is then supplied by the antenna amplifier. The only calibration of this amplifier is to adjust the 4.7K potentiometer for maximum sensitivity.
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