This circuit is from a Hands-On Electronics from 1987, but it can still be easily assembled, as all components are still common. U1 is an LM324, but equivalent operations can be used. The antenna is of the telescopic type or a piece of rigid wire at least 30 cm long. The circuit operates at frequencies up to more than 100 MHz, serving to detect the emission of signals from transmitters and even the operation of oscillators.
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