A simple field strength meter

This circuit is maybe the simplest RF sniffer you can find: Only three components: An RF choke, a germanium diode and a microampmeter.

The best for this circuit is a 50 µA microampmeter, but the cheaper ones, like 500 µA will work also, but they will be less sensitive. Use a RF choke with some inductance, For VHF-UHF, a VK-200 choke will work fine, for HF is desirable a bigger inductance. Use a small insulated wire as antenna. 15 or 20 cm will do the job.

You can mount the sniffer in about two minutes if you solder directly the components on meter's terminals. This RF sniffer is a great tool adjusting antennas, tuners, etc. Just put it near the antenna until you get some useful reading and adjust.

You can also use it with low level signals in a circuit if you use the wire antenna as a RF probe. Very useful to test local oscillators and signal levels in a receiver o transmitter stage. Be careful: High levels can destroy the microampmeter, and high levels here can be only 500 mW!!