RF meter probe

Posted on Jul 15, 2012

This probe is useful for any low level RF work, and simply connects to your multimeter. The voltage shown will not be accurate, since this is a rectifier probe, but the measurements are good enough for you to be able to determine where the RF stops, or if a stage is not giving the gain you think it should. The circuit could hardly be simpler. The diode must be a high speed type, and germanium is ideal and cheap. Do not use an ordinary silicon diode - it won't work! House the probe itself in some plastic tubing (an old pen barrel would work well), and use a sharpened nail for the probe, and an alligator clip on the ground lead.

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Keep the ground lead reasonably short for best performance. The coax can be anything that you have to hand. In fact, high capacitance cable that is useless for anything else can be put to good use. That's all there is to it. Connect it up to your multimeter, which can be used on any suitable voltage or current range, or you can use a micro-ammeter if you happen to have one lying about. For use with lower frequencies (a few MHz only), C1 can be increased in value, but I would not go above 100pF. High voltage circuits must be treated with the utmost respect, and a 500V cap is recommended for C1 unless you know that you will never use it on a valve transmitter or receiver circuit.

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