Simple RF Measurement Probe

Introduction

A few people have had difficulty with the simple FM transmitter shown on the Project Pages (see Project 54), so this simple probe can be used to determine if the oscillator is working. It will not tell you the frequency (you need a good RF frequency counter for that), but at least you will know if it is oscillating or not.

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.

On occasions, you may also have problems with an amplifier that is (or might be) oscillating. By using this probe, if you measure any DC voltage at the output, then there is some RF present.

Description

The circuit could hardly be simpler. The diode must be a high speed type, and germanium is ideal and cheap, Alternatively, a Schottky diode will work, but not as well as the germanium deiode. 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. 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 .

Figure 1 - RF Probe

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. If connected to an oscilloscope, you may also be able to see amplitude modulation.

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Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2001. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.