A RF PROBE FOR TUNING FM TRANSMITTERS

After talking to Randy Nelson, WB0VHB, Michael surmised that some very sensitive RF detector like a spectrum analyzer would be required to detect as the first stages of the transmitter were tuned. In this he was partly correct and thus the Mitrek landed on my spectrum analyzer equipped test bench. 

Naturally as luck would have it the radio was so far out of tune that not even a HT tuned to the output frequency could pick up a signal from it when I pressed the transmit button! Instead of using my spectrum analyzer, I fell back on a much simpler, cheaper, more effective tool. I got out my old RF probe! 

An RF probe is a very simple diode RF detector mounted in test probe. The output is displayed on a meter. You poke the probe into the RF circuitry somewhere on the output side of the transmitter stage you need to tune and tune for the maximum meter reading. That's all there is to it! 

The first RF probe I ever saw used a 6AL5 dual diode vacuum tube for the rectifier. Naturally the probe was size of a bratwurst and got just about as hot to hold onto while in operation! The early VTVM's often had a special jack for the RF probe to supply it with six volts for the tubes filament. 

Modern probes use the old fashioned 1N34 diode due to its good low level RF detector performance. You can build one in an hour (or longer if you are fussy like me). You don't even need a PC board since the circuit works better when the parts are mounted "dead bug" style on a scrap of bare board. 

The circuit shown is the one that has been published in the ARRL Handbook for the past few decades. The photograph of their probe shows a seven pin ceramic tube socket and metal shield being used for the probe body. That might give you an idea just how old this idea is! I wonder where you could find one today. 

For UHF and microwave work a Schottky or hot carrier type diode would be better. They are also hard to find and easier to damage from excessive voltages. The old 1N34 germanium diode has a lot to say for itself in this application! Lacking a 1N34 in your junk box, the common 1N4148 switching diode will also work well. 

The rest of the parts are just as non-critical. Almost any capacitors will work for bypasses and coupling. The probe body could be a old ball point pen or a scrap of copper or brass tubing from the hobby store. I made my test probe point from a nail. 

Read out for the meter is intended to be a analog volt meter with a 11 megaohm input impedance. That is, an old fashioned VTVM. The common modern digital meter will also work if you are able to "trend" for peaks using a digital display. If you have trouble following the rapidly changing digits on a DVM, you might want to consider buying one of the old style meters at the next hamfest. For many years my only RF probe used a 0-100 microamp meter for a read out. I placed a potentiometer in series with the meter to give me a range control. This simple arrangement worked quite well for me. Note that the 4.7 megaohm resistor shown in the Handbook circuit was replaced with a 500 uH RF choke when used with a microamp meter. 

One important area is the ground strap. Your probe needs a low impedance ground path for the RF. Use a scrap of braid from some small diameter coax like RG-178. Strip the outer insulation off and than slide the inner conductor out of the braid. Rub the braid over the edge of your work bench to flatten it and attach a small alligator clip to one end. The other goes to the circuit board your parts are mounted to. 

When using your probe remember that the stray additional capacitance that it adds to the circuit will DETUNE any resonate circuit that it is connected to. Study the schematic for a minute and locate a point that is isolated by a amplifier or small value capacitor from the tuned circuit. This way you will minimize the affect the probe's capacitance has. 

Always start on the lowest level stage you can when tuning a radio. If you can't get RF from the first stage, there is no point in twiddling the slugs of the succeeding stages! Always check to make sure the oscillator is working before anything else. Remember that screwing the tuning slugs in (clockwise) lowers the frequency of a slug tuned coil. If you are tuning a Mitrek remember your going LOWER in frequency to get to the two meter band! 

Comments or submissions for publication can be sent to muldoon@seits.org.