Simple Home Brew RF Probe

An RF Probe is an important tool to have for electronics and radio troubleshooting. They allow you to probe into different parts of circuits to find where you do or do not have an HF/VHF/UHF signal. With this information, you can find where your signal disappears. In turn, this suggests where there is a bad component or solder joint in the device you are troubleshooting. RF probes are simple devices and are easily built at home.

RF Probe Background

If you want to know more about what an RF Probe is and its theory of operation, I suggest you visit the N5ESE’s Classic RF Probe web page. N5ESE Monty has a superb explanation of how an RF Probe works and I urge you take a look at it.

RF Probe Schematic

Schematic of the Simple RF Probe.

The basic idea is to detect the RF using D1 and C1, with C1 doing the additional job of blocking DC from the test device’s power supply. R1 forms a voltage divider with your meter’s input resistance to divide the signal down. You may need to adjust this value for accurate readings (again see N5ESE’s Classic RF Probe for information on this). Even with the value of R1 off somewhat, the RF Probe will function quite nicely for most troubleshooting needs.

My RF Probe

Regardless of whether you are repairing a vintage Hallicrafters or a high tech Icom, there simply is not a lot of room for an RF Probe to fit inside a radio. I chose to build this one as small as practical. I have seen many designs, including some fine home brew ones. In my opinion, almost all of them are fat and would be more useful if they fit in a narrower package. Some are in conductive cases too. I worry about a conductive case accidentally touching another wire and doing more damage to the device I am troubleshooting. After some scrounging, I found a nice pen to use as a case to hold the RF Probe (it may have been a highlighter). Even better, it had dried up and was destined for the garbage can, so the simple RF Probe became a form of recycling too.

Assembled Simple RF Probe

Assembled Simple RF Probe

RF Probe Parts

I used a mix of things (“junk”) I had from other projects for the case, 0.01uF capacitor, 1N34A diode, 4.7 Meg resistor, coax stub, and alligator clip lead. The only thing I purchased was 1/8″ brass rod from the hobby store.

RF Probe Parts

RF Probe Parts

Building the RF Probe

Disassembled PenI began by disassembling the pen frame and discarding the interior parts. I only kept the cap, body, and rear plug.
Brass ProbeI cut a short piece of the brass rod and sharpened the end using a flat file.
Test Fit ProbeI tested the fit of the brass probe end and made a mark with a Sharpie to give myself a guide for the following steps. As I soldered the parts together the dot was a helpful reference.
Drill the Coax PlugI used a small drill bit to make a pilot hole in the pen plug. I gradually worked my way up to 3/16″ diameter, which was a tight fit for the coax. A tight fit is good so the plug will hold the coax and protect the delicate circuit inside the case after it is assembled.
Insert CoaxI inserted the coax into the plug. It was tight, so patience was required.
Cut RF Probe Alligator ClipI carefully cut the RF Probe’s alligator clip to the desired length. Making it too short will hamper the ability to find a close ground. Making it too long can negatively affect the probe’s frequency range and ability to detect specific signals.
Insert RF GroundI slowly and carefully drilled a small hole into the side of the pen body. For my alligator clip wire, a 5/64″ hole was tight, but useable. I worked the wire into the pen body and used the scrap brass like a ram-rod to turn the wire the correct direction to come out the back. I slowly pushed the wire through until it was sticking out of the back of the pen.
Solder RF Probe ComponentsI stripped the ground wire and began to solder the parts. Soldering to the brass was difficult and required a generous amount of heat. It was important to solder the parts as close together as possible so they would fit inside the pen body.
Insert RF Probe Circuit Into PenI inserted the RF circuit into the pen body to get it ready to solder to the coax. To prevent a short circuit, I placed a piece of scrap 3:1 heat shrink around the resistor lead.
Solder and ShrinkI soldered the resistor to the center conductor of the coax and heated the heat shrink. I blackened the heat shrink by accident, but it still serves as an insulator.
Solder the RF Probe GroundsI used the diode’s lead as a convenient bus-bar to tie the alligator clip ground wire to the diode and to the coax shield.
Complete RF Probe AssemblyI gently pushed the circuit into the pen body from the coax end while carefully pulling the ground wire out of the side hole until the rear plug was completely inserted. Here is the complete RF Probe assembly with the cap installed. I think it looks nice! I do not believe this pen will ever fit in my shirt pocket again though.

Testing the RF Probe

To test the simple home brew RF Probe, I used an amateur radio handheld transceiver. I put it on low power and tested with a 70cm (442.400MHz) signal. This test was with 0.5W and I did not connect the ground of the radio to the RF Probe. The probe was about 10″ from the transmitting antenna and was connected to a meter with a BNC adapter.

Testing the Home Brew RF Probe

Testing the Home Brew RF Probe


As you can see, we are capturing over a half a volt of signal. The simple home brew RF Probe works!

This was a simple, inexpensive, and extremely useful project that can be done in an hour. One thing I really like about this project is that I was able to keep the cap to protect the probe when it is in storage or being transported too.