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.
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.
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.
Building the RF Probe
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.
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.