This instructable shows how to build your own RF probe. The parts needed might already be in your parts bin!

“Probe for measuring in the Radio Frequency range built inside an earphone jack, designed to have minimum capacitance and complete shielding.

What is needed:
A diode. Germanium point contact, OA79 or 1N34 is the preferred and traditional choice. But a modern substitute will be a schottky (gold bonded) diode. These have a low forward voltage, below 250 millivolts or so, versus the 600 or so of the silicon diode.

A few capacitors, chip type, value not critical, around 1 nf to 100 nf. (or 1000 pf to 0.1 microfarad)

One resistor, 1 Megohm.

One stereo earphone jack.

One pin from a turned pin IC socket. This forms the ‘hot’ end of the probe, so that various pins can be inserted for probing. A short piece from a needle for exploratory work. A piece of wire soldered to the circuit board when using it for a tuning up session, requiring it to be in place for a long time.”