Clamp-on RF current probe

One of the useful tools a ham radio operator can easily and cheaply build is a clamp-on RF current probe. For example, this probe can help you check if you have common mode current on speaker wire or on a power cord. It can also help you find where the RF current is high and low on your transmission lines. The meter I am building here was described in the article by Steve Sparks N5SV in the February 1999 QST magazine.

This meter is suited for troubleshooting. It is not a precision tool, because it is built with a split-core ferrite that does not close consistently every time. However, it is useful in detecting the presence and strength of RF current in your radio installation.

The ferrite core used for this project is not that important, as long as it is suited for RF use. Any core you'll find in your shack of type (31, 43, 61, 75, ...), should be fine.

Schematic

RF Probe schematic

Components

  • L1 - Single turn of wire through a snap-on ferrite
  • D1..D4 - 1N34A germanium or 1N5817 Schottky diodes
  • R1 - 100 to 500 Ω potentiometer
  • C1 - 0.1 µF disk ceramic capacitor
  • U1 - 1 mA analog meter

PCB

If you want to print your own PCB you can download the Gerber file

RF Probe pcb top

RF Probe pcb bottom

Final result

I like putting my projects into these electrical PVC boxes with clear cover, where you can see all the circuitry.

On the next picture you can see an example of the RF current probe clamped on the feed line of a UHF radio transmitting at 5 Watt of power.

RF Probe pcb


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