HF current probe for EMC/EMI pre-compliance testing

This HF current probe is for testing radiated emissions from cables, for troubleshooting (potential) EMC/EMI problems.

Conclusive radiated emissions tests are done in an anechoic chamber, but this is expensive and these "far-field" results do not tell you where the emissions leak from. For that purpose, you use near-field probes and a spectrum analyzer.

The common wisdom is that radiated emissions of relatively low frequencies (below roughly 200MHz) are primarily radiated from the cables attached to the device. The HF current probe is a near-field probe for cables.

This design presented here is actually only for the enclosure of the probe. In addition, you will need:

• A ferrite ring in two halves from a standard snap-on ferrite, for example product nr. 7427141 from Würth.
• A panel-mount SMA connector, for example the RF 132164 from Amphenol.
• A stretch of insulated wire.
• Four 2mm screws.

The design of the enclosure is in OpenSCAD and it is highly configurable. So if you use a different ferrite and/or a different SMA connector, the enclosure is easily adjusted.

Construction is simple: the wire is wrapped a few times around one of the ferrite halves (I used 6 windings, the number is not critical). One end is soldered to the pin of the SMA connector, the other end goes to the base of the connector. The enclosure is put together with a bit of glue. I used a stretch of filament for the "pin" of the hinge.

There are some more details on https://www.compuphase.com/electronics/nearfieldprobes.htm.

Print instructions

I used PETG, but for no particular reason. PLA will be fine.

This design must be printed with supports, notably for the hinges.

I printed with 100% infill, so that the screws that hold the SMA connector are well fixed in solid plastic. The rest of the enclosure are mostly thin walls, so solid infill doesn't really cost much extra filament (or extra printing time).