Near-Field Probes for EMC testing

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Near-Field Probes for EMC testing


Near-field probes for EMC testing are commercially available, but fairly expensive. Yet, they are easy to make, using no more than rigid (or semi-rigid) coax cable and a SMA connector. “Do-It-Yourself” near-field probes have been covered at various sites. For example, in EEVblog #1178 David Jones showed how create a H-field probe for $10 that performs equivalently to a commercial $300 one. My contibution is not so much an improvement to the “electrical” aspect of the probes, but an adjustable design for an insulated sleeve for the probe.

Near-field probes must be made from 50Ω coax cable. Nearly all designs I found on the web use semi-rigid coax cable, because it is easier to bend. I prefer (unjacketed) 2.5mm rigid cable myself: it is thin enough to be bendable (with tools) and cutting a neat narrow slit in the shield (outer conductor) is easier to do in solid copper than in braded wire.

Since you put near-field probes very near or on components, pads and traces, the probes should be insulated. Otherwise, you risk shorting pins when the probe touches two pads at the same time. My solution is to fold the probes into a 3d-printed sleeve.

If you are creating your own probes, each will probably be unique in its dimensions: the loop diameter, the length of the wand, the size of the ferrite bead (and whether there is a ferrite bead at all)… Which is why publishing a set of STL files won't be of any use: your near-field probes will almost certainly have different dimensions than mine. And therefore, the download below is for the OpenSCAD design file. The important parameters can all be adjusted with the “Customizer” of OpenSCAD. The insulating sleeves pictured on this page were all created from the same design file, after adjusting the options.

Basic design

An H-field probe is a ring. Where the ring closes on the wand, there is a solder joint to connect the shield of the coax from end of the ring to its beginning. Furthermore, at this point, the centre wire of the coax is soldered to the shield. In the schematic figure below, these solder joints are indicated by a red dot (in reality, the solder of the shield goes all around). And finally, at the opposite site of the ring, the shield is interrupted by a narrow groove (the centre wire is continuous).

schematic design of a H-field probe

The only thing left to do is to solder an SMA connector to the other end of the wand. A ferrite bead is recommended, to reduce noise on the “ground” line, but the probe also works without it.

A word on the quality (measurement accuracy)

In literature, you may find the remark that commercial probes merit their price because of the intricate design needed to make them suitable for a wide frequency range. This refers to the design of most commercial probes: they are actually made as a four-layer PCB, where the outer layers form the shield and one of the inner layers has the center conductor. Along the edges are plenty of vias to connect the top and bottom layers and create, sort-of, a full shield. The difficulty, now, is to control the impedance over the entire stripline.

Companies choose this design, because it is easy to machine-produce. But if a bit of tinkering and hand-soldering are not an issue, there is an immediate advantage of using (semi-)rigid coax: you can readily get it with the correct impedance and optimized for a wide frequency range. These do-it-yourself probes can easily achieve the same professional-grade quality as the commercial probes.


  Design file for the sleeves     OpenSCAD