It should be obvious that the ferrite probe has many additional uses. It's worked out well for use in equipment design and test. At lower frequencies, the opportunities multiply. Later probes, built with cores up to about 3/16-in. outside diameter, helped trace signals on connector wires and on circuit-board traces. You can even define current paths in sheet conductors like ground planes. The emergence of network analyzers, high-frequency scopes, and spectrum analyzers makes this probe a potent design tool. It's also great for digital signal tracing.
Personally, I've built about 13 or 14 of these probes, most of which were left with the engineers I was helping. They've been used in avionics, missile-guidance systems, audio, analog multiplexing, FM amplifiers, and most recently in applications on the International Space Station dc-dc power converters. That's still going on as this is being written.
Since this idea was conceived in 1973, a few other probes have come on the market. They're fine for their intended purpose. But with this design, you get a home-built probe in use before you can get a requisition through purchasing.
I no longer have access to up-to-date test equipment. So the notion of a full characterization is no longer possible. Every time I built a probe for a specific application, however, it worked sufficiently to kill the problem. The range of expected frequencies determined the core size and turns count. We did check the ferrite characteristics for the choice of core. But we never were sure of just how much margin there was. If one of you readers chooses to try this probe concept, be prepared to be amazed at the extremely flat frequency response and broad bandwidth. Note particularly the absence of bothersome resonances. Perhaps someone with adequate equipment will pick up where I left off and quantify all these variables.
This article has emphasized higher-frequency applications. I built a couple of larger pickups wound on 1/4-in. cores. They provided good pickup down into the audio area. I have traced 9-kHz signals through wires, boards, racks, and chassis with one of the larger coils.
Figure 6 shows the probes made for missile-guidance-system use that are currently being utilized on the International Space Station. The application's power converters handle 75 kW in switching supplies and have a lot of radiation potential at many frequencies. But the probes are extremely flat and wideband.
If an experimenter decides to go sniffing around with one of these probes, bear in mind that your hand is poking a conductor into areas of voltage. Be forewarned. Shrink sleeving around the exposed metal could help.
In use, one finds that rotating the probe resolves the direction of current in the sensed circuit. The probe is, in effect, a transformer in which the one-turn primary is the sniffed circuit. The probe coils are the secondary. The ferrite concentrates the flux. Orienting the probe for maximum pickup is a skill quickly and intuitively learned.