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CMCC Test Rig, used in ARRL Labs?!

Hey all! Check out this video from the ARRL, showing a little in-house Common Mode Current Choke building competition they posted a couple months ago:

ARRL Choke Building Competition, featuring Halibut Electronics Common Mode Current Choke Test Rig

They have people of various experience levels, from not-a-ham(-yet) to veteran Amateur Extra, build different designs of common mode current chokes. Then (starting at about 5:45 into the video) they hook up their chokes to the Halibut Electronics Common Mode Current Choke Test Rig and measure the performance of each of the designs. This is exactly what the CMCC Test Rig is designed for. 🙂

Concerns about the measurements:

Not to nit-pick the video, but I have to point out a few issues with how they used the test rig.

The far side of the test rig (the side away from the VNA) is unshielded; that’s the whole point, to put the signal being measured on the outside of the coax so you can measure what’s happening on the outside of the coax. This means there’s no shield. This also means that the signal will interact with near-by objects. It’s best to keep the device under test (DUT) as far away from metallic objects as possible, objects that will interact with the electric and magnetic fields coming from the unshielded wire that you’re measuring.

But in the video, they show the DUT just laying on top of the metal case of the VNA, on top of other cables, all coiled up upon itself. That is definitely going to affect the response of the chokes being measured. The effects will be greater as you go higher in frequency. So the slope upward in the graph (less attenuation) as you go higher in frequency, is likely caused by this.

Concerns about how the competition was setup:

I also took issue with how they equated performance of the choke, to the build-quality from the competitors. “You did a really good job with this choke!” I’m sorry, but no. I mean, yes, everyone did a good job with their chokes, but…

The performance of those chokes is FAR MORE affected by the design of the choke that was given to each competitor, and the measurement technique, and had very little to do with how well or poorly each competitor built their choke. But the video made it sound like the poorer or better performance of each choke was caused by the competitor’s build quality, and that’s just not the case here. All the chokes were built very well (except that part about keeping the beads close to each other; that was correct.)

There were a few other small nitpicks of how they ran their tests and built their chokes, but nothing that was likely to change the outcome of the measurements.

Still awesome though

It was still awesome to see the CMCC Test Rig in use. They used it for its intended purpose and showed the results, how you can compare different choke builds and pick the design that works best for your specific use case. It made me happy to see that this morning.

Thanks to Derek N9TD for sharing this with me.

73 de N6MTS

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