Skip to content

What is the Common Mode Current Choke Test Rig?

I got a question today that I realized I haven’t answered well/publicly enough:

“What is the CMCC Test Rig?  What problem does it solve?  How would I use it?”

— Mailing list subscriber. Asked in DM so I’ll keep it anonymous.

Those weren’t the exact words, but that’s basically what they were asking.

The Short Answer:

The CMCC Test Rig is a device that routes the signal being measured by a VNA (like the NanoVNA, but it’ll work on the big ones too) from the center pin and inside of the shield (where the Differential Mode currents flow), to the outside of the shield (where the Common Mode currents flow).

If you just hook up a Common Mode Current Choke to a VNA like normal, the VNA will just see a length of coax.  The VNA won’t measure the impact of the choke at all.  That’s the whole point of coax: To keeps the signal contained inside the shield, not impacted by what’s happening on the outside.

The whole point of a Common Mode Current Choke is to stop the signals on the OUTSIDE of the coax.  So, we have to route the signal being measured by the VNA to the outside of the coax. 

That’s what this Test Rig does: It routes the signal from the VNA, to the outside of the coax.  From there, the hard part is done by the VNA.

What it is NOT:

  • The CMCC Test Rig is NOT a Common Mode Current Choke.  It is a Test Rig that allows you to measure Common Mode Current Chokes with a VNA.
  • The CMCC Test Rig is NOT a Common Mode Current Meter.  You do not leave it inline with your feed line all the time to measure Common Mode Current in a live system.  You have to take the CMCC out of your system, connect it to a VNA via this Test Rig, run your tests to measure it, modify the CMCC, make more measurements, iterate until you have the CMCC you want, then put it back into service.

The Background:

The background for the CMCC Test Rig is this video:  “Measuring Common Mode Current Chokes with a NanoVNA,” a presentation I gave at the QSO Today Ham Expo back in the fall of 2021.

In that video, I try to explain:

  • What Common Mode current is
  • How Common Mode current is different from the other kind of current: Differential Mode current
  • Why Common Mode current the BAD kind of current and why Differential Mode current is the GOOD kind of current
  • How a Common Mode Current Choke works, and how they should be used in your shack
  • Why its difficult to directly measure the effectiveness of a Common Mode Current Choke
  • Aaaaaand, finally, I describe a Test Rig I made (read: hacked together from parts on hand) that allowed me to use a NanoVNA to directly measure several Common Mode Current Chokes.
  • Then I presented a bunch of findings of a few different common Common Mode Current Choke designs.

In the video, I briefly describe how I built the test rig, and what its important features were.  But it wasn’t a comprehensive build description at all.

At the time I gave that presentation, I didn’t want to get into the business of selling kits, so I always told people No when they asked me for one.

Well, things have changed since then.  That’s PRECISELY the business Halibut Electronics is in now.  So, here’s the kit.  It’s very simple, only a few parts, relatively inexpensive to make.  I’m using it as my “Hello World!” product, in the sense that it’s the first product I’m pushing through the pipeline:

  • Does my website work?  Is it able to send emails to customers?  (Apparently not, but that’s a different story… 😉
  • How do I accept credit card payments?
  • How do I ship products to people?
  • etc..

It’s better to figure this out with a product like the CMCC Test Rig, than with SOAR.


I’m launching the CMCC Test Rig product to the mailing list before I’ve made the documentation.  That’s probably (definitely) not a good idea.  😉  I’m going to have some off-grid time this coming week, hopefully I can sit down and write up the details of the above, including the content in that QSO Today Ham Expo presentation, and release that as a PDF and possibly Blog Post.

But in the mean time, hopefully this answers your immediate questions.  Feel free to follow up with any more you may have.  🙂


Leave a Reply

Your email address will not be published. Required fields are marked *