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Re: [TowerTalk] Test Fixture for Common Mode Chokes

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Test Fixture for Common Mode Chokes
From: Steve Harrison <k0xp@k0xp.com>
Date: Fri, 23 Jan 2026 17:36:30 -0800
List-post: <mailto:towertalk@contesting.com>
On 1/23/2026 11:20 AM, David Gilbert wrote:

Any thoughts on this text fixture?

https://www.youtube.com/watch?v=AyURuo0UEdE

Having designed and built my own S- and Y-parameter test jigs back when I worked professionally in the field, I would have two concerns:

1. Those little slide switches likely have very limited lifetime, such as only 100 or 200 activations before they might become flaky. Thus, I would activate them as sparingly as possible. In fact, I would probly seek and stock a set of replacements now, just in case they become flaky later.

2. Test jigs such as these (especially at higher frequencies) only provide repeatable data when they are always placed in exactly the same configuration, each and every time, including the device under measurement. Handling and moving the jig, particularly while under measurement, is a strict no-no and must be minimized. In fact, I would be inclined to mount the jig to the nanoVNA using semirigid coax so that at least the relative positions of those two test items is always maintained the same every time. However, these precautions are really only needed if you wish to be able to repeat prior measurements easily; for rough "go/no-go" measurements (especially at lower frequencies), such rigidity isn't totally necessary.

Steve, K0XP


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