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[TowerTalk] Loading Coils

To: towertalk@contesting.com
Subject: [TowerTalk] Loading Coils
From: Brian Beezley <k6sti@att.net>
Date: Mon, 4 Aug 2025 07:44:05 -0700
List-post: <mailto:towertalk@contesting.com>
"FWIW, your number for Q (673) agrees exactly with the number I get using Medhurst's 1947 formula."


Interesting, Rick. I'm surprised you got the same result. I've tried several methods of calculating coil Q. Skin depth is straightforward. The proximity effect is another matter. I wound up using an integral equation developed by G3RBJ. I modified an approximation it uses for slightly better accuracy. The method yields the most accurate results for the set of test coils I measured with my HP 4342A Q meter.

Q is not easy to accurately measure for big coils like those sold by QCoil. The fields may extend quite a distance and interact with nearby objects, such as the instrument enclosure or even your body. The Qcoil photo shows the coil quite close to the HP 4275A. I wrote a utility to let you remote a coil with a transmission line to minimize interaction. The program recovers coil inductance and Q from what a Q meter indicates for a remoted coil. The program is part of my Q meter utilities listed near the center of this page:

https://k6sti.neocities.org/mu

I verified that the tiny zeros in the HP instrument do mean that the digit is not necessarily accurate. So 6oo means Q is closer to 600 than to 500 or 700.

It turns out that QCoils is just a few miles away from me. I'm going to contact the owner to see if he will loan me some coils that I can measure with my HP 4342A.

Brian

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