[TowerTalk] Loading Coils

[Bill Eisinger]

I was one of the first purchasers of Gaylord's (N4SF) coils...they are very well made and really the only option out there these days if you need large inductors. He also sells some nice clips for finding the tap point you need that can be tightened with a screw to make a semi-permanent connection.

In my search for coils, I contacted B&W...they are still in business but sell almost 100% to government purchasers but will do custom orders with a minimum run of 10 units...that sounded OK...I figured I could buy 10 and recoup my money by selling the extras... until they quoted me a price of $33,000!

Bill, AA7X


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From: TowerTalk <towertalk-bounces at contesting.com> on behalf of Brian Beezley <k6sti at att.net>
Sent: Monday, August 4, 2025 8:44 AM
To: towertalk at contesting.com <towertalk at contesting.com>
Subject: [TowerTalk] Loading Coils

"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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