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Re: [TowerTalk] Loading Coils

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Loading Coils
From: "Richard (Rick) Karlquist" <richard@karlquist.com>
Date: Mon, 4 Aug 2025 06:28:35 -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.

On 8/3/2025 10:19 PM, Brian Beezley wrote:
A photo at the QCoils website shows a coil attached to an HP 4275A LCR meter. At 2 MHz the coil reads 47.69 uH with a Q of 600.

I modeled the coil as follows:

Frequency: 2 MHz
Turns: 38 (counted from photo)
AWG 10
Coil inside diameter: 3.75" (website says diameter is approx 3.75")
Coil length: 10" (from website)
Annealed copper, no plating
Four 0.375" support rods (no Q degradation for any plastic)

Results:

L 47.0 uH
Q 673

I believe what the HP meter says. The small zeros shown in the Q readout may mean that accuracy is limited in those digits. Perhaps an HP expert can chime in.

The wire distributor says the tinned wire conforms to ASTM B-33 and MIL- SPEC A-A-59551. I looked up both and could not find a plating thickness specification in either. But the measured and calculated Q figures suggest that plating loss is insignificant at 2 MHz.

Coil calculator:

https://k6sti.neocities.org/coil

Brian

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