# [AMPS] Finding the Q of a Coil

**Steve Thompson
**
g8gsq@qsl.net

*Sun, 7 Oct 2001 09:36:57 +0100*

-----Original Message-----
From: Bill Smith <ko4nrbs@yahoo.com>
To: amps@contesting.com <amps@contesting.com>
Date: 07 October 2001 04:46
Subject: [AMPS] Finding the Q of a Coil
>*
*>*Hung up trying to calculate the Q of a coil I want to
*>*build. It will be 2 inches in diameter with 26 turns
*>*of #10 wire. The length is 5.3 inches. 4.9 Turns per
*>*inch. HAMCALC computes this at 11uh of inductance.
*>*
*>*
*>*If I got it right
*>*Inductive Reactance of this coil on 10 meters=
*>*2*3.14*29000000*.000011 or 2003 ohms
*>*
*>*Q of this coil should be the Inductive Reactance
*>*divided by the internal resistance of the coil's wire
*>*(factoring in the RF). This is where I got bogged
*>*down. Couldn't come up with an appropiate number.
*>*Where am I going wrong?
*I'd say you're not going wrong. Calculating a resistance number to predict
the Q can be done, but the few times I've tried it, the result hasn't
matched all that well with the measured Q. Work out the skin depth, and
given the circumference of the wire, you get the X section area of rf
current flow. You know how long the coil is, so you can work out the rf
resistance.
Steve
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