[TowerTalk] Loading Coils

[Brian Beezley]

"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