[TowerTalk] QCoil Inductor Measurements

[Brian Beezley]

Jim Lux said:

"I would guess that the measurement uncertainty (and modeling 
uncertainty) is sufficiently large that the modeled and measured are the 
same."

Jim, inductance critically depends on coil diameter. But I only have a 
tape measure, nothing more sophisticated. In addition, the diameter 
differed for two measurements 90 degrees apart on the coil. Instead of 
averaging them as I should have (or slightly squashing the coil), I took 
the easy way out and just used the nearest scale measurement. Later I 
thought about going back and remeasuring more carefully, but to me a 
decision like that after seeing the results has a whiff of drylabbing 
even though it's made in the spirit of better accuracy. So I decided to 
let the measurements stand.

Highly variable Q with hand placement at 3.7 MHz was not a surprise. But 
I was surprised to see no effect at all later at 1.8 MHz.

I forgot to mention that calculated Q peaks at 876 at 7 MHz. I couldn't 
measure that high because the meter's variable capacitor ran out of 
adjustment range. But I believe the calculated number, +/- 10%.

I originally intended to do a second measurement with the coil elevated 
about 3 feet above the meter to greatly reduce the effects of the 
enclosure. Then I'd use the software I wrote to undo the effect of the 
long transmission line to obtain results for the coil with no leads. But 
I found the stiff #10 wire so difficult to work with even for short 
leads that I decided to forego the second measurement. The 4342A has 
fussy terminals and it's very hard to get the bends just right to make 
solid connections. The 3.7 MHz Q measurement is not that far from 
calculated anyway although it did depend on magic hand placement.

The main objective was to see if a QCoil coil had reasonable Q. It 
certainly does.

Brian