[Lowfer] WA test coil

[Bill Ashlock]

Paul,

>Can you tell us a little more about your test coil?

If you all promise not to groan and mutter: "Not another plug for those dam 
loops"

>I believe you made it to try to simulate a 50 x 50 loop.

Right. I have a number of test coils with Rac ranging from 0.07 to 1.2 ohms 
and inductances close to 100uh to match the loop I happen to be using at the 
time. The physical size of these coils is typically 4" of winding length on 
a 3" dia form. They have proven VERY useful in checking out my old standby 
dual final on the bench when there is any question about the performance, 
since they load the final exactly the same as the actual loop. I use a test 
coil to test any new final that I build since a resistive load doesn't tell 
the whole story. They can also be connected in place of the loop out in the 
woods as a test. Litz is only needed when you are simulating a loop with an 
Rac of <0.2 ohms but does allow a smaller coil for the higher Rac types.

>I built a coil from Litz wire with about 100 uH and got it to resonate 
> >with about 7000 pF. Now to match it and try putting some power into it.

You have to calibrate the coil with the actual Rac. This requires an 
accurate current probe. I like to use the sine wave from my Wavetek signal 
generator (50 ohm output Z) as the signal source. Use a transformer ratio 
that is sufficient to eliminate any distortion of the generators's output 
and do a Rac = voltage/current calculation of the voltage out of the 
transformer and the current through the coil at resonance. You can also do a 
Q measurement and calculate the Rac from this. The Q is the voltage, at 
resonance, across either the capacitor or the coil, divided by the voltage 
out of the transformer.

Let me know if I've missed anything. Also, I can very easily send you a 
calibrated test coil (if the the RAC is >.5 ohms) as I have done for a 
couple of other guys. This way you can calibrate you current probe, which I 
think is a home brew?

Bill

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