Steve & others
You need to find the RF resistance of the loop at the working frequency to
see how the measured 2.3 amps compares with what you may expect for the 18
watt power level. There are several methods to do this but the easiest to
implement is the most obvious, use a scope to check the voltage across the
(measured and known)loop inductance and see if Ohms Law still applies! The
scope ground can be an issue though.
Another way is to use the incremental resistance method where you take the
resonating capacitance out of cct, maintain the source voltage constant and
measure the loop current, then insert a low value resistance in series with
the loop and do it again. A bit of arithmetic and you have the rf
resistance of the loop. You may need to allow for the rf ammeter resistance
as well if using low powers and currents.
The same method can also be used to measure the total loss resistance for a
vertical tx antenna system although I have not done this personally as I do
not run a vertical antenna for TX. Others here in ZL have done so though.
My loop antenna has a dc resistance of around 0.25 ohms but the measured
rf resistance is up about 5 ohms at 180 kHz. This checks out against the
measured current on tx of about 30 amps in the loop (measured using a
current xfmr arrangement) and the tx output power of around 750 watts, the
latter measured with a 30 dB Bird 1kW attenuator and calibrated spectrum
analyser.
Somewhere in there, all mixed up with the 5 ohms loss resistance in the
loop, is the rather meagre 0.00001 ohms or so radiation resistance of the
darn thing!! So all I am doing is providing a heated (well, vaguely so)
perch for the local bird life!
73
Dave
ZL3FJ
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