Dear LF Group,
For tuning LF antennas, I have for some time been using an oscilloscope to
monitor the current and voltage waveforms at the TX output. Resonance
occurs when V and I are in phase, and the load resistance is 50ohms when
V/I = 50. This is very easy to use provided a scope is to hand, but not
very useful if it isn't. An SWR bridge can be used, but it only indicates
the degree of mismatch, so tuning is still a matter of trial and error. So
with a mind towards future /P operations, I have built a self-contained
tuning meter.
The prototype has 2 meters - one indicates phase, while the other can be
switched between voltage and current to determine the load resistance. The
circuit is slightly more complex than an SWR bridge, but not by very much -
it uses 12 diodes, 3 toroid cores and a few other passive components. It
does not require an external power supply, and is usable with TX power
between about 20W and 1.8kW. I have written a 3 page article about it - it
is a pdf file of about 190k, so too big for the reflector, but I can e-mail
it as an attachment to those who are interested. If someone would like to
put it on their web site, they would be more than welcome!
Cheers, Jim Moritz
73 de M0BMU
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