> As suggested in a post (I believe by WN3A) these actually are Z
> matchers, and should be tuned for best efficiency. I tuned these
> things by slowly and carefully watching the current draw and power out
> simultaneously. It takes some time, but there are combinations that
> will yield considerable reductions in current draw for the same PO.
Yeah, that was probably me.
The GE method has an underlying flaw in that the voltage you are measuring
is supposed to be representative of the reflected power. However, as the
forward power is reduced, it is possible/likely that the reflected power
will likewise appear lower at the test point. So, as Andy found, it's very
easy to end up with the PA mistuned for something approaching "least power
output" rather than "least reflected power".
If you have a 200 watt station (100 watt driver feeding a pair of 100 watt
combined finals), my recommendation would be to tune the driver into a dummy
load first at about 75 watts output, then connect to the Wilkinson divider
feeding the finals. This way it's at least pre-tuned in the ballpark. Then
tune the finals for best efficiency (using the "upper" and "lower" ammeters
in the station panel) coincident with the power output level desired.
Finally touch up the tuning of the driver PA for best efficiency once you
have the finals happy.
Keep in mind that the tuning of the Z matchers will vary with power level.
Start out well below rated power (say, 100 watts total output at first),
make small adjustments to the matching, re-adjust the driver output pot,
repeat until you've reached rated power.
I have a few of the 200 watt UHF M2 stations, and as Laryn said, tuning with
this method results in 3 to 5 amps less current draw per "final" PA while
still maintaining full power output.
--- Jeff
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