excellent discussion Rick for John's query on tuners for a G5RV
Technically, the G5RV was a low SWR on 20m due to its design. Users
have found that sometimes it also is a good match at 40m, but usuallly
no better than 3:1 at 15m, and of course it is short and capacitive
reactive at 80m.
The length of feeder can make a big difference in matching to any
antenna, and can be changed longer or shorter in small increments of 5
or 10 feet to effect a match if the original length is a problem to
match on a given band.
I built a variant of the G5RV, the ZS6BKW 92 foot doublet, with 40 feet
of ladder line before switching to coax (50 ohms). It was better than
1.5:1 on 20, 40, but 3:1 on 15m, without a tuner.
You actually can run ladder line all the way to the shack and match and
tune it appropriately. While a balanced tuner is nice, you can use an
efficient Van Gorden or similar external 4:1 balun on a T Tuner, the
most common commercial tuner circuit, and effect very good balanced
antenna work with Double Zepps and other doublets or dipoles.
If ever you have matching problems with a given tuner, add or subtract 5
or 10 feet of line to incrementally find the optimum feeder to work that
band.
Most built in tuners in rigs today are L match circuits, one less
component than Tee circuits. It is desirable to have switching of the
capacitor to ground from one end of the inductor to the other, to
provide for either a step up of impedance or step down. Of couser with
some L tuners, you can simply swap input and output coax connectors, and
use a coax to binding post adapter if mating to a balanced line.
While purists will use a double L network to balance things, often the
simple L, may work adequately into the balanced line.
-Stuart Rohre
K5KVH
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