Hi guys:
Recent thread includes comments about trimming the coax to best SWR or
coiling a bit of it here or there. Check out pp. 149-151 of JOY OF QRP
for the essential theory. Neither practice actually changes the SWR on a
transmission line. You are basically trying to find a point on the
standing wave which happens to more closely match the output impedance of
your rig. A much easier method is to simply ignore the transmission line,
but insert an antenna tuner between it an the rig. When the tuner cancels
the reactive component of the reflected wave and transforms its resistive
component to the nominal 50-Ohms at the input side of the tuner, the tuner
functions as a perfect reflecting plane which reflects 100% of the
reflected wave back down the line as part of theincident or forward wave.
With the RG213 mentioned, SWR loss should be quite negligible. For a
100-ft run, line-loss ranges from 0.24dB at 3.5Mhz to about 1.2dB at
30-Mhz. Any SWR under 2:1 will add a fraction of a dB. That SWR loss will
occur regardless of whether the SWR meter shows 2:1 or 1:1. The reason:
the magnitude of the reflected wave is determined by the mismatch at the
antenna end of the line, and nothing can be done at the xmtr end of the
line to change it. If the line Zc = 50-Ohms, and the antenna input R =
75-Ohms, the SWR and associated SWR line-loss is always determined by that
mismatch. We use a tuner to present a pure resistive load to the output of
the xmtr -- a tuner does not "transform" SWR or anything like that. In a
sense, the tuner isolates the transmitter from the SWR always present on
the xmsn line. The final operates into a resistive load and is not tempted
to take off on its own in response to reactive components that are
otherwise present, as when one trims the coax. So, bottom line: insert a
tuner. Tuner loss is negligible is a hi-Q coil is part of its circuit.
Generally, toroid-wound coils are hi-Q.
Second thought: why coax if you are concerned about loss? Even with decent
foam-dialectric TV twinlead with 20-guage stranded conductor, the loss is
inevitably going to be less than with the best coax. A tuner is necessary
to match 300-Ohms to the xmtr, but once you accept that you need a tuner
with coax anyhow, you realise another advantage -- an antenna cut for 80m
can be used all the way down to 10meters. One antenna, one feedline, one
tuner, and all band operation. I thought this idea was fairly familiar,
but apparently not.
So, I'd suggest scrapping the trimmed coax + dipoles idea (as some others
have noted, its one way of creating a very efficient dummy load) and just
go with twinlead.
73, Ade W0RSp
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