Dr. Gerald N. Johnson
geraldj at storm.weather.net
Sun Feb 1 12:40:26 EST 2009
On Sun, 2009-02-01 at 14:57 +0000, Steve Hunt wrote:
> Interestingly, it isn't always true that running the ladder line all the
> way is lower loss than using coax for part of the run.
> Take the G5RV on 80m. At the end of the usual 30 odd feet of ladder line
> the impedance is about 14-j1. Any further length needed to get to the
> shack would be lower loss using RG213 with a 3.6:1 VSWR than 450 ohm
> line with a 32:1 VSWR.
Are you sure? Using data and formulae in the 1994 ARRL Antenna Book, I
see 100' of RG213 has 0.6 dB loss at 4 MHz when matched, and 1.066 dB
with a 3.6:1 SWR. 1" spaced window line has 0.027 dB (had to stretch the
loss chart by a factor of ten for that line) per 100' matched and 0.41
dB loss with the SWR of 32:1.
Could be that the difference is inconsequential, with propagation losses
varying several dB per minute.
> Where circumstances allow, I see nothing wrong with choosing a
> combination of wire length and feeder length which results in "benign"
> impedances, either directly at the tuner or for minimizing the loss in
> any further short length of coax. Actually, the G5RV isn't very good in
> this respect. The ZS6BKW is much better; with a 93ft "top" and 44 ft of
> 450 ohm line it delivers a 50 ohm VSWR under 2:1 on 5 bands - 40m, 20m,
> 17m, 12m, and 10m.
> Steve G3TXQ
Yes there are combinations of center fed flat top and feedline length
that makes for easy tuning. Tables of those were first published in the
1930s. Often the length of half the flat top plus the feed line works
out to an odd multiple of a quarter wave. That is increasingly hard to
accomplish with the addition of 15 meters, and quite difficult with the
WARC bands and 60 meters desired for the coverage. Yet with a tuner more
versatile than a Johnson Viking Matchbox, it often works out pretty good
that the center fed flat top use all of the distance between the
available supports and the balanced feed line be the length to reaches
from the middle of the flat top to the tuner at the operating position.
I've used such a flat top from 1.8 through 144 MHz with good results and
on MARS frequencies in between. In that case my supports were 95 meters
apart. It proved hard to keep one of them standing and I don't have that
73, Jerry, K0CQ
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