Thanks for the appology, Gary, but it was
unnecessary. Maybe even premature. I read it before
the TT digest arrived, so it made me curious. Some of
what you were reacting to WAS my comment.
We seem to be arguing around the physics of a situation,
and we're probably not really holding different views.
So let me say again what I said in the beginning. The
VSWR measured at the tx is not the same as at the antenna
feedpoint, at the far end of 500' of coax. Line loss is
the reason. I can point to measuring 1.1:1 at my tx, and
a 5:1 mismatch at the far end of a run of 9913. Why? It
turned out critters had chewed on the cable, and it had water
in it, and was lossy as hell. But even normal losses from
good coax will add up. rg213 is something like 0.8dB per hundred
foot at 1:1 vswr...or 4dB over 500'. So you'd lose a bit over half your
power going through that line.
The other thing which you ignore, when you say the vswr is
constant along the transmission line, is that the matching
solution is different, at every point along the line, UNLESS
the antenna presents the coaxial cable's characteristic impedance.
And the situation is different for every frequency at which the
line is used. i.e. 8' is a nominal quarter wave @ 30MHz, while
33' is the same at 40m. A dead short will look like an open, through
a quarter wave impedance transformation.
For the record, running a long length of coax to
an OCF, whether it's a carolina windom variant or not,
will tend to produce sub-optimal performance on the
higher frequencies. With 175' of feedline, mine is
quasi-deaf on 15 and 10.
I'd do much better to run the line in a matched condition,
instead of at 2 or 3:1 vswr, but the antenna is quick, easy,
and a decent enough performer on 80-17. If I give up a few
dB it isn't going to matter to me, until the sunspot count is
up and the higher bands are useful again.
From: Gary Schafer [mailto:email@example.com]
Sent: Saturday, September 08, 2007 8:38 PM
To: firstname.lastname@example.org; 'Jim Jarvis'
Subject: RE: [TowerTalk] Carolina Windom, etc.
My apologies to Jim Jarvis! Sorry for the rant Jim.
I just realized after posting that it was Jim Brown that made all the
comments on line loss and swr.
TowerTalk mailing list