On Sat, 3 Jan 2004 01:35:51 -0000, "Keven Matthews"
wrote:

I recently moved my shack from an upstairs room to downstairs, much closer
to the garden and antennas. All the antennas previously had a long run of
coax to the old shack. The obvious thing was to have a nice new short run
of coax to my HF vertical which is now only 15' away. So I cut the coax and
since then the antenna is no longer resonant on 40 Metres. Also this week I
was putting up a new HF wire antenna, it was getting dark and raining by the
time I was hoisting it up but so I could just have a listen that night a
grabbed an old (15 years) large coiled up of quantity RG213 coax complete
with rotten oxydized pl259 plugs on each end which had just sat on the
garage wall for years. I just slung the coil down and plugged in at each
end. The plugs looked so rotten it was shameful but it pitch dark by then!
However The SWR was pretty good across the band. Regardless I started my
evening doing a tidy job with some of that nice 5DFB japanese coax all ready
for the following day. Guess what ? I put on the nice new cable and plugs
and the antenna is no longer anywhere near resonant on 80M. So why am I
getting a better result with a long length of still coiled cable sitting on
my patio rather that a much shorter brand new piece. Please could some one
explain to me if the coax length does matter, it has certainly never been a
problem for me in the past on VHF and Six but I am new to HF frequencies.
If you do need to have a certain size run, what can you do with the cable if
you phisically dont need it ?


Many Thanks & 73 for 2004

Keven G7UUD


Hi Keven,

It sounds like the coiled excess of the first attempts were serving as
chokes for your antenna. For one, at 15 feet away, that is very close
and certainly puts you in the fields such that you become part of
either the ground, or its loss. As soon as you cut away that excess,
you probably now have (more or less) a straight run. Hence no choking
action and the antenna sees you more clearly now back down the
exterior of the transmission line (classic common mode issues are
revealed by change in SWR attending transmission line length changes).

Try replacing some of that lost length (probably irretrievable now) so
that you can at least build a choke of half a dozen 6 to 8 inch
diameter turns at the feedpoint. OR Add a 1:1 Current Balun at the
feedpoint. I presume you have at least some rudimentary form of
ground (half a dozen radials) to help even out the picture. This last
will stabilize any tune-ups you may need to perform; but once there
should be robust.

73's
Richard Clark, KB7QHC