Note that well-engineered broadcast stations do not accept ANY significant
affect of the length of their transmission line(s) on the SWR/impedance
match of their antenna system, as seen by the transmitter.

Greatest antenna system efficiency and least stress on the transmission
hardware are achieved when transmission line impedance is matched to antenna
input impedance. When that condition exists then the length of the
transmission line is unimportant except for the power lost due to
transmission line attenuation, and the cost to use the length and type of
transmission line selected.

The match of the antenna input to its input transmission line is a function
of their design, AND the installation environment. The physical environment
near the antenna can change its input impedance from the assumed value, and
create an impedance mismatch with a transmission line selected to match the
input impedance assumed for the antenna.

This is usually corrected by an impedance-matching network installed at the
junction of the antenna and its input transmission line. Virtually every
AM, FM and TV broadcast antenna in the US has some means of adjusting the
match between the antenna and its input line to optimise system SWR.

Impedance adjustments can be done at the input end of the transmission line
to the antenna, but will be more narrow-band. Trimming the length of the
main transmission line is another approach, but again, is more
frequency-sensitive than adjusting the match directly at the antenna input
connector.

RF

Visit http://rfry.org for FM broadcast RF system papers.

________________

"Dave Shrader" wrote in message
news:5bzJb.113315$VB2.290693@attbi_s51...
Kevin, the resonance of the antenna is determined by the length of the
antenna! It is not related to the VSWR!!!

If you are using VSWR as an indicator of resonance it is meaningless!

Since your VSWR is changing with length of Coax, I suspect you have RF
on the braid of the coax. Install a series of Chokes or ferrites.

W1MCE

Keven Matthews wrote:
SNIP
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.

SNIP