If you are having SWR change for short lengths of added coax, it is not loss
in the coax of any size, even half inch one like you have. SWR should be the
same everywhere you measure on a feedline to a given antenna.
It is RF on the outside of the coax affecting it. There are no optimum
lengths of coax, except the minimum to reach from the antenna to the tuner and
then to the rig.
Even if you used an exact half wave of coax to repeat the antenna impedance to
the rig end (which is waht they were discussing on the Collins net), you would
be good for low SWR on that one frequency only; thus, that is an old wives
tale of diminishing results when you move around even one band, to try to
tailor your line length. Now, if you used stubs with capacitors across them,
that is another issue. You were asking about plain coax runs.
What you can have, and need to avoid, is having a length that is a quarter
wave on the band of the antenna, which promotes RF to flow on the outer
shield. Keep the coax at right angles to the wire dipole to avoid RF on the
shield induced from the antenna. Use the correct coax impedance to be close
to the dipole impedance. If trying to use the single dipole on multiple bands
with coax, parallel other dipoles on the same feed line and tie them off in
fan arrangement above and below the one you have to have a match to line at
any band.
If your matching device won't tune up right on a particular coax length, it is
fine to change the length say by adding or subtracting five feet and the
matcher should be able to handle the impedance and reactance. But make sure
you have a matching device with enough L and C for the lowest bands, Many are
deficient at 75 meters or 80 and 160 M.
Hope this helps, and get a copy of Walt Maxwell's book "Reflections", it
covers this in details. Was reviewing it and some antenna handbooks just this
weekend, about how ladder line or open wire line is lower loss in any case
than plastic dielectric coaxes, and can deal with multibanding one dipole and
high SWR with low losses.
73, Stuart K5KVH
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