On Mon, 1 Jun 2009 22:50:52 +0200, "Antonio Vernucci"
wrote:

Along several decades of radio hamming on the HF bands, I noted that the
measured SWR of all the antennas I have mounted (Yagis, dipoles) slightly varies
when the feedline length is changed by several meters. For 100W of forward
power, the reflected power could vary somewhat, e.g. from 2W to 5W or so,
measured on a Bird wattmeter. This behavior would seeem to deny the theory,
according to which SWR is independent of feedline length (as long as the cable
attenuation remains constant).

Hi Antonio,

Yes, this is the accepted wisdom.

Clearly the measured SWR change cannot be due to the change in the feedline
attenuation as, at HF, adding or cutting a few meters of cable would yield a
very small change in attenuation and hence a negligible impact on measured SWR.

A good point.

Reading here and there, the most common theory explaining such phenomenon is
that, in presence of RF on the coaxial cable braid, the SWR meter reading is
influenced by the feedline length.

This, too, is accepted wisdom.

I am not too convinced of that explanation,
also because I have invariably experienced the measured SWR variation phenomenon
with all antenna I have had, and I never had hot braid problems.

You ARE describing a hot braid problem. At the slight shift of 3W out
of 100W, it is a small problem by the same degree (nothing you would
notice by other indications).

At that regard I got an idea that could explain the phenomenon, at least part of
it.

Well, I am going to skip that quote to cut to the chase. What you
describe is called mismatch uncertainty. It exists in a cable that
has a nominal Z that matches neither the load nor the source.
Depending upon the amount of mismatch at each end, you have a zone of
confusion between those ends that will result in as many different
readings as you have insertion points to measure at. As most modern
transmitters have a source Z of 30 to 70 Ohms, you might note a very,
very small perturbation when the other end of ANY line is mismatched -
but I doubt it. To provoke this condition into revealing readings
that are significantly beyond the range of error requires mismatches
at both ends on the order of 3:1. You don't describe that.

More likely your problem is Common Mode currents - what you call hot
braid. One test is to use a snap-on choke and slide it along the line
and note if the SWR meter reading moves in concert with the hand
motion (or the reading simply shifts by the addition of the choke).

73's
Richard Clark, KB7QHC