"Antonio Vernucci" wrote in message
. ..
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).

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.

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. 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.

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

Reading coaxial cable data sheet, I noted that manufacturers typically
give a
small tolerance on cable impedance (2 to 3 ohm). Let us then assume that
the
feedline cable has a 53-ohm impedance, whilst the Bird wattmeter is 50 ohm
sharp.

If the 53-ohm cable is terminated on an e.g. 75-ohm (purely resistive)
antenna,
the real SWR on the line would be 75/53=1.41 independently of feedline
length
(if the attenuation variation with length is neglected). But the impedance
seen
by the wattmeter obviously varies with the feedline length, and it can be
easily
calculated that the seen impedance range results in an apparent SWR, on
the
50-ohm wattneter, reading that varies from a maximum of 1.5 (when feedline
length is an even multiple of half wavelenght) down to a minimum of 1.33
(when
feedline length is an odd multiple of wavelenght quarters). For 100W of
forward
power, the reflected power varies from about 4W down to about 2W.

Repeating the exercise with an e.g. 85-ohm load, the apparent SWR measured
on
the 50-ohm wattmeter would vary from 1.7 down to 1.51 (reflected power
varying
from 7W down to 4W).

You can get easily convinced that such variation is only due to the
assumed
3-ohm difference in cable impedance.

With older cables having a nominal 52-ohm impedance, instead of 50, the
situation could get even more evident.

Any comment would be appreciated.

73

Tony I0JX

The Bird actually measures a combination of capacitive coupled voltage and
inductively coupled current. There is a app note on the Bird website.
Find: "Straight Talk About Directivity".