On Jun 1, 1:50*pm, "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).

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

There is a very good possibility that your analysis is correct. I see
the same effect, and in fact, it's of particular concern to me right
now, because I'm putting what effectively is an SWR meter into
production, and it's important that we have a test setup that
accurately measures the performance. I've been specifically concerned
that the test setup, as currently configured, may have trouble because
the connecting cables may not be close enough to 50 ohms.

As others have said, IF there is a problem with RF on the outside of
the line, any variation in observed SWR is most likely because the
change in line length has changed the load the other end of the coax
is seeing, NOT because the meter is directly responding to the
"outside" RF. The meter measures transmission line current and
transmission line voltage, and the line itself is all the reference it
needs to do that. Direct response to RF on the outside of the line
could result from poor construction of the meter, but I wouldn't
expect that from a Bird.

One other possibility that I haven't seen mentioned, too, is that the
impedance of the line is not constant along its length. With line of
good construction that hasn't been abused, the variation should be
small. You can detect it by running a network analyzer sweep of just
the line, across a broad frequency range. But a line with
polyethylene dielectric (and especially one with foam polyethylene)
that's gotten too hot--perhaps because of high power at high SWR--can
have the center conductor go "off-center" and change the impedance.

If the effect you are seeing is the result of a line that's not quite
the same impedance that the meter is calibrated to (which itself may
be noticably different from 50 ohms), you could plot the change in
indicated SWR as a function of line length and see it vary in a smooth
and predictable manner. Most likely, though, what you're seeing is
the sum of several effects, and the variation in indicated SWR or
reflected power may not be all that smooth. Very often when I expect
to see a nice smooth spiral centered on one point on my network
analyzer's Smith chart display, what I see is a spiral that follows
along some arc, because of various imperfections. (Sometimes it's fun
to try to figure out just what the imperfections are...)

Cheers,
Tom