On Wed, 20 Jun 2007 09:14:04 -0400, Chuck
wrote:

who believe common mode RF getting into
some electronic metering circuits
produces erroneous SWR readings.

This would be an exceedingly strange meter. Of course, there are any
number of ways to do something wrong. If this were the case (the
"infected" meter) then the antenna's being tuned is also suspicious.

Could
that be the reason small changes in
transmission line length sometimes
result in apparent changes in SWR?

No.

Of course, if there are common mode
currents on a coax transmission line,
the impedance of the line seen by the
SWR meter (50 ohms in parallel with the
impedance between the shield's outer
surface and ground) is no longer 50 ohms
and the meter calibration is no longer
correct regardless of whether RF is
getting into the electronics of the
meter.

What you have done is tuned the entire antenna/feedline system to 50
Ohms (this includes the common mode effects). Changing the length of
the line (which should not change the SWR in a CM free system) also
changes the reactance of the this length that was formerly tuned out.

Changing the length of the coax
gives you a new, out-of-calibration
measurement. ;-)

By giving you an out-of-50-Ohm load (antenna plus unchoked line).

Fellows, this is all classic stuff and has been fodder for discussion
for years. The solutions have met the test of time.

73's
Richard Clark, KB7QHC

Richard Clark wrote:


What you have done is tuned the entire antenna/feedline system to 50
Ohms (this includes the common mode effects). Changing the length of
the line (which should not change the SWR in a CM free system) also
changes the reactance of the this length that was formerly tuned out.


Thank you for that
clarification/correction, Richard. What
I should have said was that with CM
effects, the SWR meter will not show the
actual SWR "in" the coax, which could be
wildly different from the 1:1 measured
for the entire antenna/feedline system.

73,

Chuck

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