Antonio Vernucci wrote:

Hi Walt,

I have always used a good balun on all my antennas, and therefore I am
not too convinced that, in my case, the SWR change I observe when adding
(or removing) a piece of coax in my station could be due to RF presence
on the coax braid.

Anyway, I have not yet read a clear and convincing explanation of why
the presence of RF on the coax braid would cause the SWR meter to give a
different reading when moving it along the line.

I appreciate that, with a hot braid, the coax cable becomes part of the
antenna and then radiates, but I cannot clearly focus why this can cause
the SWR meter to see different impedances at different points of the
line. Impedance is the ratio between RF voltage (between center
conductor and braid) and (differential mode) RF current. So, I do not
well visualize how the presence of a common mode RF current can
influence the meter reading.

73

Tony I0JX

If you keep the same total line length and move your meter to different
points along the line, current on the outside of the line won't change
the indicated SWR except to the extent that the SWR meter is poorly
shielded and affected by the current. However, I believe the problem was
stated in such a way that the SWR was seen to change as line was added
or removed, that is, as the line length changed. And that could change
the actual line SWR for the reason you described.

Roy Lewallen, W7EL