Roy Lewallen wrote:
The feedline (and other metallic structures) adjacent to an FM broadcast
transmit antenna will affect the radiation patterns of the antenna even
though the measured match between the feedline and antenna input is
extremely good (even 1:1 SWR) -- in which case the line should have no
differential current to produce such an effect. What is the explanation for
that, please?

We've been down this path before, and you've shown that you won't
accept the fact that SWR has nothing to do with whether or not common
mode current exists on a feedline, and there's nothing I've been able
to do to convince you otherwise. You also either haven't read or won't
believe that it's common mode, not differential, current that causes a
line to radiate and thereby contribute to the overall pattern.

I agree with Roy that SWR itself is not the *cause* of radiating
feedlines.

However, if the feedline is allowed to carry currents and radiate, it
effectively becomes part of a new and different antenna configuration.
That new configuration will have a different feedpoint impedance, so the
SWR will change.

But the SWR didn't cause the feedline current and radiation; in fact it
was exactly the opposite.

Also, Richard is assuming that unwanted feedline currents will always
change the SWR for the worse. That's often true, but it doesn't have to
be - sometimes the SWR gets worse when the feedline current is choked
off.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek