Owen Duffy wrote:

I really don't know why there is more TVI with a high swr. But my
experience has been that there is, especially on 6 meters.

If you can't explain the mechanism by which SWR causes TVI, perhaps
they correlate by some other cause. For example, an antenna may develop
loose oxided connections which both change the load impedance (and
hence VSWR), and create intermodulation causing TVI.

If VSWR *does* cause TVI, surely someone will be able to explain how?

When people report "high SWR", they are usually talking about a coax-fed
system, and they usually mean "a higher SWR than I expected for this
antenna".

That is a big clue that the antenna is not performing correctly... but
the high SWR is only a symptom. It shouldn't be mistaken for a cause.

One very common cause of RFI is common-mode RF current on the outside of
the coax - in other words, the coax has become an unintended part of the
antenna. The outside of the coax comes back down into the house, and can
be a potent conductor of RFI. The higher than expected SWR is simply
because the addition of the coax makes this a *different* antenna from
the one you thought you were using.

With something like a 6m yagi, the cure is generally to change to a
truly balanced feed system, and to add a feedline choke.

Obviously common-mode current is not the *only* possible connection
between "high SWR" and RFI, but it's more common than many people
suspect.


Just caught Roy's second post about the mistaken belief that high SWR
and feedline radiation. It should be clear from the above that
higher-than-expected SWR and feedline radiation are two separate
*results* of unwanted common-mode currents. Once again, SWR should not
be mistaken for a cause.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek