Richard Fry wrote:
"John Ferrell" wrote:

A poor match will be inclined to generate TVI...

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Could somebody please explain why feedline SWR should produce any more
general TVI in the neighborhood than if the match is perfect?

If the match is perfect, the antenna radiates all the power from the tx
(less line loss). If anything else radiates part of that tx power (the
feedline, you say), the total radiated power still can be no more than
when the antenna radiates all of it. So if tx harmonic levels are the
same in both cases, why should "feedline radiation" have much affect on
TV receivers in the neighborhood?

Are you talking about some very localized effects (tx feedline running
next to your own TV set or TV antenna, etc)?

RF

It sounds like you've confused feedline SWR with feedline radiation.
Those are two different effects. A line can have 1:1 SWR and radiate, or
10:1 and not radiate. SWR is determined solely by the differential load
impedance and the line Z0. Feedline radiation is determined only by the
amount and distribution of common mode current on the transmission line
and of course its length.

The line SWR can be altered only by impedance matching at the load or
changing the line's Z0. Feedline radiation is reduced by using one or
more current baluns (common mode chokes) or a tuner with an inherently
balanced output such as link coupling. While changing the feedline
radiation can change the SWR (by changing the radiating part of the
antenna), changing the SWR by altering the differential load impedance
has no effect on the common mode current and hence the line radiation.
(Anything which alters the common mode impedance of the antenna, which
depending on its design a differential matching circuit might do, can
change the line radiation. But it's not because of the change in SWR;
it's because of how the matching circuit alters the common mode impedance.)

Only feedline radiation is associated with TVI; SWR isn't.

Feedline radiation can cause increased TVI and other RFI if the
transmission line is routed in a way that it can couple to power lines,
telephone lines, TV cable, and other conductors which take RF into homes
and close to TV sets and other electronic devices. (I've had more
trouble with modern telephones than anything else.) Radiating antennas
can, of course, do the same thing and to an even greater degree -- an
attic transmitting antenna can often cause problems, for example. But
feedline radiation can increase the RFI potential of an antenna that's
otherwise far away from other conductors. The problem is simply one of
effectively bringing part of the antenna into the house. It is, as you
say, a localized effect, although it can affect a neighborhood by
conduction and radiation from the conductors into which the feedline
radiation is coupling.

Roy Lewallen, W7EL