Roy:

Even if you have me bit-bucketed, thanks for that info.

I am one who was ignorant to what you present in your text--I always expected
increased line radiation with a mismatch. Now that you have stated it in text,
it is plain a balun would prevent most if not all of this...

I will re-evaluate and brush up on what I have certainly missed before.

Warmest regards,
John

"Roy Lewallen" wrote in message
...
Richard Fry wrote:
"John Ferrell" wrote:

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

_________________

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