Richard Fry wrote:
As a discussion point, consider 1/2-wave parasitic radiators sometimes
positioned within a foot or two of FM broadcast transmit elements, to
"shape" their patterns. This is the common technique used in "sidemount"
antennas that must meet FCC requirements for directional FM broadcast
assignments.

A parasitic itself is suspended mechanically in space by a non-metallic
support. It has no direct connection to the transmitter, and no conductive
physical path to any part of the antenna, its feedline, mounts, or
supporting structure.

Such parasitics do affect the net radiation pattern(s) of the array.

Isn't a "non-radiating" feedline with a balun just an arbitrary length of
conductor, but now with a metallically conductive path to the driven
element(s), as well?

No. A "non-radiating" feedline is one which has no significant amount of
common mode current. This can be accomplished by making the feedline a
length such that the induced current is minimal; by inserting a balun or
baluns; and/or by placing the feedline symmetrically with respect to the
antenna. I thought I had explained this -- I don't seem to be
communicating well.

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. But hopefully
other readers have learned from this exchange. Once the basic principles
are grasped, these phenomena lose their mystery, and they're no longer
"unpredictable", but readily measured, modeled, and understood.

Based on past experience, nothing I say will sway you from the way
you've chosen to interpret observed phenomena. And I believe I've done
enough explaining so that any other readers, who are open to learning
some fundamentals, can come away with a better understanding. So that's
enough for now.

Roy Lewallen, W7EL


RF
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"Roy Lewallen" wrote in message
...

The radiated field in which a feedline is immersed produces a common
mode, not differential, current on the feedline(*). ETC