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Owen Duffy · July 17th 10, 06:56 AM posted to rec.radio.amateur.antenna

"Sal M. Onella" wrote in news:cc6de1f9-fa1d-4a1c-bf7f-
:

Does the velocity factor of the coax come into play when locating the
second
choke or does the outer shield behave like an open wire, VF ~ 1.0?

Perhaps I am slicing the baloney too thin for an average sandwich ;-)

You will often see emphatic advice as to which side of the thing to
attach the shield, and which the centre conductor.

In fact, the antenna structure lends itself to an effective integral
balun. If you take the feed coax through the bottom wall of the U
section, bonding the shield to the tube, and up one side (doesn't
matter), exiting at the appropriate point. At this exit, trim the shield
and bond it to the tube, and take the inner horizontally across to the
other tube and connect it.

If physical symmetry is very good, the balun will be very good.

Nevertheless, as Roy observes, the feedline and supporting structure
below the U section is coupled and there is still potential for some
level of common mode current.

(To preempt Roy, the velocity factor of the outside surface of the shield
clad with a thin layer of vinyl will be close to 1, close enough for the
purpose at hand.)

Owen