"David Platt" wrote in message
...
In article ,
Sal salmonella@food poisoning.org wrote:

How would you change the feed method? I've had generally good performance
from my J-poles but I'll gladly improve what I do, if you have some ideas.

One of the older ARRL guides or antenna books shows a balanced method
of feeding a J-pole. A standard half-wave coaxial balun is used. The
two balanced outputs of the balun are tapped onto the two sides of the
J-pole matching section, some distance above the usual "50-ohms-or-
thereabouts" attachment point.

My understanding is that the impedances "seen" on the two sides of the
matching section won't be identical; the short side ends at an
open-circuit point and the other side "ends" at the beginning of the
half-wave section, where the impedance is high but not quite an open
circuit. Hence, you won't achieve complete balance this way - there
will probably be some current flow on the outside of the halfwave coax
balun section.

I don't know which one would be the best but I have seen 3 methods of
feeding the J-pole. If you insulate the bottom then you hook the feedline
to the bottom with the center of the coax to the long side. If you do not
insulate the bottom you tap up the matching segment so that you get a 50 ohm
(if that is the coax used) match with the center of the coax connected to
the long leg. Then there is the balun made out of coax that is hooked up to
the matching segment so that a low swr is obtained.



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