Roy:

My "bumbling goof-abouts" with antennas has proven to me you are once
again right on the money...

I have gotten to where I don't even worry about the loss of a well
constructed choking--balun/unun in the circuit--better to be safe than
sorry...

John

"Roy Lewallen" wrote in message
...
Modeing shows that radiating current can occur on a J-Pole feedline
due to mutual coupling from the antenna. The amount depends heavily on
the length and orientation of the feedline. The main effect of the
current on the pattern is to distort the main lobe, reducing the
amount of signal directed horizontally. I suspect that the widely
varying reports on the effectiveness of a J-Pole are due, in part, at
least, to different feedline lengths and orientations and therefore
different amounts of feedline radiation.

If there is significant current on the feedline, it might take a
couple of current (choke) baluns, placed about a quarter wave apart,
to reduce it to a small value.

Most people don't realize that ground plane antennas are subject to
the same phenomenon.

Roy Lewallen, W7EL

Larry wrote:
ml wrote:

second

I have a arrow 2/440 j pole works great---- it dosn't have any
radials hanging off naturally, so since basically just the center
conductor is connected would you suspect my coax shield is 'part of
the antenna' radiating alot of stuff?? (course the shield does
attach to the bottom mounting plate of the antenna but i wasn't
counting that


In the typical J-pole antenna, the shield connects to the long
element and the center to the stub. I don't know your antenna in
particular, but I would suspect this is the case and don't believe it
is considered a radiating element.

I have gotten to where I don't even worry about the loss of a well
constructed choking--balun/unun in the circuit--better to be safe than
sorry...

If the decoupling is designed into the antenna, using cones,
bells, sleeves, etc, there is little if any loss. Being as the
feed radiation skews the pattern up off the horizon, adding
decoupling usually ends up with a "gain" at that low angle.
Also, I've heard different types of "J" feed methods, naturally
decouple better than others..This also applies to the ringo's,
etc. The gamma loop matching device on a ringo does a
halfway job of decoupling the line, but it can always be improved
by a 2nd decoupling section. I don't use conventional ferrite
chokes when decoupling VHF/UHF antennas. IE: If I were to
add additional decoupling to a ground plane, I would use a
2nd set of radials, or a sleeve, vs a ferrite choke. MK