RadioBanter - View Single Post

[email protected] · July 7th 06, 01:56 AM posted to rec.radio.amateur.antenna

wrote:
Tom, I didn't know that the feedline decoupling issues with J-poles
were so bad.

I'd be interested in more information about why it's such an issue.
Would a sleeve stub work better than an open wire stub? It's certainly
stupid to make the feedline MORE of an issue.

Look at what the J-pole really is.

1.) You have a half-wave end-fed antenna. There has to be as much
common mode current leaving the end of that point and flowing down the
feedline as there is flowing out onto the antenna at that point. There
isn't any exception to this rule.

2.) While that current may be small with a perfect half wave, it is
never zero. It gets worse fast of the antenna is not 1/2 wl long
electrically, or if it is thick.

3.) Now you have a 1/4 wl stub feeding that half-wave on the end. If
you perfectly floated that 1/4 wl stub, common mode current in the stub
would DECREASE as you move away from the feedpoint. But if you ground
the 1/4 wl stub, current common mode INCREASES as you move away from
the stub. This is why end-fed Zepps and J-poles model very good when a
perfect ground independent current source is used to feed them.
Unfortunately we can't do a perfect ground independent feedpoint in the
real world, so depending on the CM impedance the amount of pattern
distortion will be all over the place.

This is why Zepps, antennas that are really just the same as a J-pole,
are notorious for RF in the shack. The lack of feedlines in models are
why people who do not include the feeder or feedline matching device to
the 1/4 wl closed stub conclude they aren't so bad.

Why would anyone go through all that bother to complicate the feed
system in a Yagi is beyond me, when there are a half dozen easy
solutions that were mentioned here.

73 Tom