Thanks for the responses.

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.

I like the sleeve/skirt decoupled dipole idea.

I hadn't thought about routing the feedline out on a sidearm. I was
thinking of doing that with the antenna, but with a large yagi it would
be a problem. Putting the feedline out a few feet and then dropping it
down to a lower point on the mast could certainly work.

As far as putting the feedline in the plane of the elements, understood
on the modeling and placement issues.

If one were to do this, would it help to have ferrite beads all along
the coax from the feedpoint to some distance down the mast? Of course
good feedline decoupling is important for any directional array, but I
imagine that a wire with a string of ferrite beads on it is pretty much
invisible to RF... is this right?

Maybe a long bead balun dropping away in between elements would be a
good way to go with no sidearms?

Thanks for the responses; I'd be interested in further ones.

73
Dan

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

wrote:
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.

It certainly isn't that simple in a distributed network.
The currents into the end of a half-wave section are certainly
unbalanced at X and Y but the current amplitudes are pretty low.
5 watts into 5000 ohms is only about 30 mA. 1/4WL back at the
shorted matching section at ++, the currents have to be close to
balanced because of the short. The coax connection at A and B is
closer to the short than it is to to X and Y. The current down
the coax braid at B does NOT have to be equal to the current at
the feedpoint at X since they are a good percentage of 1/4WL
different in location. And common-mode current can form standing
waves so common-mode current maximums and minimums exist depending
upon location on the feedline. The lumped circuit model strikes
(out) again.

X
+--A----------------------------
|
+--B--------Y
--
73, Cecil http://www.qsl.net/w5dxp

In message .com,
" writes
Anyone ever get around the VHF FM vertical yagi feedline routing
problem by voltage feeding the bottom of the driven element with a stub
like a J-pole? A quick EZNEC run seems to indicate that it's not a bad
idea.

So it seems like it might work OK... but I wonder if anyone has
actually done it. Would you expect a coaxial stub to work better than
a parallel wire stub in terms of preserving the pattern of the yagi?

Any commercial products doing this?

There's an example of a 4ele 2m J-fed yagi in the MMANA files. It has a
gain of 7dBd. Free space pattern is skewed up by 1 degree, but a real
ground will do that anyway.

I don't know of any commercial versions.

Brian GM4DIJ
--
Brian Howie