Tom,
Yes, I modeled the coax as a grounded piece of wire connected to one side
of the source. My goal was to establish boundary conditions that
discouraged feedline currents to a large degree using a choke balun at the
feedpoint and another one at end of the coax that reached the ground. I
grounded the coax on the shack side of the choke balun near the ground.
With the right choice of feedpoint polarity you can get the currents on the
feedline to be low.
One thing about this particular configuration, though, is that a little
feedline radiation is not too harmful to the pattern because it is of the
same polarization of the vertical element it is near to. And it is either
in phase or 180 degrees out of phase with that element.
I did all this modeling quite a few years ago on NEC3 running on a VAX
8600. I was lucky enough to have access to that at the time. I will set it
up again in Nec-Win Plus when I get a chance and see if I can duplicate it.
My opinion of the performance of the antenna is purely anecdotal.
However, I felt it was operating as a bobtail curtain because the element
lengths at resonance and the bandwidth in both of the modes I mentioned was
close to the model's predictions. It was also pretty insensitive to the
length of feedline from the grounded point to the shack, but I suppose that
last point is not very indicative.
You see, living in New England at the time, I had an intrinsic distrust
of feeding a vertical against the ground. Here in Ohio, with the rich, wet,
clay that is all around me, I have joined a 12 step program to change my
attitude about that.
Let me state as a disclaimer that Tom has a very valid point (as usual).
Without careful consideration of design, feeding any unsymmetrical antenna
with coax at a point on its structure that is far from the ground is asking
for trouble.
Dudley - WA1X
-----Original Message-----
From: Tom Rauch [mailto:w8ji@contesting.com]
Sent: Tuesday, February 08, 2005 7:05 PM
To: Dudley Chapman; towertalk@contesting.com
Cc: n9wx_dan@yahoo.com
Subject: Re: [TowerTalk] RE: Bobtail Curtain
> One thing I did differently was to feed it at the top
of the center leg
> with coax, rather than using the classic parallel LC to
ground. I brought
> the coax off at a 45 degree angle to ground and used a
choke balun at the
> feedpoint.
Did you model this antenna with the coax in the model?
I'm not saying the antenna didn't "work" and make contacts,
but it is extremely difficult to get a feedline off an
antenna like that without greatly affecting pattern. A
simple balun at the feedpoint generally won't cut it if you
want a true clean Bobtail pattern.
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