Tom,
Yes, sometimes the cure is worse than the disease, especially if you are
not adept with the cure.
In those days, Chuck and I conversed about antennas almost daily on 2m
during commute time as we had a repeater and a number of friends in common.
As for the dual mode bobtail, either he got the idea from me, or I got the
idea from him. But either way, his design was much more interesting because
it avoided the problems you mentioned, and it was useable on other bands.
Mine was an attempt to avoid work, I think.
We were unlikely collaborators, Chuck and I, because most of the time I
would lash-up stuff in the trees and try them out over a weekend. But
whenever Chuck built something, it was more like real product development
than breadboarding. Everything he built looked like it was completely ready
for market and could be used in deep space or in a battle situation. So
when it came to antenna collaboration, I usually stuck to the modeling on
the VAX while Chuck cranked out works of engineering art. I am sure he is
still doing that kind of stuff.
Dudley - WA1X
-----Original Message-----
From: Tom Rauch [mailto:w8ji@contesting.com]
Sent: Wednesday, February 09, 2005 6:03 AM
To: Dudley Chapman; towertalk@contesting.com
Cc: n9wx_dan@yahoo.com
Subject: Re: [TowerTalk] RE: Bobtail Curtain
> 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.
Well, you did things correctly.
My bet is most people would simply connect any length of
coax at almost any angle (or even parallel with the leg)
with almost any terminal phase at the coax-to-antenna
connection point.
If someone doesn't understand feedline decoupling and IF
they want a clean pattern (they may not care), they would
probably be much better off to not get the coax get the
coax into the main radiating aperture of the antenna. That
means a classis feed would be much better overall for those
who want a clean pattern without planning the feed system.
It's ironic that the coax, while making matching easy,
brings hidden problems that are often as difficult to cure
as building a simple parallel tuner would be.
One very novel approach to this antenna was by ex-WA1EKV
many years ago. Chuck installed a classic Bobtail but used
open wire for the center drop. By paralleling the center
wires he had a classic Bobtail. By feeding the wires in
push-pull he had a low center fed dipole (with the ends
dropped vertically). That's a pretty good idea, because the
system is easy to feed with a tuner at the feedpoint. It
becomes a multiple band antenna that works well on the band
below the design frequency of the Bobtail and it provides a
high angle radiator for local ragchews if you switch the
feed.
73 Tom
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