On 6/5/13 1:50 PM, Jim Brown wrote:
On 6/5/2013 1:33 PM, Larry Loen wrote:
Are you going to contend that dipoles don't work in city circumstances as
well?
Gee, I thought I made my point quite clearly at the very beginning of
this thread. Perhaps you should re-read it. Indeed, I observed that a
true dipole (defined by most textbooks as a CENTER FED antenna -- that
is, two equal lengths) is a far better choice, because it can be choked
effectively.
The problem is that the Windom is badly unbalanced, the feedline picks
up noise, and it needs a serious common mode choke at the feedpoint to
kill the noise. But because it is so badly unbalanced, there is a lot of
common mode voltage across the choke, and a choke is likely to fry with
high power.
I've always worked on the theory that you start with "close" to equal
legs and then thrash from there. Sure, you're going to have asymmetries,
but all the other stuff is less of an issue, the closer you are.
I think the gripe that Jim might have is with "deliberately" going OCF,
when there's no good reason to.
About 10 years ago, I was looking to put up a new wire antenna and I ran
across the whole OCF thing. I ran the models: hey, about the same
several hundred ohm feedpoint impedance on all bands: all I need is a
suitable transformer and I'm good to go.
Then I started doing some parametric perturbation (that is, I started
tuning it a bit off, or throwing some other conductors in the model
near, representing things like rain gutters, etc.)
All of a sudden, that "works on every band" went away. sometimes it was
horrible, sometimes it was ok. This, to me, indicates that the design
is "over sensitive" to small things.
So I went back to a center fed multi wire dipole (like a DX-CC), and
analyzed what the effect of a tuner at the feedpoint would be
(calculating the losses, etc. using a version of AC6LA's spreadsheet
turned into matlab code). Hmm, not much loss for the "nominal" case.
And what's better, when I perturbed the model with other things (other
conductors, changing the droop, changing the length of one leg, etc.).
Not much difference. The pattern stayed reasonable, the loss in the
tuner was small, etc.
So now, my "simple go-to" design is "tuner in the middle, wires close to
the right length, don't hassle the last 6 inches of pruning"
And for those with a taste for heat.. a resistively loaded dipole with a
big amp. That 3-4 dB loss makes everything a decent match.
There's an extensive discussion of this in my Power Point on Coaxial
Chokes. http://k9yc.com/publish.htm
73, Jim K9YC
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