I've been messing with EZNEC, modeling stacked C-3s at various spacings (top
antenna at 97-102 ft), and the results indicate that while the stacking gain
may be relatively constant over a fairly wide range of spacings, the pattern
formed by the stack is anything but. Some spacings produce definitely
enhanced patterns at one frequency (14 mHz and 37 feet, for example, or 21
mHz and 25 feet), while others have a devastating effect on the pattern (21
mHz and 37 feet, for example -- note the problem?).
If anyone wants the details, I'll be glad to provide them -- as well as the
C-3 model).
I've checked what I have from the amateur literature, and the closest thing
to describing what I'm seeing is the the K8CC article on stacking dissimilar
yagis in NCJ a few years back. K1VR and N6BV seem to like 30-foot spacing,
but my models using that spacing don't look particularly good. Maybe that
is the best compromise available, particularly to cover 20-10 meters, but
what about for the next couple of years?
I really have two questions:
To what extent are these spacing effects dependent on particular tribanders,
or are they generically true?
If they are generic, can anyone suggest a spacing that might be a better
compromise between 20 and 15 (ignoring 10 for now) than the 30-foot one?
I'm pretty well stuck with having the top antenna between 97 and 102 ft, by
the way.
Thanks for your help. Please reply direct, and I'll summarize if there is
interest.
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