Bob:
You want to use identical antennas as N3RR pointed out. Also, to
insure equal currents, you want to use feedlines from the antenna to
the common point (t/b/l switch, etc) that are equal AND a 1/4 w.
multiple. There is much in the published literature on this point.
Anecdotal experience: I had a dissimilar stack, 3/2 on 40 mtr. That
was mistake #1. Mistake #2 was having equal length but NOT 1/4 w.
multiple feedlines to each antenna. It worked "ok" on cw, but I could
work ABSOLUTELY NOTHING on ssb using both. The impedence of the
antennas didn't track the same over the large frequency excursion.
Never tried to figure out exactly what was happening at the common
feedpoint, but suspect major phasing errors (from 0 deg) and major
current imbalance. I played around a little with a model, and found
that the physical misalignment of the antennas was causing phasing
errors, correctable by either moving the bottom beam forward (not
mechanically practical) or via a feedline phasing correction (which
would introduce current balance problems). I concluded that this
stack would only be marginally effective over a very narrow bandwidth
- if I did a lot more work electrically. Given the inherent
mechanical problems of so much antenna, the above electrical problems,
and other interactions, I took the bottom beam down and wrote off the
project as an interesting experiment and learning experience.
I think if you stack the 3 antennas you mention, you will be quite
disappointed. You may find a frequency or situation that they like
but certainly your performance over the band will suffer.
To get an idea of the tradeoffs, do a little modeling. Echoing n4kg,
you may find that 3 antennas on 10 will work as well as 4. Boom
length will be a big factor. The f/b is much easier to maximize with
3 antennas vs. 4. Finally, an antenna needs to be optimized in the
stack, NOT as a stand-alone beam. An antenna optimized in a stack
will perform well by itself. One optimized by itself will have
degraded performance in a stack, particularly f/b. See Lawson's book
on stacking.
Good luck!
Greg
na8v/4
tallahassee, fl
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