HFTA is fantastic software for its purpose. I use it extensively here. One
thing that it DOES NOT do from my observation is model stacking distances. It
seems to assume that you will get a combined 2.8dB gain at the center of the
stack spacing. To confirm this, model 2 20M yagis with 5 feet of spacing on
flat ground vs 2 20M yagis with 50 feet of spacing. Intuitively (and by
modeling) we know that the 50 feet spacing will beat the 5 foot spacing. But
on HFTA, the reverse is the case because it seems to think that the 2.8dB gain
will occur at the center point of the yagis and that center point is higher at
5 foot spacing than at 50.
I would try to understand the best heights for your antennas at the individual
heights and then make the best compromise decision on tri-band stacking
distance vs. information learned on HFTA for your individual location. Good
stacking distances should also equal balanced individual lobe coverage of each
antenna. A stack with one antenna at a bad individual height to the target
area is not going to be a good stack.
"Guying Rohn 25 at 20/40/60/80" why? Why not just buy 45G and guy it at 40 and
80 with 1/4EHS guys? Seems a lot simpler and probably cheaper. You had better
use Philly guys if you are going to have 4 guys over 80 feet of tower. The RF
reflecting off of that much close metal in the air is not going to help your
"in practice" performance at all.
Let us know how the project progresses.
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