My experience has been that you cannot use HFTA to decide on the actual
optimum gains of antenna systems. HFTA seems to have a formula that simply
adds 2.5dB gain to 2 phased yagis, no matter what the spacing. Doubt it?
Plug in 2 yagis at exactly the same height and it will still give you a 2.5
dB gain vs one at the same height. I believe that what HFTA does is apply
the 2.5dB gain to the height that is midway between the 2 yagis and then
calculates the take off angles depending on the terrain.
I love HFTA but not for deciding optimum spacing for gain. Instead for
deciding different heights of the antennas. For instance, if you have 2 -
44ft boom 15M yagis at 13m spacing, is the best heights of low antenna at 8,
10, 12, 15, or 20M? For that, its fantastic.
By the way, I have 2 - 15M 6el 44 ft long boom yagis at 28 and 70 ft. And
they "kick tail!".
Ed N1UR
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