Jim,
43' was chosen as a length because it represents 5/8 wave on 20m,
which is the point at which
a higher angle lobe starts emerging, but groundwave is maximized.
Clearly, the antenna is the biggest it can be, to produce best 80m
performance, while retaining
best 20m performance. 160 is compromised. 15 and 10 are
compromised by multiple lobes.
Curiously enough, though, on both those bands, there are times when
signals arrive at high angles.
At a sunspot peak, for example, at mid day, if you don't have a 10m
yagi @ 25', you won't break
contest pileups.
So, the only answer is, "it depends".
Regardless of the electrical length of the antenna, though, getting
it up high and in the clear
is the important factor. A 40m vertical dipole plays amazingly well
on multiple bands, with
a tuner, if you can get the high current portion of the antenna up
50' or so, in a tree.
N2EA
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