Stacking different-frequency monobanders isn't as easy as it seems. If
the monobanders are too close, it's awfully easy to end up with less,
rather than more, performance than a good tribander. On the other hand,
a monobander has fewer failure mechanisms.
For monobanders, stacking at 1.0 and 1.5 wavelengths is one of the very
best setups, with an addition of a lower one for the occasional
high-angle situation. The 1.0/1.5 hieghts give the best match to the
5%-95% range of elevation angles on the high bands (2 degrees to 16
degrees, if memory serves).
Since it covers three bands, the triband stack needs more range of
physical heights to get the best electrical heights, but a 3-stack would
be a great starting point. Whatever you do, the experience will lead to
other projects, so I wouldn't consider either choice the last you'll
make. I agree with Steve, K7LXC: Go for it and get the first-hand
experience of what happens, but put the antennas together well enough to
avoid the usual trap, element, feedline, balun or boom failures.
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