A clear disadvantage of phase reversal schemes is that the ALL
discriminate AGAINST low angle radiation. I've tried several of these
schemes at my station, and found them useful only for breaking the
occasional pileup into the Carribean and NEVER as a good combination for
running stations into Europe or anywhere else for that matter. A far
more important set of switching variables is to be able to select antenna
HEIGHT rather than reverse phasing. Variable antenna height trades of
gain at low angles for a broader main lobe with increased high angle
coverage.
For example, with a two-high stack (by far the most prevalent among
stacks), switching should be provided to select BOTH/UPPER/LOWER.
The lower antenna has a nice broad main lobe with reasonable low angle
radiation, good high angle radiation and absolutely no nulls at useful
angles. The upper antenna will have excellent low angle gain, at the cost
of a narrower main lobe and discrimination against higher angles. A
well designed stack (two identical monobanders with at least 0.7
wavelength spacing) produces 2-3 dB of honest gain (not smoke and
mirrors...) over either antenna alone, its low angle radiation is as good
as the upper antenna alone (except at grazing angles), and its high angle
radiation is better than the high antenna alone, but as good at high
angles as the low antenna alone. Unlike phase reversal schemes, none of
the BOTH/UPPER/LOWER combinations produce a null at low angles.
73
Frank
W3LPL
donovanf@sgate.com
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