Dave Brown writes:
That said, the use of larger verticals will not necessarily give the best
takeoff angle for the relatively poor skywave that they tend to produce.
Bigger ain't necessarily better in such cases. This is of course in direct
conflict with 'conventional' wisdom that is based on commercial practice
using groundwave only, not amateur 'limited' comms
Not exactly. Commercial and military practice at MF and below often entails
use of skywave, too, and we specifically use large verticals for their
low-angle radiation characteristics to achieve best long range coverage.
The objective is twofold: To minimize high angle radiation that sometimes
results in destructive interference to our groundwave (not a prime
consideration for this effort, of course), and to maximize it at angles where
the fewest possible hops will be required to reach the target, as multiple
hops are disproportionately lossy.
Angles of departure well under five degrees will be necessary to cross the
Atlantic in two or three hops. An angle of 20 degrees could easily entail as
many as 10 hops, with consequent extra loss. However, one must get beyond a
quarter wave antenna height before the vertical radiation pattern begins to
concentrate significantly below 20 degrees.
I doubt many amateurs, even in the wide open spaces of Scotland, are likely
to have, say, a half wavelength vertical at their disposal for 136 kHz. <g>
But bigger can still be better. Anything that minimizes the number of hops
will prove worthwhile; as, for that matter, will anything that makes it
possible to achieve a full one watt ERP. Larger antennas will certainly
help, as will any and all loading tricks to linearize current in the vertical
run.
73,
John KD4IDY
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