Hi all,
Taking up the thread of the ERP discussion, and consideration of
verticals or a dipole:
At the rather long wavelengths of interest, amateur antennas at LF are
usually very short in electrical length terms. The presence of "earth"
can not be separated from the antenna environment. The "earth" is a
combination of conductance and dielectric. The launching of a radiated
wave is greatly facilitated by using the ground conductivity, and is why
VERTICAL POLARISATION is the main mode of antenna operation, and ensuing
GROUND WAVES are relied on for reliable coverage (at least that is the
reasoning for most of the broadcast and military LF systems). Usual
practice is to have a "big antenna with lots of top loading" and "lots
of ground radials". LF amateurs are mostly building small scale
versions of what the "big boys" do i.e. a Marconi T or variant.
However, the best of amateur LF DX is usually a result of SKY WAVE
propagation, even if the amateur antennas are vertically polarised (the
polarisation reflected by the ionosphere could be random, but only the
vertical component is efficiently received). If sky wave mode is what
is actually being sought after, then it may well be worth experimenting
with a horizontal "dipole"? The ground conductivity will try to "short
out" the launching of a ground wave, but who cares if sky waves are the
desired main objective? A horizontal wire electrically close to ground
provides a "near vertical incidence" radiation pattern, with most power
"going upwards", but with sufficient angular spread to provide
reasonable DX from a one hop path via an ionospheric reflection.
One New Zealand amateur, Bruce ZL1WB, uses a very long wire strung out
over a gully, and with only 30 watts of RF power applied, is the most
often copied ZL LF amateur station in Australia (spanning the Tasman
Sea, a definite sky wave path, signals are zero much of the time during
daylight hours). So there is a suggestion that the very long wire that
is mostly horizontal in nature does provide good LF DX.
So, casting aside the preconditioning about vertical polarisation and
LF, I suggest that it would be well worth investigating from the point
of view of sky wave DX potential.
Does anyone have a back yard that is a half wave across on LF :) I do
not :(
Regards,
Bob ZL2CA
PS I would ask Finbar if he could erect a second big mast for the other
end of an LF dipole?
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