Rik Strobbe wrote:
Hello Dave and the others,
I think that the discussion about antennas with a significant horizontal
part is not 'if' they work (they do work, signals of OH1TN, DJ9IE etc.
prove so), but 'why' or 'how' they work.
The simple 'toploaded vertical' approach seems not correct to me, because
according to improved current distribution the gain (versus a pure vertical
of the same height) can be no more than 6dB. Also beyong a certain lenght
of the topload (of approxemately 5 times the height) there should be very
little gain.
Some possible solutions were already given :
- horizontal polarized component
- larger 'footprint', reducing losses
As this can be a very interesting antenna (for those have the proper space)
it might be worth trying to understand the 'why' and 'how'.
73, Rik ON7YD
My two pennies worth: Conductive soil "shorts out" the horizontally
polarised component anywhere near the conductive surface. However, at
LF the "skin depth" can be tens of metres so effective ground level may
be somewhat below physical ground level? The antenna wires could have a
considerable horizontal component but I believe the substantive
radiation component (energy leaving the antenna environment near field)
is vertically polarised. Having long lengths of top loading does a lot
for "reactance compensation" and minimising the need for a loading coil
(and associated coil loss). The "effective height" is still mostly
related to the length of the "up wire" and it is the vertical component
of the upwire that does most of the radiating.
I have never been clear about how the sky wave "starts off" from a
transmitting antenna such as a (vertically polarised) LF antenna. While
there is a "ground wave" that propagates across the surface of the earth
with currents physically present in the soil, it would seem the sky wave
could leave directly from the environment of the transmitting antenna.
LF DX is achieved by sky wave, so how the wave departs from the
transmitting antenna (and reciprocally is received elsewhere) as well as
the vertical radiation pattern is where the keen interest exists. This
is also where there may be a horizontal component to radiation, probably
at higher angles, but it may still do a good job for sky wave DX.
Most of the text books on LF are for broadcasting type of applications
that plan for reliable ground wave coverage (necessarily with vertical
polarisation), and largely ignore the sky wave issues. Amateur DX is
otherwise, exploiting the sky wave, so there are some interesting
differences in "how to get best results".
In New Zealand Bruce ZL1WB consistently pulls the best DX reports,
including in VK, and he uses an end fed long wire strung across a gully,
and fed with relatively low power. The wire is about 1000 metres long
and droops considerably, but always has some ground clearance. Exactly
how it "works" remains unexplained to date, but there is no denying that
it DOES work.
73, Bob ZL2CA
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