Dear Peter, LF Group,
I can assume the
> current is flowing down the tower because there's simply nowhere else it
can
> go. If a meter at the base of the whip registers 1 amp then that's 1 amp
> flowing down the whole height of the tower. I can then just measure the
> height of the tower above the surrounding terrain, derive the radiation
> resistance (using the full height, not half of it), and estimate the erp
> from that.
I don't think this is quite true. If you consider a small vertical in the
middle of a large flat roof, most of the electric flux lines emerging from
the vertical element will terminate on the "ground plane" formed by the
roof - thus most of the antenna current will be circulating between the roof
and the antenna. A small proportion of the flux will miss the roof and
extend to the "real" ground around the building, so some current will flow
from the real ground vertically up the building, but only a small
proportion. of the total antenna current. However, if the
antenna-plus-building is electrically short, and the vertical element is
small compared to the building height, this small current will make a
disproportionately large contribution to the overall radiation resistance,
so still result in a large increase in ERP.
Even if the antenna consists of a tall, thin, grounded mast with a small
vertical driven element on top, a lot of the field would exist between the
driven element and the top part of the mast, so the current would be much
higher at the top of the mast than the bottom. But some of the field would
reach ground level, so there would be a contribution from the whole mast as
before.
To optimise the LF building-antenna, you would want to arrange things so the
largest possible fraction of the antenna current flowed through the whole
height of the building, in other words so that the largest part of the
electric flux extended between the antenna element and the ground
surrounding the building. This would mean having the antenna element
sticking out as high and/or as far from the sides of the building as
possible. This is really the same objective as top-loading a monopole
antenna. So David's sloping wire transatlantic antenna is probably a good
approach - one would want to make the wires as long and high up as possible,
as usual.
I would certainly agree with Peter that the CFA, EH and their ilk are in
many cases simply acting as devices for loading up the building or mast on
which they are mounted. Because these are fairly high-Q devices, with
presumably large voltages and circulating currents, and with the square-law
relation between short antenna height and ERP for a given current, it
wouldn't take much current "escaping" from the antenna for the mast/building
to be the major contributor to the radiation. My experience some years ago
at work with HF measuring loop antennas revealed that almost any feeder
cable or preamp power lead several metres long attached to the antenna could
make 10dB or more difference to the received signal, even if the cable was
just lying on the ground. It was neccessary to add lots of clamp-on lossy
ferrite cores and take care with layout to keep this sort of effect to a
reasonable level.
Cheers, Jim Moritz
73 de M0BMU
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