From G3PLX:
Dave/Brian/Jim/all..
Thanks for the feedback. I also received one from Bob VE7BS describing his
pal's results from the top of a tower block with a whip on top-band.
Brian's point about ships antennas is one I also recall seeing mentioned in
G3LNP's article about his top-fed tower. It seems that the early workers
just calculated the effective height of the antenna measured from the radio
cabin, not from the sea surface, and were surprised at how good the results
were. I think there has been a blind spot in conventional understanding of
how a small antenna fed against 'ground' works when it's sitting ontop of
something else, and it seemed to me that if we took this into account, a lot
more interesting locations might suggest themselves for LF working.
We don't need to be able to actually see and touch the conductors involved
in the process of carrying the current up and down the structure in order to
know that we are using it as an antenna. If I sit on the top of my tower
with some sort of whip or capacity hat out in clear space, and I can measure
that I am poking a useful amount of RF current into it, then I don't need to
go to great lengths to trace how this current is getting back down the tower
to ground. In particular I don't need to run a wire down the side of the
tower and worry about how to ground it at the bottom. 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. As I said in my RadCom letter, a small antenna fed 'against
ground' at a height of H metres is a vertical antenna H metres tall. This
is clearly only true if the structures in question are small compared to a
wavelength, but this is certainly going to be true for 2.2km waves.
What would be useful now is to see how easy or difficult it might be to push
a decent amount of RF current into a whip from the top of something tall.
73
Peter
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