James Moritz wrote:
snip
The main motive for this expedition is to do some back-to-back
comparisons between a big antenna (the 100m Decca mast) and a
small, amateur-type antenna (an inverted L about 9m high and 50m
long). The idea is to set the antenna currents so that the same
effective radiated power should be obtained from both antennas,
and then see how signal strengths compare.
There has been some vigorous debate lately about the advantages
and disadvantages of big and small antennas, with some holding
the view that big antennas have superior radiation patterns to small
ones, and some holding the opposite is true. The aim of the
experiment is to put this to a practical test. We did this before
during the previous trip to Puckeridge, but due to time pressures
and the dreadful weather, relatively little operating was done using
the two antenna setup. The results then were that the two
antennas gave roughly the same results, when transmitter power
was set for same ERP from each antenna (this meant feeding
about 500W into the small antenna, and less than 0.5W into the
main mast!).
I think it was also raised before, but for doing such a test you should
have a convincing way of showing that mutual coupling to the big mast is
not swamping the result of transmitting from the small antenna.
A simple additional experiment could be to transmit on the small
antenna, with the big mast "floating", and then short the base of the
big mast to ground, and see what happens to far field readings.
73, Bob ZL2CA
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