Tom, W8JI wrote:
"The person would could figure out how to use the pattern tables that are
exported by Eznec in a spreadsheet to determine S/N ratio would be a
top-band hero, because the data would express the effectiveness of an
antenna for receiving. We need directivity, not gain."
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I'm in total agreement with Tom on this. S/N ratio of the receive
antenna depends on its pattern. K6STI's programs and W7EL's programs
compute F/B and F/R ratios, but that isn't enough. If F/B was the only
thing that mattered, the Pennant with 50+ dB F/B would be the ultimate
receive antenna (and it's not)! A simple bidirectional receiving loop
antenna has no F/B at all, yet it has a better S/N than a vertical simply
because it has nulls off to the sides.
K6STI states that his AO program can be used to design an antenna to
minimize lobes to the rear and to the side, which can be quite useful in
optimizing a design for best S/N on receive, but it doesn't specifically
give a S/N number. He suggests that you just look at the pattern plotted
to get an idea of what the S/N might be.
The width of the main lobe and other lobes off to the side and rear are
all parts of the S/N calculation. The total area of the azimuth and
elevation patterns and their shapes must be taken into consideration. As
Tom mentioned, using the pattern tables produced by EZNEC provides the
needed information, but what equation is needed to use them?
K0HA posted an interesting list of S/N ratios for his antennas using his
own empirical method of calculation. He used an "omni antenna" as the
0.0 dB S/N reference. Knowing that a 1/4-wave vertical receives equally
in all
horizontal directions is one approach we can use if we compare its
azimuth plot with the azimuth plot of a low-noise receive antenna. This
ignores the elevation plots, but should be of some use.
It seems to me that to arrive at a precise number that represents the S/N
improvement of a receive antenna compared to a reference antenna
(vertical) would be to calculate the area enclosed by the receive antenna
compared to the area enclosed by the reference antenna, with both antenna
patterns in the same scale, regardless of antenna gain. This, no doubt
would require use of calculus in the algorithm, which I haven't used
since college. Any takers for "Topband Hero" award?
73, de Earl, K6SE
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