Topband: RDF in the real-world
carlluetzelschwab at gmail.com
Fri Mar 4 09:07:03 EST 2016
Rick N6RK said:
> In this webinar, it was asserted (without explanation) that
> for every 1 dB increase in RDF, you get 1.5 to 2.0 dB
> improvement in S/N ratio. I've never heard that before
> and don't even see how it makes sense. Actually, I don't
> even know how you can make generalizations like that
> unless you are describing a theoretical QTH with uniform
> isotropic noise. I'd like to believe this is true.
> Can someone educate me as to why I should believe this?
I believe the issue is that RDF is a theoretical calculation comparing the
main lobe gain to the overall average gain. The overall average
gain essentially results in the assumption that noise is coming in
uniformly from all azimuth angles and elevation angles. But in the
real-world man-made noise doesn't abide by this assumption, and neither
does atmospheric noise. As for atmospheric noise, there was an interesting
article about the directional characteristics of atmospheric noise in Radio
Science in 2002 (by Prof Chris Coleman, who is a VK5 but I don't remember
his call at the moment). Chris even did a plot for me of atmospheric noise
coming into W4ZV's QTH on a winter morning, and it was obvious that
antennas with the same (or very similar) RDF could provide different SNR
improvements depending on where the nulls in the pattern were.
I can't vouch for JC's numbers (his numbers may be QTH specific), but the
concept is believable since the theoretical assumption of isotropic noise
falls apart in the real-world. My BOG *at times* gives much more of an SNR
improvement than the SAL-20 (using measurements on a calibrated S-meter) in
spite of the small difference in RDF between the BOG and SAL-20.
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