> I'm told, by many people who know a zillion times more
about this stuff
> than I do, that increasing antenna gain doesn't improve
received SNR
> under the following conditions:
> The received noise is greater than the internal noise of
the receiver
> The noise has a uniform spatial distribution
> i.e. no stronger in one direction than any other
>
> This doesn't seem to agree with my experience so I
conducted the
> following thought experiment.
That explanation still misses a few things in cases where
external noise sets the noise floor (the normal HF case).
1.) If the noise is EVENLY distributed, S/N improves by the
directivity change.
2.) If the noise comes from one direction or a few
directions, the ratio of response in those direction(s) to
response in the signal direction and angle determines S/N
ratio up to the limit where the noise in other directions
finally dominates.
3.) If the noise all comes from the same direction and has
the same polarization as the signal, you can't do anything
to improve S/N.
In a city, for example, a VHF 1/4 wl GP can have about the
same S/N as a collinear vertical because noise comes from
the horizon, the same elevation and directions as desired
signals! A omni-direction collinear increases gain in the
direction of noise as much as it does signal level. We had
that experience with VHF systems in urban areas.
I have some stuff related to this on my web site. I
initially put it there because people would claim changing a
feedline over to lower loss line would help them hear DX on
HF, and people would claim two Beverages close-spaced in a
broadside configuration would improve signal to noise. (If
I add a second Beverage, gain goes up at least 3dB almost
regardless of spacing. It is only at wide spacings (over 1/2
wl) pattern changes and S/N can change significantly!)
One thing for sure, without a pattern change there cannot be
a S/N change. The exception would be if you are using
something like a Knight Kit Star Roamer on ten meters.
I wrote QST about this after they published an article where
the author assigned a figure of merit to antenna gain for
receiving based on gain, but they never really responded.
IMO, the idea transmitting gain pays an equal dividend on
receiving S/N is a very common myth that deserves to be
corrected. Pay attention, and you will see that myth
repeated over and over again.
73 Tom
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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