[TowerTalk] Why gain isn't everything.

[Jim Lux <jimlux@earthlink.net>]

A

> Then on the other hand we have antennas with easily
> documented high loss that people just love.
>
> Oh well, one of the great mysteries of radio.
>
> 73 Tom

Probably because nulls and directivity are practically speaking, more 
important than absolute gain?  I've been doing some analysis on the impact 
of doing things like pointing the beam off to the side of the desired area, 
to reduce an interfering signal on the other side, and it's interesting...

If your goal is to optimize Signal/Interference ratio, and the two sources 
are within, say, 10-15 degrees of each other (or even more), you may get a 
bigger S/I (or S/J in most of the literature, but Jamming seems 
perjorative) by pointing 20-40 degrees to the side (because it puts the 
interferer much farther down).  A lot of the patterns of the antennas I've 
been looking at are pretty broad across the nose and don't start falling 
off fast until you get well out past the 6-10 dB down point.

As a practical example (going off memory here, because the paperwork is 
elsewhere), I was looking at a signal from Boston and an interfering signal 
(of the same strength) in NYC (or DC, I can't remember) from Los 
Angles.  It's something like a 10 degree spread.  I found that if you use 
the 20m 3element model in the ARRL antenna book, your best pointing is 
something like 30 degrees to the left of Boston, to maximize S/(N+I)...

Pointing off decreases S, but also decreases I, while N remains basically 
the same.


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