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TopBand: s/n

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Subject: TopBand: s/n
From: (w8ji.tom)
Date: Sat, 21 Nov 1998 22:24:43 -0500
Hi Bob,

> Whatever the numbers are, do you reduce noise more with better f/b or
with a
> narrower BW?  

Depends on where the noise is coming from Bob.

If the noise primarily comes from one direction, you want maximum
signal-direction to noise-direction ratio in the pattern. If the noise is
evenly distributed, you want maximum directive gain (as opposed to forward
gain). If the noise is a combo of the two, you need a combo that is between
the two.

Let's say during operation there is a severe thunderstorm off the back of
the antenna, propagating in at a wave angle of 20 degrees. You'd want the
deepest possible null at that angle and direction. If the null was 30 dB
below the response in the desired signal direction with an antenna with 3
dB of directive gain, you'd improve S/N 30 dB. 

If the noise was evenly distributed, that null would barely help. The
improvement would be 3 dB.

If the noise came only from the direction and angle of the signal, nothing
would help.

> Like most topband topics I'm sure there may not be an easy answer - noise
> not that simple.  Also another factor is reduction of signal hash from
> strong stations/signals.  One advantage of the broadside array is that
> is no critical phasing involved. 

One advantage of the end-fire array is, for a given array size, directive
gain can be made higher than any other array.

Since the goal is both directive gain and deepest null (in unknown random
directions), the ideal pattern would be a very clean pattern with a narrow
lobe and little or no spurious responses.  That would be a
broadside-endfire combo or an endfire array with perfectly controlled
distribution, with maximum directive gain without side lobes.

The desired receive pattern is often different than the desired transmit
pattern, which is just maximum forward gain in the right place. Which is
why the best S/N ratio antenna is often not the best TX antenna, or vice

73 Tom

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