-----Original Message-----
From: Vic Rosenthal <rakefet@rakefet.com>
To: W8JI@contesting.com <W8JI@contesting.com>; Amps reflector
<amps@contesting.com>
To: <amps@contesting.com>
Date: 11 March 2002 09:23
Subject: Re: [Amps] Antenna vs. amp (was: al-1200 question)
>Tom Rauch wrote:
>>
>> Noise does not multiply by peak antenna gain, unless the noise is
>> in the peak response of the antenna. My web page outlines how to
>> do this, but what you do is take the ratio average gain to gain in the
>> desired direction to determine antenna system signal to noise
>> response for evenly distributed noise. If noise is concentrated in
>> one direction you compare gain in that direction to gain in the
>> signal direction.
>
>I don't disagree with this. I was trying to explain intuitively why a
directive
>antenna has a better s/n ratio than a non-directive one. I was assuming
that the
>noise was evenly distributed. Then I reasoned as follows: if the antenna
is
>directive, then the noise that comes from the direction in which the
antenna is
>pointed increases while the noise from other directions decreases.
>
>Then -- other things (i.e., what you refer to as 'average gain') being
equal --
>I would expect to hear the same noise level when switching between a full
size
>dipole and a beam at the same height. The difference is that the beam
would be
>hearing more noise from the direction in which it was pointed than the
dipole.
>However the dipole would make it up by hearing noise from other directions.
Of
>course a signal coming from the direction the beam is pointed would get
>stronger. That's why a directive antenna improves s/n ratio.
>
>> Noise power of noise evenly distributed around the antenna relates
>> only to average gain of the antenna compared to absolute gain in
>> the direction of the desired signal.
>
>This is exactly what I was trying to say. If you divide the pattern of the
>antenna into little slices, the total amount of noise presented to the
receiver
>is the result of integrating the noise level in each slice multiplied by
the
>absolute gain in each slice. So you could imagine two antennas, one that
was
>highly directive and the other not, with the same average gain. The
directive
>one would have a better s/n ratio, but both would hear the same absolute
amount
>of noise, always assuming that the noise is evenly distributed.
This is where I became confused earlier. The descriptions above seem
intuitively correct to me, but experience deviates. On a 144MHz contest
station, set up on a site with sea takeoff from E to SW, swapping 2x9 yagi
for a 50R resistor does not change the noise we hear, swapping 50R for 4x19
yagi results in a clear increase in noise, I estimate about 3dB. This
increase is constant over at least 90deg. of antenna rotation pointing out
to sea and has been there on checks months and years apart. The noise is
smooth, not electrical interference type. I think this excludes it coming
from sidelobe pickup or being a remote localised source (the antenna
beamwidth is around 25deg).
The earlier reasoning implies that we should get the same noise from both
antennas, but we don't. What are we hearing?
Steve
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