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Re: masthead preamp question

At 11:34 PM 5/3/2005 +0000, John B. Stephensen wrote:
>It won't make a difference at 144 MHz where the sky temperature is over 600
>At 432 MHz there will be at least another 0.1 dB loss in the connectors so
>the loss of 1 dB is about 75 K. Sky temperature is also about 75 K at that
>frequency. A 1 dB NF preamp adds 75 K so the system noise temperature will
>be about 150 K with the preamp at the antenna or about 225 K with the preamp
>at the receiver. You get 1 dB extra SNR by mounting an inexpensive preamp at
>the antenna. The very best preamps are 0.4 dB NF or 30 K so you could get a
>2 dB improvement.

Since Jack is running LMR-600 he will not see a lot of improvement on 144
by placing the preamp at the antenna, but John I will argue with your
comment of 600K sky temp.  Since this is a satellite station, Jack will see
lower sky temps when elevated above the horizon.  I operate 144-eme and sky
temp can be as low as 250K when looking at colder parts of the sky.  When
operating with the antenna on the horizon you pick up 150K more ground
noise, so it is less important for terrestrial work.

It makes a difference when you are trying to dig out that last tenth of a
dB of a weak signal.  With local strong signals it will not matter.  I will
admit running with an internal preamp for a short time and it works pretty
good even off the Moon...but it still is better at the antenna!  

Certainly as you increase frequency the sky noise drops a lot making much
more difference where the preamp is located.  Remember that every dB of
loss ahead of the preamp is directly added to your system NF.  Line loss of
1dB plus preamp of 0.5 dBNF = 1.5 dB system NF.  <this is for jack and
others benefit, as I know John is aware of this> 
Ed - KL7UW  <new call>
144-MHz EME - BP40iq:
FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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