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Re: preamps--mast mount vs. in shack



>Mark L. Hammond  [N8MH] wrote:
>Assume I put a preamp with the following specs in the shack vs. on the mast 
>(at the antenna).
>
>Frequency 		144-148 MHz
>Noise factor(dB)	0.5
>Gain(dB)		24

Mark,

Finally got to your question {getting caught up to >160 mail/day from
amsat-bb}:

I'm going to run the numbers for the two examples you gave, then point you
to a program that you can use to play "what-if".

Case-I: placing your preamp at the antenna:

I assumed:
a receiver bandwidth of 2.1 KHz for ssb 
receiver noise figure of 6 dB {actually most multi-mode 2m rigs are worse!}
line loss 1.2 dB
sky temperature of 200K {or worse}
ant temperature of 251K {depends on antenna design}
your system noise figure becomes 0.56 dB and your signal level threshold
will be approx. -138.5 dBm.

Case-II: place the preamp at the receiver:
your system noise figure becomes 1.75 dB and your signal threshold will now
be -137.6 dBm

The reason the poorer NF doesn't affect your system sensitivity more is
that at 144 MHz you are looking at rather warm sky.  In other words it
doesn't matter much {about 1 dB degradation}.

At microwave frequencies the sky is about 20K and antenna temp maybe about
the same.  Your signal threshold now becomes -164.2 dBm vs. -161.3 dBm
depending on where you put the preamp.   Notice this is almost a 3 dB
degradation in your receiving ability if you don't place the preamp at the
antenna.  Also the overall ability to hear is over 25 dB better than 2m
because of the quieter sky!  {discounting path loss change with freq.}

Now this was not a fair comparison since I used the same 1.2 dB line loss
at both frequencies.  Actually 75 feet of 9913 at 1296 MHz has about 5.0 dB
loss and your 1296 NF would be ~5.5 dB, signal threshold -136.2 dBm with
the preamp at the receiver.   {for microwave receivers you have no choice
but to place the preamp at the antenna}

Now this all being said, I notice a definite improvement by placing my 2m
GasFet preamp at the antenna.  Here's a program that will give you system
NF.  Its called T-Calc and is at:
http://www.setileague.org/software/spreadsh.htm

Ed
PS: Now that I have spent the last two hours wading thru all the amsat-bb
mail, I see Jon Ogden and John Stephensen have covered the above.  I agree
with Jon that you can hear the difference when the preamp is at the antenna
even if the numbers say it may not be a significant difference.  How so?
Well real life never is as good as calculations on paper {and usually due
to faulty assumptions}!


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