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Re: preamps--mast mount vs. in shack



>on 11/30/00 2:45 AM, Edward R. Cole at al7eb@ptialaska.net wrote:
>
>> 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.
>
>Did I say that?  :-)

It wasn't meant as an exact quote ;-)  I was agreeing with what I thought
you and John Stephesen both said {in different manner}: that a person gets
better noise performance when their preamp is at the antenna.

>> How so?
>> Well real life never is as good as calculations on paper {and usually due
>> to faulty assumptions}!
>
>This is true!   I don't know if you can "hear" the noise more when the
>preamp is inside the shack or not.  I do know that when I tried my 2m preamp
>inside the shack, it did seem to have a higher S-meter reading for the noise
>then when I had it on the mast (before it went kaput).  This is why I was so
>surprised to see that the theoretical difference is just 1 dB.

My observation too!  But I still see a rise {though smaller} in noise when
I power up the preamp located at the antenna.  Its when trying to dig out
those weak mounbounce signals that the preamp location is everything!  Most
of my NF calcs have been for eme or radioastronomy application where tenths
of dB improvement can really make the difference.

On my four M2 crossyagi 2m eme antennas I unfortunately have about 20 feet
of phasing line before i reach the preamp.  The challenge is eliminate
every bit of loss to get the system NF down.  For calcs I still assume
about .5 dB loss before the preamp and that raises my .15 dBNF preamp to a
system NF of .79 dB.

Your noise environment really makes a big difference in performance as John
pointed out.

Just a sidenote:  When I worked with s-band recievers for NASA {Goldstone
Tracking} back in the 1970's we used cryogenically cooled masers with noise
temps of 4.3K {this was the physical temp, too} and system temp of 16K.
That meant {from memory} a signal threshold near -198 dBm {at narrow bw}
and an operating threshold of -178 dBm {5Mhz bw}.  These days cooled
GasFets are the norm.  Put that in front of a 210 foot dish and you can
really "hear"!

Ed

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