RE: [amsat-bb] Antenna noise temp

["Lars Mortensen" <labmo@xxxxxxxxxxxxx>]

Hi
May I suggest You do some experiment with a patch feed,... it is a fact
that helix ant.'s DO have some very nasty side lobes, and in Your case,
Your side lobe will pick-up a lot of noise from the ground, and ground
based equipment.
You may also be able to shift the position of Your feed slightly closer
to the primestar, and find a better compromise between gain and noise.

Try it out, good luck

73 de OZ4UI / Lars



David,

I'm not sure of the gain for the primestar dish but I would guess more
like 25 dBi.  A good tool for evaluating your system is W3PM's
spreadsheet program: 
http://www.amsat.org/amsat/ftp/software/spreadsheet/w3pm-ao40-v2.1.zip
Just enter the parameters for your station and it will show you your
system NFor Temp, Min detectable Signal (MDS), and beacon S/N.  He also
shows you graphical representation of signal strengths.

When I entered Ant Gain=25, zero for everything ahead of the amp/conv
entered as 1.7 dBNF and 25 dBG, cable loss of 3dB and receiver NF of 8
dB (typical for commercial 2m radios), range 65,000 km and squint of
10-degrees, I get a system NF=1.8 dB, MDS=-143.4 dBm, and beacon
S/N=22.8 dB.  That is not bad....the Prime star does well.

Next I introduced a preamp of NF=0.6 dB with gain=16 dB (typ of a DEM
preamp). 
Your System NF=0.6 dB, MDS= -148.5 dB, and beacon S/N=27.9 dB   ...that
is
roughly 5 dB better sensitivity!

I cannot measure your dish noise temp, but the 5-1/4 turn helix should
do a good job as a feed.

As you can see a good preamp will make a significant improvement in
sensitivity.  It will not help with man-made interference.

73 - AL7EB 


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