Re: [amsat-bb] Antenna noise temp

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

At 11:37 PM 11/2/2003 -0600, DC wrote:
>Now that I've gotten my primestar based AO-40 recieve setup on the air 
>I'm trying to tweak it a little.  Currently I have a 5.25 turn helix 
>mounted on a oval primestar dish feeding a 1.7db nf California Amplifier 
>based downconverter.  According to W0LMD (ultimatecharger.com), the gain 
>of the dish is about 27db (this seems a little optimistic).  Signals 
>seem okay to me (I've never heard AO-40 on any thing but my own 
>equipment) but SSB voice seems more difficult to copy than in recordings 
>I heard.  The noise figure of my downconverter is a little higher than 
>most so I was thinking a 2.4Ghz preamp might help things along a 
>little.  Firstly though, my antenna is located in a high noise 
>environment, downtown Houston, and my feed currently points towards the 
>ground due to the offset feed angle.  This combined with 
>over-illumination resulting from the oval shape of the dish leads me to 
>believe that I have a relatively high antenna noise temperature.  I can 
>hear 2.4Ghz spread spectrum cordless phones very well.  It is my 
>understanding that a preamp won't help me as much in this situation 
>because the noise will be amplified along with my signal.  What steps 
>could I take to reduce the noise temperature of my dish?  Also what is 
>the easiest way to measure my current S/N ratio and if possible my 
>antenna noise temp?
>
>73s
>David Carr
>KD5QGR

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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