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[Fwd: SUN Noise]



>From: Jim Sanford <wb4gcs@amsat.org>
>Ladies and gents:
>Seeking some info.  Finally managed to make crude, but repeatable
>sun-noise measurement on my ao-40 S-band rcv setup.
>
>Set up below:
>
>2ft dish ==>DEM .7dbnf preamp==>2ft 9913==>Drake downconverter==>50ft
>RG-6Quad sheld==>variable attenuator==>DEM 144-28==>IC-751 (agc
>off)==>HP RMS voltmeter
>
>I get about 0.5 to 1db sunnoise, either by using attenuator to match
>voltmeter or simply reading the voltmeter scale.
>
>Question:  Is this a reasonable number?  
>Qualitatively, I hear the bird fine, tho not the noise floor.
>
>Have used the spreadsheet from SETI for G/T, but it wants flux in
>Janskys, and th sources of flux data they point to provides flux in
>unknown units (or at least I didn't see it anywhere.)  Making a guess
>about the units on one site maybe being Janskys, I get a spreadsheet
>value of G/T of 0.3.  
>
>Question:  IS this a reasonable number?  What are you guys with better
>AO-40 S-band receive setups seeing?
>
>All comments, suggestions, enlightenment appreciated.

Jim,

I'm going to post my reply back to the -bb in case others are interested.

First, I took my system and ran both cascade.xls to analyze my system noise
figure (temperature), and then ran g-t.xls to both find my G/T ratio and
predict what I think my sun/cold sky Y-factor should be.  I actually did
this some months back and had the saved results:

My system [best guess]: 33-inch (85cm) offset feed dish...gain ~25.7 dB
with est 65% eff., preamp 0.6 dB NF, 35 dB gain, 4 dB coax loss to dual
Drakes thru a splitter, Drake 6 dB NF, 15 dB gain, 2.5 dB coax loss to Rx,
FT-847 14 dB NF.

Cascade.xls gives a receiver NF=.61 dB (44K noise temp).  Assuming a sky
temp of 10K and ant temp of 50K (mostly sidelobe spillover of hot ground
noise), I get a total system noise figure of 1.33 dB (104K noise temp)
using my 2.2 KHz ssb bw.  From this my MDS (S/N=0 dB) is -145 dBm at the
preamp input, or -170.7 dBm signal falling on the dish.

OK fine.  What about the G/T.

When I run the g-t.xls spreadsheet I get a predicted Y-factor of 3.0 dB of
sun over cold sky noise.  In fact that is close to what I see (about one
s-unit).  Inputing 3.0 for Y-factor gives a G/T=3.4 dB for a flux of 100
Jy.  Well, infact I believe the flux was higher on the day when I made
these measurements so my G/T woud be lower more like 2.3 dB.  This also
means that my estimate of system noise temp of 104K is probably low.  I
probably am not doing quite this well (I suspect my helix feed sidelobe
energy is much higher and dish eff is less with coresp less gain).

OK.  Now lets run the number for your system, Jim:

Your system: 2ft dish ==>DEM .7dbnf preamp (16 dB gain)==>2ft 9913 (1
dB)==>Drake downconverter (6 dBNF, 156 dB gain)==>50ft RG-6Quad sheld (2 dB
loss)==>variable attenuator==>DEM 144-28 (1.5 dBNF, 30 dB gain)==>IC-751 (agc 
off)==>HP RMS voltmeter

Cascade.xls gives a receiver NF= 1.17 dB (89.8K noise temp).  Asuming the
same sky temp (10K) and ant temp (50K), your total system noise figure is
1.7 dB (139K). MDS is -143 dBm

Running g-t.xls your antenna should have 22.2 dBi gain at 55% eff.,
predicted Y-factor is 1.4 dB and at a flux of 100 Jy that would produce a
G/T= -0.7 dB.  But you state the best Y-factor you see is 1 dB and that
produces a G/T= -2.4 dB at 100 Jy.
===================================

Note that using g-t.xls to determine G/T ratio is entirely dependent on the
accuracy of the solar flux density value inputed.  The flux number needs to
be read from one of the web sources within one hour of your sun measurement
for your measurement to be meaningful.

BTW G/T measurements are difficult to make with our narrow band width
satellite systems and simple step attenuators, so one should use it as a
benchmark of performance, making subsequent observations every couple
months will indicate if your system performance is still good or
deteriorating.

It looks to me that 20 dBi antenna gain is the minimum needed to see sun
noise, and probably some thing like >23 dBi gain is needed to make
meaningful measurements with this technique.  Anyone using a helix or
18-inch DSS dish or the like probably will not detect sun noise with their
system.

What does this mean?  To build an AO-40 2.4G downlink receiving system able
to hear the xponder noise floor probably should see at least about 3 dB sun
noise.  If you do have a couple dB sun noise then using sun noise will
allow you to determine whether modifications to your antenna or receiving
system are productive.
===================================

Jim:  You could improve your system MDS by adding 12 dB more gain to your
first preamp, but this only gives marginal improvement in MDS of ~1.4 dB.
A bigger dish would have a more observable effect.  This is why I maintain
that the min requirement for "good" reception is 24 dBi ant gain and a 1
dBNF.  The 0.5 to 1 dB Y-factor you see indicates that your ant. gain
and/or system NF are not optimum, yet.

Ed 


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