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Re: [Fwd: SUN Noise]



Hi all

I am using a 2' bbq dish and a dem lnb 2400-144.  the delta noise cold sky
to sun is 2 db

The SETTI spread sheet call for an input of Jy but I think it relay is SFU
which differ from Jy by a factor of 10,000.  Typical the SFU vary from 130
to 160 .  My source is gopher://solar.sec.noaa.gov/00/latest/curind   The
real question I have that measuring sun noise how much snr should we see on
the beacon at a range of 50000 km and squint angle < 20 deg???

nick k5qxj



----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, October 21, 2001 3:13 PM
Subject: [amsat-bb] [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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