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Re: [Fwd: SUN Noise]



Maybe this is useful to someone:

An approximation for G/T at S-band is:  G/T=(Y-1)/0.635.  I believe that
is from a past G3RUH paper (pls forgive if I've hosed the attribution). 
In any event it is reproduced on AMSAT-DL under the 'links' section.
This formula assumes a value for SFI, I believe it is around the 130-135
mark.  A higher SFI increases the denominator "constant", so that a
larger sun-noise measurement would result for a given G/T.  At least,
you can use this simplified version to verify if the units you're using
in the published SETI/Ed Cole equation are correct (the denominator
should be on the order of one or so).

As to what's required, of course you can use Gene Marcus' link budget
spreadsheet to run some scenarios.  An example:  at 60,000km with
squint=0, a 4.5dB G/T is required to match the transponder noise floor. 
This would yield a maximum S/N of 18dB (28dB beacon SNR - 10dB
beacon/QSO ratio).  Included in this calculation is 13dB multi-user
margin (20 users), so with low activity quite a bit lower G/T will be
"at" the transponder noise floor.

Along those lines, G/T=-1.5dB would be required for a max. 12dB S/N,
generally the minimum value I've seen cited for intelligible SSB.  Of
course, CW can run much lower than that, even.

PS I'm preparing an article for either the AMSAT Bulletin or QEX
expounding a bit more on this subject, but I have some field
measurements to perform yet for illustration...soon I hope!   
Nick Pugh wrote:


> 
> 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
> >
> >
> > ----
> > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
> 
> ----
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