RE: [amsat-bb] Sun noise

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

Tom et al:

The calculations made by Dick Flagg on the SETI-League site (I merely
implemented it on a spreadsheet) are directly derived from Kraus, Radio
Astronomy.  The correction factor apparently is empirically arrived at
(from experience).  Most hams won't need the correction factor unless using
an eme-sized dish.  I would hope that the two approaches arrive at similar
results.  I am just a couple hours from getting on a plane so can't take a
look right now.

Ed - AL7EB

At 06:13 PM 12/21/2002 -0500, Tom Clark, W3IWI wrote:
>> ----- Original Message -----
>> From: "Tom Clark, W3IWI" <w3iwi@toad.net>
>> To: "'Eric Van Offelen'" <eric@vanoffelen.com>; "'Amsat-bb'"
>> <amsat-bb@AMSAT.Org>
>> Sent: Saturday, December 21, 2002 14:15 PM
>> Subject: RE: [amsat-bb] Sun noise
>>
>>
>> [snip]
>> >
>> > The radio sun at 13 cm, depending on sunspot count, is
>> about 40,000 Kelvin
>> > and is about 1 degree in diameter. Therefore the sun fills
>> about (1/10)^2
>> or
>> > ~1% of the beam. Therefore, the sun should contribute about
>> (1/100)*40000
>> =
>> > 400 degrees Kelvin if you dish has perfect (100%) aperture
>> efficiency.
>>
>> Tom,
>>
>> I was under the impression that the antenna beam width
>> correction was to
>> compensate for very narrow beam widths in which some of the
>> sun's energy
>> falls outside the view of the dish. The formula I've been
>> using is from the
>> SetiLeague and seems to match my measurements on both the 60cm
>> and 90cm dish.
>> Their web site  which is what I've been using for my g/t
>> measurements is
>> http://www.setileague.org/articles/g-t.htm
>
>For this case, the radio sun is small compared with the beam of the antenna
>(Visible sun = 1/2 degree, radio sun a bit under 1 degree while beam is 10
>degrees). Therefore only ~1% of the beam hits the sun and 99% of the beam
>sees cold sky. Taking the sun as 40,000K and cold sky as 10K, then the total
>noise contribution is (0.01*sun = 400K) and (0.99*sky = 10K). Then I added a
>fudge factor called aperture efficiency, saying that not all the power is
>squeezed from the main beam -- imperfection in dish illumination, sidelobes,
>aperture blockage, roughness in the dish's surface, losses from being out of
>focus, etc. Based on experience, I guessed at an aperture efficiency of 50%,
>which then took the sun contribution down to about 200K.
>
>The empirical correction that the SetiLeague discusses is for the case of
>larger dishes. When the size of the antenna beam gets comparable to size of
>the source, then such corrections are needed.
>
>Let me assume that the sun is a uniformly bright disk -- In the limit of an
>antenna with a beam much smaller than the source (like a 10M dish at X-band
>looking at the sun), then making more gain (like replacing the 10M dish with
>a 20M dish) will make no change in the sun noise you measure. You will
>measure the true brightness of the disk -- like 40,000K. If you had a ~0.5
>dB NF rcvr, the sun would yield ~30 dB of excess noise.
>
>So for a 60-90 cm dish, the SetiLeague "rule" would say to set L=1. I
>concur. Except for the fact that I chose to use temperature units and I
>didn't give the final equation for G/T, I don't see any difference between
>what they said and my "let's learn from first principles" tutorial.
>
>Tom
>
>
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