RE: [amsat-bb] Sun noise

["Tom Clark, W3IWI" <w3iwi@xxxxxxxx>]

> ----- 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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