Re: [amsat-bb] Sun noise

["Lee" <lee-fl@xxxxxxxxxx>]

Ed,

Yes both approaches give the same results.  I initially misread Tom's post
and confused a ratio for a decibel.  That made me think he was getting a
smaller noise rise.

Still Learning,
Lee-KU4OS

----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: <w3iwi@toad.net>; "'Lee'" <lee-fl@cfl.rr.com>; <amsat-bb@AMSAT.Org>
Sent: Saturday, December 21, 2002 21:43 PM
Subject: RE: [amsat-bb] Sun noise


> 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
> >
> >
> >----
> >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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