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RE: Moon



Bill, WA2TQI said

> Has anyone tried looking at the noise from the moon using the 2.4ghz
> AO-40 setup. I can see about a half S-Unit of noise from it. Using the
> 3733 and a Myers Cast Bar B Q dish.

Not too likely. Let me walk you thru the BOTE (Back Of The Envelope)
calculations:


A 1 meter dish at 13 cm has about 8 wavelengths in the aperture. Therefore
the beamwidth (Full Width @ the -3dB level) is somewhat more than
lambda/D=1/8 radian [the normal Airy criterion would say 1.22*lambda/D, but
the Myers dish is not circular]; without doing in depth calculations, I'll
call it 10 degrees.

The moon subtends 0.5 degrees (about 1/20 of the beam in each direction), so
the moon fills ~(0.5/10)^2 = 1/400 of the beam. At all microwave
frequencies, the moon has a brightness temperature ~250K, and your dish/feed
are not perfectly efficient, so when you point at the moon, you should see
should see your system temperature (Tsys) rise by ~0.5K

Your system temperature consists of the noise made by the LNA + ohmic losses
in the feed + contributions from atmospheric absorption, plus the 3 degree K
microwave remnant of the Big Bang. The very best Tsys at 13 cm wavelength
achieved by the Deep Space Net  in the dry desert is ~10 K with cryogenic
receivers and shaped cassegrain dishes.

Assuming that an S-unit is 6 dB, the half S-unit you report would be ~3dB.
This in turn means that the 0.5K of moon noise has DOUBLED your system
temperature! Hence your measurements would indicate that you started with a
Tsys ~0.5K.


This in turn would mean that you have set 15 billion years of cosmology
deduced from studies of the Big Bang on its head by proving that the cosmic
background is much less than 3 Kelvin. And you have beaten the DSN's
performance by about 10 dB to boot!


Levity aside, if instead you had seen this performance with the sun, it
would have been plausible. The radio sun is bigger than the moon, it fills
~1/100 of the beam. The radio brightness of the sun is ~25,000-50,000K, so
could have expected 250-500K of sun noise. I would guess that this is more
like your true Tsys, so I would have expected you to see several dB of sun
noise.

73 de Tom, W3IWI


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