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VK2ZRG > TECH 13.04.06 19:09l 48 Lines 2366 Bytes #999 (0) @ WW
BID : 2362_VK2ZRG
Read: DL1LCA GUEST
Subj: Moon noise in EME path?
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Sent: 060413/1300Z @:VK2WI.#SYD.NSW.AUS.OC #:11739 [SYDNEY] FBB7 $:2362_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : TECH@WW
VK2ZRG/TPK 1.83d Msg #:2362 Date:13-04-06 Time:12:51Z
Hello all,
I've been working on my latest programme over the last ten days or so.
It's a programme that calculates the MDS and SNR for a terrestrial radio
link or a EME path. The programme is more or less finished now; it's to
the test and refinement stage. One of things that my programme does is
to calculate a Moon noise component for a EME path, given the RX antenna
beamwidth and other system noise.
To my way of thinking, Moon noise, in Kelvins, should just be added to the
antenna noise and amplifier noise to determine the SNR of the EME circuit.
However on reading the DOC file for a BASIC programme that came with
ARRL UHF/Microwave Experimenter's Manual, I found this statement.
> If your beamwidth is narrower than the moon (0.5 degree) then,
>your antenna temperature will be about 200 kelvins. If the moon does not fill
>the entire antenna field of view, then the antenna temperature will be an
>average of the lunar temperature and the sky temperature. The sky temperature
>depends strongly on frequency, weather and elevation above the horizon.
>Lacking a better value, use 50 kelvins as the sky temperature. If, for
>example, the moon occupies one quarter of the antenna field of view, then
>the antenna temperature will be 0.25*200 + 0.75*50 = 87.5 kelvins.
The above would be true for a perfect antenna that has no side or rear
lobes, and does not receive any ground noise. i.e. The main beam of the
antenna is aimed at a cold part of the sky; but then the effective antenna
temprature would be around 3 Kelvins at 1296 MHz.
However for real world UHF antennas with lobes, most of the 50 Kelvins
referred to above, will be due to side/rear lobes, spill over from the dish
feed or leakage through the mesh of the dish, picking up noise from a warm
Earth at a temperature of 290 to 300 Kelvins. Having the Moon in the field
of view of the antenna isn't going to alter that noise pick up.
So I reckon that the Moon noise, in Kelvins, should just be added to the
antenna tenperature without any proportioning according to antenna
beamwidth as the BASIC DOC file suggests. What do you think?
73s from Ralph VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
Taglines by Colin Coker G4FCN
C Program run, C Program Crash...ReWrite in Pascal!
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