

Re: Receiver Noise temperature

At 02:22 PM 12/22/98 , Bob Bruninga wrote:
>If I have an omni directional dipole at 36,000 km
>above the earth surrounded on all sides by the 4 degree coldness of space
>and am listening to an earth bound weak transmitter sitting on a 300
>degree kelvin planet earth.  Is the Signal to Thermal noise ratio that I
>hear from him relative to the 300 deg or the 4 deg?

Neither.  The total noise your antenna receives is the WEIGHTED average of 
the noise in all directions.  Take the fraction of solid angle that 
contains earth, and multiply it by the temperature of earth.  Take the 
remaining fraction of solid angle pointing toward cold sky, and multiply by 
temperature of sky.  Add.  (More precisely, one should integrate over the 
solid angle, multiplying the noise temperature of the object by the antenna 
gain in the directon in question at each direction.)

Actually, the picture is more complicated than just earth and cold sky, 
because there are other hot things in your field of view besides the 
earth.  The sun comes to mind.

Also note that temperature of sky is not 4 degrees.  It depends on 
frequency and direction.  Center of the galaxy is quite hot, for 
example.  Maps of radio sky temperature are available in many reference books.
