Ask Dr. SETI ®
Say I have an antenna consisting of a parabolic dish reflector, which is pointed away from the earth. Imagine I have a low noise amplifier, say 30K, and the signals from space are reflected from the parabolic dish into a feedhorn, and then into the preamp and receiver. Why doesn't my receiver see the 273 K temperature of the reflector, instead of the low temperature in space (plus atmospheric noise)?
David (via email)
The Doctor Responds:
A properly designed, well fed parabolic reflector acts as a thermal mirror, rather than a thermal blackbody. Let's answer by way of an analogy involving colors. Imagine a perfect, flat optical mirror that's shiny silver in color, lying on the green grass, and pointing up at blue sky. Now, look into the mirror, and what color do you see? Blue sky reflected back to you, of course, not the green of the grass or the silver of the surface.
Perfectly illuminated parabolic reflectors do something similar to color temperatures. Only, they're not ideal reflectors (because their surfaces are imperfect), nor are they perfectly illuminated. So, the system temperature you actually see is a combination of mostly sky temperature, a little noise contribution from the reflector surface (due to its imprecision), and a little noise contribution from the Earth (due to spillover, or over-illumination). Both the ground and the dish temperature contributions are (as you might expect) worse for a mesh dish than they are for a solid one. But, unless you've done a miserable job with your reflector or feed designs, sky noise will clearly dominate.
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