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Chapter 6: Technology

Antenna Temperature

Dear Dr. SETI:

I am attempting to warp my head around the concept of Antenna Temperature, and would like to ask if it could be simply summarized. I am curious to know is a higher or lower K better in terms of relating to the antenna performance in general for best results in the system. I understand all the variables such as radiation pattern, gain, object reflections, angles and such but simply is the result of the K value better to be higher or lower?
Casey, NE3KO

The Doctor Responds:

Short answer: the lower the K, the better.
Longer answer: K is effective antenna temperature, in Kelvins. Thus, it relates to heat. Heat in a system generates noise, and the lower the noise a weak signal has to compete with, the easier it is to detect.
Longest answer: the equation for noise power is Pn=kTB, where lowercase k (not to be confused with capital K) is Boltzman's constant (a conversion factor to make the units come out right), T is total system temperature, and B is bandwidth. Since K (antenna temperature) is one of the factors that contributes to the total system temperature T, you can see that the lower the K, the less overall system noise, and therefore the better the signal-to-noise ratio achieved with any given signal. So, low K is a good thing!
Hope this helps.

Casey continues:

Thank you for the quick reply and explanation. After all these years as a Microwave RF Engineer I never actually had much of concern for "Temperature" as all designs were short point to point terrestrial links. I am considering moving the microwave experience into the realms of satellite communications / DSN reception and researching small radio-telescopes. I always knew cooling of pre-amps was used in some practices and understood the reason behind this as electrical based heat but with the antenna side of theory was more of a weak spot for me beyond the typical to intermediate design theory.
I believe I got most of the theory behind the antenna temperature principals but was getting confused on which was was better for K. Thank you for the simplified version.
Based on the fact idea of heating is one of the bigger factors adding system noise, if a design of such would be created to physically cool the entire antenna as a whole (the whole antenna sealed in a very controlled cold environment) or even the elements of the antenna would this have much beneficial effect vs the large cost involved in this method?

and the Doctor Replies:

Physically cooling the antenna would have little advantage, because the major part of antenna temperature comes from the 290 Kelvin temperature of the Earth itself, picked up by the antenna backlobes and sidelobes.
Now, if you could just cool the whole planet...

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