Ask Dr. SETI ®

Dear Dr. SETI:
Dry ice has recently become readily available at local supermarkets in my area. Dry ice evaporates (sublimes) at - 109 F or 195 K so would it be useful in radio astronomy for cooling the receiving pre-amplifier or the entire feedpoint container ?? (Keep in mind that amateurs do not have a big budget for cryogenics and dry ice is about one U.S. dollar per pound at retail.) How does physical temperature of the pre-amplifier figure in to overall noise temperature for the system ?
Warren C., SETI League member

The Doctor Responds:
Yes, thermal cooling of the Low-Noise Amplifier (LNA) will indeed improve your system performance, but not as dramatically as you might expect. First off, all else being equal the total system noise temperature (in Kelvins, or degrees above absolute zero) must be reduced by a factor of four in order to double communications range. A good GaAs PHEMT preamp may have a 50K noise temp at room ambient temperature; frozen carbon dioxide (dry ice) cooling might lower preamp noise by perhaps 35%. [The exact result is: (50K * 195K / 290K) = 33.6K].

OK, so that is indeed a slight improvement. But now the problem becomes the rest of the system -- antenna noise temperature might be another 50K or so, mostly dominated by sidelobes. And sky temperature (which varies with where you're pointing) may add, say, another 10K or so. So unless you dry-ice the Earth, your system noise goes down only from 110K to maybe 94K, a 0.7 dB improvement in sensitivity (which translates to a 8% increase in range). Well, every light year counts, so don't let me discourage you from trying dry ice! (Liquid Nitrogen would be even better, with something like 77K evaporation temperature at standard pressure. In the prior example, this would lower LNA noise to 13K, system noise to 73K, and improve our range by about 23%!)

Here is a spreadsheet to help you with such computations.

Other cooling options also come to mind. Some of our members have successfully employed solid state Peltier Effect cooling devices to lower the ambient temperature of their preamplifiers. Like cryogenic cooling, these devices can do nothing to reduce antenna noise temperature, sky temperature, or Earth-temperature sidelobe pickup.