The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

Ask Dr. SETI ®

Chapter 6: Technology

Dicke Switches

Dear Dr. SETI:
Do you have any information about Dicke-switching? We have been suggested to use such a switching with the object of eliminating noise generated by the system itself, but we are not sure how this would work.

EN, Norway

The Doctor Responds:
The Dicke Switch (named for its late inventor, the noted astrophysicist Robert Dicke of Princeton University) is much used in total-power radio telescopes, not so much to eliminate noise, as to eliminate the effects of receiver gain fluctuations during a measurement. The idea is that you repeatedly (and very rapidly) switch the input of the receiver between an antenna and a constant-temperature load resistor. You then look at the difference in receiver output amplitudes between the two conditions (it's rather similar to the technique used to measure receiver noise temperatures by switching between a "hot" and a "cold" load).

Dicke Switches are not much used in amateur SETI, mainly because we use other techniques (software Fast Fourier Transform analysis, for example) to differentiate between signals and noise. Nevertheless, they are quite popular in astrophysical observation. One of our suppliers of amateur SETI equipment, Radio Astronomy Supplies, provides a Dicke switch kit to use with their various radio telescope designs. You can find a link to them from our online Technical Manual.

Incidentally, Bob Dicke had quite a unique sense of humor. I once heard him deliver a lecture in which he mentioned (in passing) that the universe is 15 billion years old. Afterward, a grad student asked him how he knew that. Now Dicke could easily have given the traditional explanations, about Doppler shift of the radiation background from the Big Bang, measuring rates of expansion, calculation of the Hubble Constant, and so on. Instead, he said (with a straight face): "Our studies show that, from the beginning of time, it takes about 15 billion years for astrophysicists to evolve. I am an astrophysicist. I am here. Ergo, the universe is 15 billion years old."

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