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 5: Sociology

SETI in the City

Dear Dr. SETI:
I live in a rather densely populated suburban area in a city of 400,000 people. Lots of radio emissions around here, to be sure. How much interference can I anticipate? and would this interference be enough to drown out (or otherwise make indiscernable) the signals I'd be looking for?
Chris, The Suburbs

The Doctor Responds:
Radio frequency interference (rfi) is a very real problem in and near cities, Chris, but not an insurmountable one. Of all the antenna designs which have been used for SETI, the most immune to radio frequency interference is the properly aimed and illuminated parabolic reflector, or "dish".

What many of our urban members do is aim their dish straight up ("birdbath" mount). Since any direction in the sky is as likely as any other as a source of at least nearby signals, this keeps you very much in the running. (Out to about 300 parsecs [1000 light years] or so, stars are pretty uniformly distributed anyway.) Under-illuminate your dish (see this article) to minimize sidelobes. Keep your dish low (on the ground is fine; putting it on a tower exacerbates the rfi problem). Use plenty of filtering between the LNA and receiver. And hope for the best. Remember that if you can program your signal analysis software to recognize and ignore know sources of rfi, your false alarm rate goes down and detection of a valid SETI hit is facilitated. Radio frequency interference is only fatal to SETI if it convinces us not to search.

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