The search for extraterrestrial radio signals


Trying to find radio signals originating from extraterrestrial civilizations presents huge technical challenges, one of which is path loss between other stars and our home planet. Neglecting the possible effects of obstacles such as dust clouds, basic geometry creates a detection problem all on its own.

In the February 2021 issue of IEEE Spectrum, we find “SETI’s Million Star Search” by Danny Price (page 34). We read therein of the technological efforts being brought to bear on the search for extraterrestrial radio signals.

In the March 2021 issue of Scientific American, there is an essay “Traveling Photons” by Joanna Thompson (page 18), which states, “A radio beam fired from the Moon to Earth ‘would typically diverge to the size of a continent’.” This statement provokes a few thoughts.

North America is approximately 3000 miles across, so we could take the illumination area of the article’s hypothesized radio beam to be circular with a 1500-mile radius, while the Moon itself is roughly 240,000 miles away. From these numbers, imagine a cone of 240,000 miles distance from its tip to a circular plane, where the radius of that circle is 1500 miles. We may then further assume that the plane of that cone receives radio signals at some power density, perhaps measured in watts per square mile.

annotated diagram of a radio path cone from the Earth to the MoonFigure 1 This cone represents the radio path from the Moon to Earth.

Project SETI (search for extraterrestrial intelligence) is looking for extraterrestrial radio signals coming from other star systems. If we construct similar cones from those star systems, each with its point at a star in question and with Earth at the center of the circular plane of that cone, we can compare the path attenuation from that far away star to the Earth versus the normalized path attenuation from the Moon to the Earth.

Table 1 Extra solar radio paths

Distance
Light Years
Distance
Miles
Radius
Miles
Illum. Area
Square Miles
Norm. Power
Atten. in dB
~1.29 light seconds 240000 1500 7.069E+06 0.00
1 5.879E+12 3.674E+10 4.241E+21 −147.78
4.3 2.52797E+13 1.580E+11 7.842E+22 −160.45
50 2.9395E+14 1.837E+12 1.060E+25 −181.76
1000 5.879E+15 3.674E+13 4.241E+27 −207.78
1.00E+06 5.879E+18 3.674E+16 4.24147E+33 −267.78
1.00E+09 5.879E+21 3.674E+19 4.24147E+39 −327.78
1.00E+10 5.879E+22 3.674E+20 4.24147E+41 −347.78

The extra solar path from Proxima Centauri, our sun’s closest interstellar neighbor, is approximately 4.3 light years. A radio beam traveling from there to the Earth would experience approximately 160 dB more signal attenuation than the beam path to the Earth from the Moon.

The so-called local star group consists of those stars within 50 light years from here, for which the attenuation worsens to approximately 180 dB versus the beam path from the Moon. At distances of 1000 light years to 10 billion light years, the attenuations worsen further.

If an extraterrestrial civilization were using radio communications as we do, their antennas would not be especially different from our own because everyone would be constrained by the same laws of physics. I’m sure they would also have phased arrays and parabolic reflectors.

Any transmissions they would likely make would not be especially targeted to reach us here. We would be searching for radio signals that just incidentally happen to come our way and such signals would experience the inferred path attenuations of Table 1.

Does the SETI project have receiver sensitivities sufficient to do signal detections via multi-hundreds of dB of interstellar path attenuations?

I honestly don’t know, but it is one tall order indeed!

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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