Question: Hi. I’m doing some research for a piece of fiction and was wondering: if you had a radio transmitter/receiver aimed at a far star 1) what kind of equipment would one need and 2) how would the equipment’s behaviour vary from a conventional satellite receiver? I know it’s not quite an astronomy question and more a question of radio astronomy technology, but I do hope you might be able to help. Any information would be very much appreciated. — Steve
Answer: As a prelude to the answer to this question, let me dispel a myth about the detectability of Earth’s radio transmissions. Even though it is true that many of our radio and television broadcasts are “leaking” into space at the speed of light, those signals are likely too weak to be detectable by any of the inhabitants of planets orbiting even the most nearby stars. This is due to the fact that the strength of a light signal, such as a radio broadcast signal, decreases as the inverse-square of the distance that it travels from its source.
To illustrate this inverse-square law let us assume that we transmit a radio signal with a power of one megawatt (1 MW) into space. For reference, the Arecibo Observatory has a radar system that it uses to map the surfaces of the Moon and nearby planets that transmits a signal power of about 1 MW at a frequency of about 2.4 GHz. Let us further assume that the signal is transmitted at a frequency that can penetrate the Earth’s atmosphere and travels unimpeded through space. This 1 MW signal would be just barely detectable at the nearest star, Proxima Centauri, which is at a distance of 4.243 light years. Furthermore, keep in mind that this distant civilization would have to be staring at Earth with its receiving system tuned to exactly the same frequency that we are transmitting. Therefore, it takes a rather power transmitter to send a detectable radio signal to the nearest star. This sort of equipment is far more sophisticated than your average home satellite signal receiving system.
Furthermore, as you can probably imagine, this scenario has been investigated by quite a few researchers. There are many factors that affect the detectability of radio signals emitted or transmitted from the Earth, complicating the rather simplistic scenario discussed above. For a nice summary of these investigations see the io9 article on the pros and cons of sending beacon signals into space.