A large radio telescope built in a lunar crater on the "other side" of the moon would be much better than any terrestrial telescope. Even better, an array of antennas spread across a wide distance and use interferometry techniques would be better than any one antenna.

As for the frequencies to listen to, the radio astronomers know which signals are best to travel through interstellar space with the lowest background noise levels and probability of detection. One commonly mentioned frequency is 1.420 GHz, known as the Watering hole in the SETI community.

* Modern SETI efforts began with a paper written by physicists Guiseppe Cocconi and Philip Morrison and published in the science press in 1959. Cocconi and Morrison suggested that the microwave frequencies between 1 and 10 gigahertz would be best suited for interstellar communications. Below 1 gigahertz, "synchrotron radiation" emitted from electrons moving in galactic magnetic fields tends to drown out other radio sources. Above 10 gigahertz, radio noise from water and oxygen atoms in our atmosphere tends to also become a source of interference. Even if alien worlds have substantially different atmospheres, quantum noise effects make it difficult to build a receiver that can pick up signals above 100 gigahertz.

The low end of this "microwave window" is particularly attractive for communications, because it is in general easier to generate and receive signals at lower frequencies. Lower frequencies are also desireable because of the "Doppler shifting" of a narrow-band signal due to planetary motions.

Doppler shifting is a change in the frequency of a signal due to the motion of the source of that signal. If the source is approaching, the signal will be shifted up in frequency, while if the source is moving away, the signal will be shifted down in frequency. The rotation of a planet and its orbit around a star causes a Doppler shift in the frequency of any signal generated from that planet, and over the course of a day the signal can drift in frequency far out of its intended bandwidth. The problem gets worse with higher frequencies, and so lower frequencies are preferred.

Cocconi and Morrison suggested that the frequency of 1.420 gigahertz was particularly interesting. This is the frequency emitted by neutral hydrogen. Radio astronomers often search the sky on this frequency to map the great hydrogen clouds in our Galaxy. Transmitting a communications signal near this "marker" frequency would improve the chances of its detection by accident. This frequency is sometimes called the "watering hole" by SETI enthusiasts.