Radio transmissions are best because the photons are low energy (cheap) and radio signals travel unimpeded through the dust and gas of the Galaxy.

In 1959 Philip Morrison and Giuseppe Cocconi showed that there is a certain part of the radio spectrum that is ideal for interstellar transmission: around a frequency of 1420 MHz (or a wavelength of 21 cm) that corresponds to the emission line of atomic hydrogen.

Subsequently Bernard Oliver suggested that higher frequencies associated with the OH radical (up to 1721 MHz) might also be useful. If all life is based on water (H-O-H), then symbolically a civilization might broadcast between the emission frequencies of H (1420 MHz) and OH (1721 MHz). This region is often called the "water hole".

At frequencies below this range, the radio spectrum is dominated by Galactic sources, while at higher frequencies, the cosmic background radiation becomes important (see Fig. 20.2 on page 489). All communicative species in the Galaxy would recognize these same limitations and select the "water hole" for transmissions.

Signals may be transmitted over a relatively small frequency range (easier to detect), so receiver systems need to record in many finely divided channels.