Figure 27-12: The Water Hole This graph shows the background noise level from the sky at various radio and microwave frequencies. The so-called water hole is a range of radio frequencies from about 103 to 104 megahertz (MHz) in which there is little noise and little absorption by Earth’s atmosphere. Some scientists suggest that this noise-free region would be well suited for interstellar communication. Within the water hole itself, the principal source of noise is the afterglow of the Big Bang, called the cosmic microwave background. To put this graph in perspective, a frequency of 100 MHz corresponds to “100” on a FM radio, and 103 MHz is a frequency used for various types of radar.
(Adapted from C. Sagan and F. Drake)