Moonbounce is a common name for a technological practice called Earth-Moon-Earth (EME.) The process involves using the moon as a passive communication satellite, bouncing radio waves from an earth-based transmitter off the moon back to earth. Although moonbounce sounds like a science-fiction technology, it is in practice today and is a favorite trick of many amateur radio operators.
The premise of moonbounce is based on the science of electromagnetic waves, radio and otherwise. With a radio transmission, sound waves travel away from their point of origin, a person speaking into a microphone, for instance. If at some point, the waves encounter an electromagnetically reflective surface, the waves bounce back. With EME, a transmission from earth travels to space on a specific frequency, bounces off the surface of the moon, and its reflection is detected by another point on earth using the same frequency.
In 1940, Mr. W. Bray of the British General Post Office theorized that moonbounce was possible. After the end of World War II, the American military conducted EME experiments. The first successful moonbounce happened on 10 January 1946. Before the propagation of communications satellites, EME was a useful means of secure, wireless communication, including a teletype satellite link between the naval base at Pearl Harbor, Hawaii, and the navy’s Washington D.C. headquarters.
Moonbounce is of limited use because of several complicated factors. Two way conversations are inconvenient through EME because of the lag distance between the earth and the moon. The moon is nearly 250,000 miles (402,336 km) from the earth. A radio wave travels at only 186,000 (approximately 300,000 km) miles per second. For a question to reach a respondent through moonbounce there is a 2.7 second lapse between the time the questioner starts their sentence and the respondent hears it. A simple question and response takes 5.4 seconds of lag time to complete.
The surface of the moon is not a good reflector of electromagnetic waves, tending to scatter reflected energy. Because of this, advanced equipment is needed to successfully moonbounce, including a highly sensitive and high power antenna. Additionally, because the moon is irregularly surfaced and shifts slightly in place as it turns, returning waves are distorted as the signal bounces off lunar features such as craters or mountains.
Amateur radio operators have been using EME since only a few years after its inception, beginning in 1953. In order to establish an moonbouncing transmission, you will need a transmitting location that has a clear view of the moon, preferably in a location with few man-made radio transmissions. Experts recommend using a sensitive narrowband transmission receiver, a rotatable antenna and a transmitter that can produce at least 1500 watts of radio-frequency output. The user must also be sure that there are no local ordinances prohibiting the use of this equipment, and that neighbors do not mind the practice.
With time, patience, and skill, moonbouncing is an achievable goal for an amateur operator. More than 60 years after its inception, moonbouncing remains popular among radio fans. The possibility that a human being can use a clear night and the proper equipment to reach out and touch the moon with their voice seems unlikely to lose its poetic attraction.