Earth-Moon-Earth (EME), or moonbounce, uses the Moon as a passive reflector. You aim a powerful signal at it and listen for your own echo, or for another station's, a couple of seconds later. Any two stations that can both see the Moon can work each other, which on Earth means up to about 12,000 miles apart.
The path is brutal
Not to scale. Both stations need the Moon above their horizon, so they can be up to about 12,000 miles apart.
Round-trip delay2.56s
Extra path loss vs perigee+1.3dB
The Moon is roughly 384,000 km away, and the signal has to cover that twice. Free-space loss alone, Earth to Moon one way, is about 187 dB on 2 m. The Moon is a big target but a poor reflector, and the whole round trip, with 0 dBi antennas and the Moon at average distance, costs about 252 dB: your echo is about 1 part in 10²⁵ of what you sent. Compare a 100 km terrestrial VHF path, at about 116 dB.
Loss is for isotropic (0 dBi) antennas: antenna gain at both ends comes off it dB for dB. Not to scale.
Band
Delay2.56 s
Loss252.1 dB
When the Moon is closest (perigee) the path is shortest and the loss least, by about 2 dB against apogee. The loss rises with frequency, but so does the gain of a given antenna, and gain at both ends counts. At a fixed antenna size, the higher bands do better on paper; the cost is narrower beams to aim and harder equipment.
What a station needs
High gain: arrays of Yagis on 2 m and 70 cm, dishes on 23 cm and up. Doubling the antenna area (say, twice as many Yagis) adds about 3 dB at each end, so 6 dB for the link.
Power: hundreds of watts to the legal limit on the low bands, although small stations can still hear their own echo or work big stations.
A quiet receiver: a low-noise preamplifier at the antenna, and a narrow filter, because the echo is far below the noise.
Tracking: the Moon moves across the sky, and the antenna must follow it in both azimuth and elevation, usually automatically.
Operating it
Because the echo takes about 2.5 seconds, contacts cannot work like a conversation. Stations agree on a shared clock and alternate: one period you transmit, the next you listen.
Long delay, weak echo: both stations follow a clock and take turns.
Round-trip delay2.56s
The Moon's rocking (libration) smears the echo in frequency, causing a fast, flutter-like fading. On the lower bands the ionosphere can also twist the polarization (Faraday rotation), fading linear antennas in and out. The Earth's rotation gives Doppler: hundreds of hertz on 2 m and kilohertz on 23 cm, so software corrects for it. Weak-signal digital modes from WSJT-X, such as Q65 or JT65, can decode signals below the noise; strong stations can also work CW.