Field Guide · term

Also known as: moonbounce, EME, earth-moon-earth

Moonbounce, or Earth-Moon-Earth (EME), is a propagation mode that uses the Moon as a passive reflector: a station aims a radio wave at the Moon, and the faint echo scattered back reaches any other station that can also see it, linking points anywhere on the Moon-facing hemisphere.1 Because the round trip is about 770,000 km, EME must overcome roughly 250 dB of free-space path loss, making it the most demanding “natural reflector” mode in amateur and experimental radio.

Moon station A station B uplink weak echo
The Moon scatters a tiny fraction of the incident energy back toward earth; the returning echo is roughly 250 dB weaker than the transmitted signal.

How it works

The Moon is a poor, rough reflector — only a few percent of the energy that hits it scatters back, and it scatters diffusely rather than mirror-like. The dominant challenge is distance: free-space path loss grows with the square of range and frequency, and over the Earth-Moon-Earth round trip it totals about 250 dB. Closing that link is a brute-force exercise in the link budget:

  • Big antennas. Stations use high-gain arrays or parabolic dishes with as much antenna gain as they can steer and track across the sky.
  • Power and low noise. Legal-limit power amplifiers on transmit, and very low noise figure preamplifiers on receive, since the echo can be at or below the noise floor.
  • Weak-signal coding. Modern EME leans on error-correcting digital modes that dig signals out of the noise, replacing the extreme operator skill once needed for CW.

The path also imposes its own distortions. Doppler shift from the relative motion of earth and Moon offsets the echo frequency by tens to hundreds of hertz and must be tracked. Libration fading — a slow flutter caused by the Moon’s apparent rocking, which makes different parts of its rough surface add and cancel — modulates the signal over seconds. And Faraday rotation in the ionosphere twists the polarization, so a signal that left vertical can arrive horizontal and momentarily vanish on a fixed antenna.

In practice

EME is primarily an amateur-radio and experimental pursuit on the VHF, UHF, and microwave bands, prized as the ultimate long-haul path: a single Moonbounce contact can span half the planet with no infrastructure between the two stations. Digital weak-signal software has widened access enormously — modes designed for sub-noise decoding let modest stations make contacts that once required enormous arrays. The Moon was also an early passive relay in military and research communications before active satellites existed.

Relevance to SDR

EME is a natural fit for software-defined radio: narrow, weak echoes buried in noise, with a Doppler offset that shifts over a pass, are exactly what SDR-based weak-signal decoders handle well, and a waterfall display makes the faint trace visible. It has no connection to trunked land-mobile radio, so GopherTrunk neither targets nor encounters it. EME earns a place in an SDR reference as the extreme case of the same path-loss and link-budget arithmetic that governs every RF link — the demonstration that with enough antenna, power, and coding, even a 250 dB path can be closed.

Sources

  1. Earth–Moon–Earth communication — Wikipedia, on lunar reflection, ~250 dB path loss, libration fading, Doppler, and Faraday rotation. ↩

See also