Moonbounce
| Moonbounce | |
|---|---|
| Also called | EME (Earth–Moon–Earth) |
| Distance to the moon | About 239,000 miles (384,000 km) on average |
| Round-trip time | About 2.5 seconds |
| Path loss | Over 250 dB on 2 m; more on higher bands |
| Operating | |
| Popular bands | 2 m, 70 cm, 23 cm (1296 MHz) and higher microwave bands |
| Modes | Q65 and JT65 in WSJT-X; CW for larger stations |
| Antennas | Arrays of Yagis on VHF and UHF; dishes on microwaves |
| First amateur contact | 1960, on 1296 MHz |
Moonbounce, or EME (Earth–Moon–Earth), is the art of making contacts by bouncing radio signals off the moon. Any two stations that can see the moon at the same time can, in principle, work each other, even from opposite sides of the world. The catch is distance: the moon is about 239,000 miles away, so a signal travels nearly half a million miles and comes back extraordinarily weak. EME was long the domain of stations with huge antennas and maximum power, but digital modes now let surprisingly modest stations take part.
The path
The round trip takes about 2.5 seconds, so an EME station can hear its own echoes return after it stops transmitting, the classic test that everything is working. The moon is a poor reflector, sending back only about 7 percent of the energy that reaches it, and the signal spreads out over the whole distance in both directions. The total path loss is over 250 dB on the 2-meter band and grows by about 6 dB each time the frequency doubles, although the extra gain a dish of a given size has at higher frequencies more than makes up for it. Because the moon's orbit is elliptical, the loss also changes by about 2 dB between its nearest and farthest points.
Fading, Doppler and noise
EME signals suffer from effects that earthbound signals rarely meet:
- Libration fading. The moon's surface is rough, so an echo is the sum of reflections from many points. As the moon appears to rock slightly from the observer's point of view, a motion called libration, those reflections drift in and out of step, causing rapid, deep fading that is faster on higher bands.
- Faraday rotation. On its way through the ionosphere, the signal's polarization rotates by an unpredictable amount, which can make signals vanish for a while on 2 meters and 70 centimeters. Microwave stations commonly use circular polarization, which avoids the problem.
- Doppler shift. Earth's rotation and the moon's motion shift the frequency of the echo by hundreds of hertz on 2 meters and far more on the microwave bands. Software helps track it.
- Sky noise. When the moon passes in front of the Milky Way, background noise rises, especially on 2 meters.
Stations
Traditional 2-meter EME stations use arrays of four, eight or more long Yagis, steered in both direction and elevation to follow the moon, with high power and a low-noise preamplifier mounted at the antenna. On 1296 MHz and the microwave bands, parabolic dishes take over, and on the highest bands even fairly small dishes can be effective. Software calculates the moon's position and shows when it is visible to both stations at once.
Digital modes and small stations
The biggest change in EME came from the WSJT software of Joe Taylor, K1JT. JT65, designed with EME in mind, decodes signals well below the audible noise level by sending a slow, structured message over most of a minute. Its successor, Q65, copes better with the fading and frequency spreading of the moon path and is now widely used for EME on VHF and higher bands. Both run in WSJT-X, the program used for FT8 and meteor scatter.
With these modes, a single long Yagi, a few hundred watts and a preamplifier can work the large stations on 2 meters. Stations without elevation control operate at moonrise and moonset, when the moon is near the horizon and reflections from the ground in front of a horizontal antenna add several decibels of gain. Contacts between two small stations are harder but possible. Experienced operators still make many CW contacts, mostly between larger stations.
History
The US Army's Project Diana first bounced radar signals off the moon in January 1946. Amateurs heard their own moon echoes in the 1950s, and the first two-way amateur EME contact was made in July 1960 on 1296 MHz, between club stations W6HB in California and W1BU in Massachusetts.