Why Earth Moon Earth - EME ?
Radio signals at frequencies above about 30MHz do not normally travel far beyond the horizon. Indeed above about 1000MHz obstructions such as hills and even trees can severely block radio waves.
Man made earth orbit satellites, acting as repeaters, provide a means of overcoming this problem but they are costly and do not last for ever. The moon can be used as a passive reflector and the technical problems that must be overcome to accomplish this have fascinated Radio Amateurs for some years now.
The reflecting surface of the moon is far from perfect and before arriving back on earth, the signals must travel some 500,000 miles. This means that EME receivers must be very sensitive for a system to be practical. At the higher frequencies, above about 5000MHz the naturally occurring (black body) radiation of the moon becomes a limiting factor and man made signals must be at a greater level than this to be heard back on earth.
The challenge of achieving Earth Moon Earth communications at ever higher frequencies gives Radio Amateurs the opportunity to push forward the frontiers of the science of communications.
See the G4NNS
10GHz EME and 24Ghz EME pages
See Also the G4NNS
Radio astronomy page