EME communication has become a popular form of amateur space communication. The concept is simple; the moon is used as a passive reflector for two-way communication between two locations on earth. The adoption of modern coding and moulation techniques has made this mode of communication much less demanding than it was just 10 years ago.
EME is a natural and passive propagation phenomenon, and EME QSOs count toward WAC, WAS, DXCC and VUCC awards. EME opens up the bands at VHF and above to a new frontier of worldwide DX .
Professional demonstrations of EME capability were accomplished shortly after WW II. Amateurs were not far behind, with successful reception of EME echoes in 1953 and pioneering two-way contacts made on the 1296, 144 and 432 MHz bands in the 1960s. Increased EME activity and advances to other bands came in the 1970s, aided by the availability of reliable low-noise semiconductor devices and significant improvements in the design of Yagi arrays and feed antennas for parabolic dishes. These trends accelerated further in the 1980s with the advent of low-noise GaAsFET and HEMT preamplifiers and computer-aided antenna designs, and again after 2000 with the introduction of digital techniques. EME QSOs have been made on all amateur bands from 28 MHz to 47 GHz. Many operators have made WAC, WAS and even DXCC on one or more of the VHF and UHF bands.
EME is now within the grasp of most seriousVHF and UHF operators
Path loss in free space is caused by nothing more than the spherical expansion of a radiowave as it propagates away from the antenna.Radiowaves incident on the surface of the moon are often said to be “reflected,” although in fact they are partly absorbed and partly scattered by the irregular lunar surface.
Reflection from a smooth surface preserves linear polarization and reverses the sense of circular polarization. At shorter wavelengths the lunar surface appears increasingly rough,so reflections at 10 GHz and above contain a significant diffuse component as well as a quasi-specular component. The diffuse component is depolarized, and significant portions of it arise from regions farther out toward the lunar rim. The median time spread can then be as much as several milliseconds. In all practical cases, however, time spreading is small enough that it does not cause significant smearing of CW keying or intersymbol interference in the slowly keyed modulations commonly used for digital EME. Time spreading does have one very significant effect. Signal components reflected from different parts of the lunar surface travel different distances and arrive at Earth with random phase relationships. As the relative geeometry of the transmitting station, receiving station and reflecting lunar surface changes signal components may sometimes add and sometimes cancel, creating large amplitude fluctations. Often referred to as liberation fading, these amplitude variations will be well correlated oveer a coherence bandwidth of a few kHZ, the inverse of the time spread.
EME signals are also affected by Doppler shifts caused by the relative motions of Earth and moon. Received frequencies may be higher or lower than those transmitted; the shift is proportional to frequency and to the rate of change of total path length from transmitter to receiver. The velocities in question are usually dominated by the Earth’s rotation, which atthe equator amounts to about 460 m/s. For the self-echo or “radar” path, frequency shift will be maximum and positive at moonrise, falling through zero as the moon crosses the local meridian (north-south line) and a maximum negative value at moonset.
The basic circumstances described so far ensure that frequencies from 100 MHz to 10 GHz are the optimum choices for EME and space communication. Over this region and a bit beyond, a wide variety of propagation effects and equipment requirements provide a fascinating array of challenges and opportunities for the EME enthusiast. The enormous path-loss variability encountered in terrestrial HF and VHF propagation does not occur in EME work, and some of the remaining, smaller variations — for example, those arising from changing lunar distance and different sky background temperatures — are predictable.We can therefore estimate with some confidence the minimum antenna sizes and transmitter powers required for EME communication on each amateur band.
Perhaps you already have a weak-signal VHF or UHF station and would like to make a few EME contacts before possibly undertaking the task of assembling a “real” EME station.On 144 and 432 MHz — probably the easiest EME bands on which to get started — you can visually point your single long Yagi at the rising or setting moon and work some of the larger EME stations. Your daily newspaper probably lists the approximate times of moon rise and moon set in your vicinity; many simple web-based calculators can give you that information as well as the moon’s azimuth and elevation at any particular time. These aids may be all you need to make your first EME contacts, especially if you take advantage of the weak-signal capabilities of an efficient digital mode. To optimize your chances of success, adapt your operating procedures to the prevailing standards that other EME operators will be using. For your first attempts you may want to make prearranged schedules with some established stations.
More detailed information is available on the web, just google moonbounce or EME.