Bouncing One Off The Moon

The Concept Behind Earth-Moon-Earth Communications

By Jonathan Imberi

Stations that can simultaneously see the Moon can communicate by reflecting VHF or UHF signals off the Moon's surface. This is called Earth-Moon-Earth (EME) or Moonbounce Communications. Stations communicating through EME can be separated by 11,000 miles on the Earth's surface. 

There is one drawback to this communications method. The Moon's average distance from the Earth is 239,000 miles, and EME signals must travel that distance twice, to the Moon and then back to Earth. These path losses are huge when compared to local VHF paths. Path loss refers to the total signal loss between the transmitting and receiving stations when compared to the total radiated signal energy. In addition to the long distance to the Moon and back, the Moon's surface is also irregular and not a very efficient reflector of radio waves. 

A typical EME station uses high-gain antennas and high-power amplifiers. A high-gain array of Yagi antennas would be a good choice for a moonbounce station. You would not even want to try moonbounce with a simple ground-plain antenna, no matter how transmitter power there is. 

You might think that you would have to take the Doppler effect into account for EME communications, just as you do with satellites. But since the Moon really doesn't move very fast relative to the Earth, you don't need to the concerned with the Doppler effect. Consider that a typical satellite or a space station, like a shuttle for example, goes from horizon to horizon in ten to twenty minutes, while it takes the Moon many hours to make a similar path. Although the Moon is actually speeding through space, it is moving very slowly compared to the Earth. 

Choosing An EME Band 

50 MHz EME is quite a challenge since the required antenna arrays are very large. Sky noise also limits receiver sensitivity at this frequency. Because of power and licensing restrictions, it is not likely that many foreign countries will be able to get on 50 MHz EME. 

144 MHz is probably the easiest EME band to begin on because it supports the largest number of EME operators. Commercial equipment is widely available and a 144 MHz EME station can almost be completely assembled from common equipment. 

222 MHz is a good frequency for EME, but there are only a few active stations. 222 MHz is available only in ITU region 2. 

432 MHz is the second most active EME band. Libration fading is more of a problem than at 144 MHz, but sky noise is much less than on 144 MHz. The improved receive signal-to-noise ratio may more than make up for the more rapid fading. 

902 MHz and above should be considered if you enjoy experimenting and building equipment. If you plan to operate at these frequencies, an unobstructed moon window is a must. The antenna used is almost certain to be a dish. 902 MHz has the same problem that 222 MHz has, it's not an international band. Equipment and activity is expected to be limited for many years. 

1296 MHz currently has a good amount of activity from all over the world. Recent equipment improvements indicate 1296 MHz should experience a significant growth in activity over the next few years. 

2300 MHz has received renewed interest. It suffers from nonaligned international band assignments and restrictions in different parts of the world.