Extra Class Question of the Day: Earth-Moon-Earth communications

One of the more exotic things that amateur radios do is earth-moon-earth (EME) communication, sometimes called “moon bounce.” As this name implies, radio amateurs actually bounce their signals off the moon. This is the ultimate DX. The approximate maximum separation measured along the surface of the Earth between two stations communicating by Moon bounce is 12,000 miles, as long as both can “see” the Moon. (E3A01)

Because the signal travels such a long way, you need to do everything you can to avoid signal loss. So, for example, scheduling EME contacts when the Moon is at perigee will generally result in the least path loss. (E3A03) Perigee is the point at which the Moon is the closest to Earth.

Because the signals are so weak, it’s also important to use equipment with very low noise, so that the signals don’t fall below the noise level. That being the case, the type of receiving system that is desirable for EME communications is equipment with very low noise figures. (E3A04)

EME communications can take place on both the 2m band and the 440 MHz band. The frequency range that you would normally tune to find EME signals in the 2 meter band is 144.000 – 144.100 MHz. (E3A06) The frequency range that you would normally tune to find EME signals in the 70 cm band is 432.000 – 432.100 MHz. (E3A07)

As you can imagine, there are not many operators working moon bounce. You don’t just get on an call CQ—generally you set up a schedule with another operator to contact one another via moon bounce. At the appointed time, the operators take turns transmitting, while the other listens. Time synchronous transmissions with each station alternating describes a method of establishing EME contacts. (E3A05)

One interesting phenomenon is libration fading. Libration fading of an Earth-Moon-Earth signal is a fluttery, irregular fading. (E3A02) This fading is caused by the irregular surface of the Moon, and the peaks can last for up to two seconds on the 2m band. These peaks can actually help operators make contacts when they would otherwise be impossible.

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