EME (Moonbounce)

See also Getting Started in EME, a brief discussion on how to make your first EME contacts on 2 meters.

Earth and the moon

Earth and the moon as seen by the Galileo spacecraft (NASA)

Although not really an efficient reflector (reflecting only about 7% of the signal that hits it), the moon is capable of reflecting enough of a VHF or UHF signal back to Earth to allow communication between well equipped stations. Working range on EME is primarily limited by moon visibility. If two stations can simultaneously see the moon, they will be able to make contact (assuming adequate power, antennas, etc. for EME work). However, due to variability in conditions, several attempts may be required to achieve success. The round trip path is nearly half a million miles. Path loss varies with frequency, ranging from about 243 dB at 50 MHz to 289 dB at 10368 MHz.

EME signals fade in and out for a number of reasons. Sometimes you hear nothing for long periods of time, other times you will be amazed what you can hear.

The polarization of EME signals is constantly changing. Except for a few stations who can rotate or electrically switch the polarization of their antennas, this causes very deep QSB that can last from several minutes to several hours or even days. There is also such a thing as true one way propagation on EME, largely due to polarization shifting. Sometimes, both stations hear each other; sometimes, A can hear B but not vice versa; sometimes B can hear A; and sometimes neither can hear the other. There are two basic polarization affects. Spatial polarization : two stations unsing linear polarization arrays with azimuth/elevation mounts will usually not have their antennas aligned in polarization as seen by the moon. Faraday rotation : actual rotation of the wave front as it passes through the inonosphere. Spatial polarization is the dominant factor at 1296 MHz and above where ionospheric affects are minimal.

The moon follows many cycles. The distance between the Earth and the moon is not constant. It varies, and generally there will be a perigee (moon closest to Earth) and an apogee (moon fartherst from Earth) each month. Path loss to the moon and back is roughly 2 dB less at perigee than at apogee. This can make a very noticeable difference for small stations. Also, the sky behind the moon can be very noisy at certain times. All planets, stars, etc. emit noise across the radio spectrum, and most EME systems are sensitive enough to hear this noise. Sky noise is generally at its worst when the moon is crossing the galactic plane (moon appears in the milky way), which occurs twice each month. Practically all software intended for EME use includes this data. You can download Z-Track from this site, but there are several other eqaully good programs available. Sky noise is more of a problem at the lower frequencies.

Signals also tend to exhibit a rapid, almost fluttery fading known as libration fading. This is caused by the irregular surface of the moon, which "rocks back and forth" slightly as viewed from Earth. Libration can cause signals to go goth above and below the average level. Libration peaks, which can last up to a couple of seconds at 2 meters, can actually help the small station make contacts they would not be able to otherwise. This phenomenon is of little use at higher frequencies, as the rete of libration fading is directly proportional to frequency (the fading is faster on higher frequencies). Thus, a libration peak that lasts 3 seconds on 144 MHz will only be one second on 432, and 1/3 of a second on 1296.

Because the moon moves in relation to Earth, there is a slight doppler shift on EME signals. The amount of doppler shift is proportional to frequency. It is about 350 Hz maximum on 144 MHz, more on higher frequencies. At moonrise, the doppler shift is upward in frequency, reaching zero as the moon passes overhead, and then going negative as the moon heads toward set. AF9Y, Mike, has written a program which can help you find the exact fequency of a station even when you can't yet hear the signal. It's called FFTDSP and is quite useful.

Because the round trip distance is nearly half a million miles, it takes over 2 seconds for a signal to travel from Earth to the moon and back to Earth again. Well equipped stations can actually hear their own signals echoed back from the moon when conditions are favorable.

 EME Operating Techniques

EME is weak signal work and almost all contacts are made with CW. Some of the well equipped stations occasionally try SSB just for fun, but it's the exception not the rule. As for CW speed, most operators are comfortable somewhere between 10 to 20 WPM. Fast CW tends to be difficult to copy when signals are very weak, and too slow CW gets chopped up by libration fading, so there has to be a compormise somewhere. Good clean sending is an advantage, and many operators recommend increasing the length of the dits (increase the weight) slightly to make them stand out more. A dit lengh of 1.2 times normal seems to work well here.

Random operation (calling CQ or answering CQ's) is common, but for very small stations better success will be had on prearranged schedules. Schedules are generally run for 30 minutes, but may vary. Random operation is at the low end of the band with schedules taking place above that.

Because signals are weak and not always out of the noise, almost all contacts are made with accurately timed transmit and receive sequencing. Standard sequencing is 2 minutes on 50 and 144 MHz, 2.5 minutes on the higher bands. Sometimes one minute sequences may be used for random contacts.

Long ago, the so-called TMOR signal reporting system was used on EME. On 6 and 2 meters, T and M are almost never used. T, if used at all, indicates traces of signal have been heard; M indicates some letters received but not enough for a contact. On higher bands, T indicates traces or partial calls, M complete calls (a valid report for a completed contact), and O complete calls, good copy. For a valid contact, complete calls, signal report and acknowledgement must be received by both stations.

 Some examples of EME Signals

Here are some recordings of actual EME signals.

AUDIO A typical 144 MHz EME singnal with no audio filtering. Very little QSB on this one. Can you identify the station? This signal isn't too strong, but most experienced EME'rs can easily copy it.
Provided by AF9Y

AUDIO A 144 MHz EME signal with narrow filter. Notice the QSB. This is quite typical of 2m EME signals.
Provided by VE7BQH

AUDIO Hearing your own echoes at 144 MHz.
Provided by N1BUG

AUDIO Hearing your own echoes at 432 MHz. Note how sometimes a CW character gets chopped up (only part of it can be heard) due to libration fading.
Provided by WB6IMC

AUDIO Strong singnal on 23cm (1296 MHz). Not all EME signals are weak! This is what one of the 23cm big guns sounds like. W4OP was receiving this on a 12 foot dish.
Provided by W4OP

AUDIO Strong SSB singnal on 23cm (1296 MHz). On 23cm big stations can ragchew easily.
Provided by KB2AH

AUDIO A 3cm (10368 MHz) EME Signal Notice the rapid "flutter" caused by libration at this frequency!
Provided by OH2AUE (OH2AXH EME Station)


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