Earth-Moon-Earth, EME or Moonbounce propagation is a really challenging, but interesting form of radio propagation for radio amateurs to use.
Moonbounce propagation presents a number of significant technical and operating challenges, but in this is provides a real sense of achievement and enjoyment when a contact has been successfully achieved.
Using VHF and UHF amateur radio bands, along with relatively high powers, high gain antennas and sensitive receivers, it is not a mode for all, but with today’s technology it is a mode that is within the reach of a large number of amateur radio stations with those wanting challenge.
The basis of operation of Moonbounce or EME, Earth-Moon-Earth is the use of the Moon as a passive reflector. In view of the very large distances involved and the fact that the Moon's surface is a poor reflector the path losses are colossal, but nevertheless it is still a form of communication that is theoretically possible to use, and one that many radio amateurs regularly use.
With radio signals being very low, it is found that galactic noise becomes a significant factor. This noise emanates from a variety of sources in the galaxy - planets, stars, etc. emit noise throughout the radio spectrum, and EME systems are very sensitive and will be able to hear this noise. The level of noise is not constant across the sky and this means that some times the sky around the Moon can be very noisy and at other times it can be much quieter.
It is found that sky noise is normally worst when the Moon is crossing the galactic plane (i.e. the Moon appears in the Milky Way) and this occurs twice each month. Fortunately software used for amateur radio EME Moonbounce indicates this and this helps choose the optimum times for any activity.
Equipment considerations for EME Moonbounce
To overcome the losses and enable amateur radio radio communications to be established using Moonbounce, very high radio transmitter powers, directive antennas and very sensitive receivers are required.
With the distance of the Moon from the Earth being between 360 and 405 thousand kilometersand its diameter being 3475 kilometres it subtends an angle of only 0.52 degree to observers on the Earth. In order to illuminate the Moon with little wasted power either side, enormously directive antennas are required. Also these antennas must be completely steerable to be able to track the steadily changing position of the Moon.
Frequencies used for Moonbounce are generally in the VHF or UHF portion of the spectrum. This allows antennas with sufficiently high gains to be used to overcome the path losses. Although frequencies as low as 50 MHz have been used, it is more normal for the 144 MHz, 432 MHz or 1296 MHz amateur radio bands to be employed.
Moonbounce propagation effects
Any signals transmitted for EME, Moonbounce communications are subject to a number of signal propagation effects:
- Huge path losses: To quantify the path losses the distances and reflection efficiency of the Moon are required. The Moon is around 385 000 kilometres distant from the Earth. The surface of the Moon is also reflects only about 6% of the radio signal power that reaches it. Added to the path loss for the signal travelling to and from the Moon, the overall path loss is at best approximately 252 dB on 144 MHz and 271 dB on 1296 MHz.
- Variable path losses: The level of loss also varies because the distance between the Moon and Earth is not constant. There is a "perigee" when the Moon is closest to Earth when the Mon is said to be a large Moon. There is also an "apogee" when the Moon is at it furthest point from the Earth each month. This distance variation results in a difference in path loss of around 2 dB between the apogee and perigee positions. For small stations where making contacts using Moonbounce may be marginal, the choice of time in the month can make a difference.
- Faraday rotation: At frequencies of 1296 MHz and above it is not a problem, but on 432 MHz it is believed that rotations up to 360 degrees are common, and below this the signal may rotate through several complete revolutions. This may result in stations only being able to communicate in one direction at times.
- Libration fading: This effect occurs on EME Moonbounce signals because the surface of the Moon is not flat and the reflected signal consists of a variety of wave-fronts each with differing phases because the distance travelled by each one is slightly different due to the rough Moon surface. The received reflected signal is therefore a sum of all the wave-fronts. As the Earth and the Moon are moving relative to each other the sum of these wave-fronts is always changing and this results in a signal onto which is superimposed a rapid flutter as well as deep fading (sometimes up to 20 dB) and some peaks. These peaks can often be very helpful to stations with less power or smaller antennas.
After reflection by the Moon, wavefronts have a variety of phases which sum to give the overall signal. As these change with the relative movements of the earth and the Moon this results in libration fading.
- Doppler shift: The relative movement of the Earth and the Moon can result in some degree of Doppler shift being add to the signals. This will vary according to the relative movements of the two bodies, and also to the frequency in use. As an example of how Doppler shift affects a Moonbounce / EME signal it is found that at "Moonrise", a 2 Metre signal may be shifted up in frequency by as much as 350 Hz. It is then found that this figure reduces, reaching zero when the Moon is passing the particular longitude in which you are located (due south or due north azimuth heading) After this, the Doppler shift starts to move in a negative direction, reaching an offset of around 350 Hz LF by "Moonset." As signal levels are low using EME Moonbounce, very narrow bandwidths are often used and as a result Doppler shifts can be of importance.
- Signal polarisation changes: Another problem that can occur with Moonbounce is that as stations are located at different positions around the globe, a horizontally polarised signal in one area of the globe will be at right angles to a horizontally polarised signal a quarter of the way round the globe. This spatial polarisation problem adds to the difficulties caused by Faraday rotation.
Equipment for ham radio EME, Moonbounce
Comparatively few EME, Moonbounce contacts are made on 50 or 70 MHz in view of the local noise as well as building he sort of antennas that would be needed to provide the required gain.
For 144 MHz the sensitive receivers and transmitter exciters are relatively commonplace. A good pre-amplifier is needed at the antenna, and a high power linear amplifier needed to develop the maximum legal power. Antennas are manageable even at 144 MHz, but high gains are required. Also feeder losses must be kept to an absolute minimum.
As the selection of the relevant amateur band moves into the UH portion of the spectrum, there is a steady move from Yagi antennas to parabolic reflector or dish antennas and it becomes more difficult to generate the levels of power needed to drive the antenna.
Calculations indicate that antenna gains of around 20dBd are needed on 144 MHz and 23dBd on 432 MHz are needed to achieve Morse contacts. To achieve these levels of gain one arrangement is to use four long Yagi antennas that are stacked and bayed. With 3dB additional gain being required on 432 MHz to cover the additional path loss, eight long Yagis are required.
That said many stations are able to make contacts with stations with huge antennas, effectively piggy-backing off their antenna gain. Also using low signal modes available now, there are significant reductions in the antenna requirements.
Modes for amateur radio EME Moonbounce
Although SSB has been used on some occasions by stations using exceptionally large antennas, the majority of contacts used to be made using Morse. Now with computer technology and specialised data modes, these are widely used, and because there are low signal modes, this has considerably reduced the requirements on the equipment, bring Moonbounce within the reach of many radio amateurs.
- Morse EME contacts : When using Morse speeds are generally kept to speeds between about 12 and 15 words a minute. The reason for this is that if the speeds are too high then copy becomes difficult in the presence of noise whereas if the speeds are too slow then the characters become affected by the libration fading which again makes copy difficult. Additionally the weighting on the individual dots and dashes is often increased slightly to aid copy. Normally when making Morse code amateur radio, the signal levels are low and therefore only the basic contact details are exchanged. For a contact to be deemed to be completed a minimum of the callsigns, and reports must be exchanged with a confirmation that the report has been received.
- WSJT: WSJT is an ideal and the most widely used format for ham radio EME Moonbounce contacts. Normally the JS65 variant within the WSJT software suite is used and this provides some considerable advantages. The WSJT screen has been designed for amateur radio Moonbounce, EME and as and even provides Moon direction information. WSJT operation on 144 MHz typically takes place between 144.100 and 144.150 MHz.
- Moonbounce via SSB: For stations with very high gain antennas and high power levels, it can be possible to use single sideband. However the propagation characteristics mean that copy can be distorted at times.
When using any mode, a good first check of a station can be gained by listening for echoes of one's own signal. If these can be heard then there is a good chance of others hearing. However even if the echoes cannot be heard, it is possible that others with higher gain antennas may hear.
Although many stations call CQ, this is only viable for stations using high powers and high gain antennas. For stations where signal strength may be marginal arranged contacts produce a far better possibility of a contact. These arranged contacts use accurately timed transmit and receive periods to enable the participating stations to have the best chance of communicating.
The use of E-M-E propagation or Moonbounce is a challenge to any radio amateur wanting to use this mode of radio propagation, but it can yield some excellent results. Those with the right equipment are able to make contacts with stations in many different areas of the globe when the Moon is in the right position relative to the Earth. In this way it is a particularly interesting form of propagation to use.
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