This month Steve White G3ZVW takes a look at a propagation mode that is very, very challenging.
This month Steve White G3ZVW takes a look at a propagation mode that is very, very challenging.
There’s a mode of propagation that very few radio amateurs ever use. Not because it only occurs rarely but because it’s so difficult to exploit. I refer to Earth-Moon-Earth propagation, also known as EME or Moonbounce.
This feature will not be a guide on ‘how to’ and neither will it go into the mathematics, but hopefully it will serve to explain why it is so difficult to make a contact by bouncing VHF/UHF radio signals off the Moon.
First though, some facts about Earth’s only natural satellite. The Moon has no atmosphere and is covered with impact craters. It orbits the Earth slightly elliptically, i.e. it is not always the same distance away. The Moon is rocky and – like Earth – has iron at its core. This means it also has its own magnetic field but because the Moon is much smaller than the Earth, its magnetic field is a lot weaker.
A Small Target
If you look up into the sky and see the Moon, it appears to be quite large, but if you were to measure the distance across it in terms of degrees you would find that it is just 0.5°. Its average distance from Earth is about 380,000km (240,000 miles), which alone means on VHF radio the path loss is high. Now double the path loss, because the signal has to go there and come back.
Consider a typical long Yagi antenna. Its 3dB (half power) beamwidth is likely to be around 30° (both horizontally and vertically). Fig. 1 shows this, the relative sizes being to scale. If you were to calculate the area of the Moon, compared with the beamwidth of the transmission, you would find less than one tenth of one percent of the transmitted signal will actually reach it. The rest will miss and disappears out into space.
A Poor Reflector
Unlike something like a radar target, which is intended to reflect radio waves with some efficiency, the moon is a big lump of rock and isn’t. On average, about 7% of a radio signal that reaches the Moon is reflected by it. The other 93% is simply absorbed.
The next thing we need to think about is that unlike the antenna, which transmitted the original signal in a tight(ish) beam, the Moon doesn’t do the same in the reverse direction.
A radio signal that strikes the Moon dead centre is likely to be reflected in a roughly hemispherical pattern, Fig. 2. Certainly, more is going to be reflected back towards the earth than in other directions but the only direction in which you can guarantee no signal will go is through the Moon itself. Meanwhile, a radio signal that strikes somewhere near the edge of the moon will also end up being scattered in all directions but now the strongest of those directions will take the signal further on. From near the edge of the Moon precious little signal will be reflected earthwards, although some should be.
Phase and Multipath
Now we run into the next problem! All the signals that were reflected back towards Earth will have travelled slightly different distances. This means they will not arrive in phase (and therefore not add together). I discussed phase distortion and multipath reception in this column in July 2017, so please refer back to that for more information.
There’s also a special kind of signal fading that only occurs on Moonbounce signals, caused by the relative motions of the Moon and Earth.
Think it’s got difficult enough so far? Read on!
Suppose you transmit a horizontally polarised signal at the Moon. By the time what pitifully small amount arrives back at Earth, the polarisation of it will have been changed. This is due to two things.
1. The polarisation of a radio signal is changed as it passes through a magnetic field. The Earth has a magnetic field, so the polarisation is twisted. This is known as Faraday Rotation, which is a topic I wrote about in July 2018. The Moon also has a magnetic field, although it is very much weaker than Earth’s.
2. The relative positions on Earth of the two stations trying to communicate also has a big effect. See Fig. 3. In the most extreme case, the signal polarisation can be ‘twisted’ by as much as 90°, which results in a huge loss of signal. It is not practical for stations to physically align the polarisation of plane polarised beam antennas for each and every contact. One fix for this problem is to use circular polarisation but a more popular method is to use a second antenna at the receive end of the link that is polarised differently, or just suffer the loss that less than perfect polarisation alignment results in.
Electrical noise comes from the sky, some of it being an ‘echo’ of the Big Bang, which created the Universe. The noise has different levels in different directions. Consequently, there are times during the Moon’s orbit when it transitions areas of the sky that are noisier than others. This means that at times, making a contact is even more difficult, but at least the more noisy parts of the sky are known about, so people avoid trying to make contact when the Moon is in one of them.
The Sun is also a noise transmitter, so people avoid trying Moonbounce contacts when the Moon appears close in the sky to the Sun (i.e. around the times of the New Moon).
A Moving Target
When the Moon is above the horizon it moves across the sky. It always rises roughly in the East and sets roughly in the West. The antennas required for a Moonbounce contact need to track the Moon, not only horizontally (azimuth) but vertically (elevation). Az/el rotators for the amateur radio market have been around for a long time and are mainly used for tracking man-made satellites. To track the Moon (or indeed any satellite) efficiently, they need to be computer linked.
Most rotators for tracking amateur satellites are designed for modest antennas, which means they are not sufficiently strong to point large antenna arrays. To track the Moon it is possible to use a strong ‘normal’ antenna rotator in combination with the kind of screwjack used to point a satellite dish but some additional engineering will be required.
While I am on about moving targets, the rotation of the Earth results in Doppler Shift of the signal, so the receive station also needs to track the signal as it drifts in frequency.
How it’s Done
Given all the difficulties of making a Moonbounce radio contact, how do people actually do it? Be warned, there’s a price tag associated with making a Moonbounce contact! Moonbounce enthusiasts need…
1. The best possible receiver. The receiver of an off-the-shelf multimode VHF transceiver is very unlikely to be good enough. Without criticising any particular makes or models – after all, only a relatively few amateurs place such extreme demands on their station – the performance of commercial amateur VHF/UHF transceivers is limited by the design. Phase noise from the frequency synthesiser is the most common limiting factor that people tend to know about but PIN diodes used for transmit/receive switching can also limit performance. Good quality coaxial relays are better than PIN diodes. Excellent frequency stability is also needed. The best receiver is likely to be a top-notch HF transceiver, coupled to a top-notch transverter – but even then some modifications/upgrades might be needed to get the very best out of the system.
2. High power. In terms of power, try to run the legal limit.
3. The best possible antenna system. A multiple long Yagi array will be better than a single Yagi.
4. The best quality coaxial cable. Aiming for the minimum loss is crucial.
5. A masthead preamplifier, to boost the incoming signal before the inevitable loss in the coax makes it even weaker.
6. A low noise environment. If you live in a city, your local noise environment is likely to be worse than if you live in a rural area. Be prepared to move house! Bear in mind though that – unlike normal VHF/UHF operation, where a hilltop site is preferred – for Moonbounce (or satellite) communication you don’t need to live on a hill. British Telecom did exactly this when they were planning their second satellite earth station, back in the 1970s. They built it in a valley in Herefordshire (which gave it some screening from man-made terrestrial interference).
7. The mode of transmission used to conduct a Moonbounce contact is important. Forget FM or any form of digital audio. CW (Morse) is traditionally the mode of choice, because it is a narrowband mode (tight filtering will reduce the noise at the receive end). SSB can also work, but not as well. These days most Moonbounce contacts are conducted using one of Joe Taylor K1JT’s WSJT (Weak Signal by Joe Taylor) suite of computer programs.
Having laid out what makes conducting a Moonbounce contact difficult, is there anything which is actually easy? Oddly enough the answer is ‘yes’.
1. The orbit of the Moon is very predictable, so it’s easy to find out which countries are contactable and when. The Moon needs to be above the horizon at both ends of the contact but it doesn’t matter if it is daylight or cloudy.
2. If you have a good quality multimode VHF station, you can still make Moonbounce contacts when the Moon is close to the horizon. Mainly you are likely to work the ‘big guns’, who have powerful stations and large antenna arrays.
3. Using the internet to discover the best propagation times and schedule a contact has made life a lot easier – with the majority of Moonbounce contacts being scheduled.
4. Using modern software (JT modes) can result in a contact with someone you cannot actually hear by ear.
This article was featured in the March 2019 issue of Practical wireless.