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A proper 'gator, or a real crocodile?
EME is conducted on VHF, UHF, and microwave bands.
Originally EME (Earth-Moon-Earth) involved Morse code, using high power and multiple Yagis. "Super-stations", with a large array of many Yagis allowed SSB contacts, including with the normally CW stations. Nowadays, modest power, about 100 watts; and a single Yagi of a reasonable length can make these contacts, using digital modes such as JT65.
Both the length of the path to and from the moon; and fact that the moon has poor reflectivity for signals, is spherical, and has a rough surface means that the signal returning to earth is very small. The signal also undergoes Faraday rotation, meaning circularly polarised antennas are ideal.
The path is also shortest at the moon's perigee, the lowest point in its orbit. This is thus a time the examiners suggest scheduling contact attempts.
Beginners with antennas which can only be rotated horizontally can work off the moon when it is rising or setting. In any case, when the moon is low is when it is possible to work from Australia to the Americas, or to Africa or Europe. The maximum distance around is around 20,000 km.
Despite the comments above, the ability to use legal power (1500 watts) can be valuable. Australian stations may be able to obtain a "high power permit" for EME use.
160 metre and HF signals can travel more than half the way around the world, such as south from Sydney, and up the Atlantic to Britain, rather than across Asia. My TAFE teacher said he used to work the UK long-path from Sydney regularly before work. I can't remember definitely if it was on 20 metres, but the exam certainly says this is the most common band on which long-path occurs.
This occurs as the conditions for propagation over the short path do not exist, but they exist over the long path. In the example above, Asia would still be in darkness, while the Atlantic would still be in sunlight, or dusk.
While the exam does not discuss this, long path occurs on 6 metres as well, although this is an occasional event.
When regular linearly polarised radio waves hit a refractive medium, they refract, but a portion refracts in an unusual way, resulting in the generation of elliptically polarised waves. The Wikipedia article on Ordinary and Extraordinary Waves has interesting demonstrations of this, affecting light waves.
Until recently unexplained, this is a phenomena in which stations, especially on 6 metres, can communicate if they are a similar distance from the geomagnetic equator, a line which varies somewhat from the regular equator. Tubular formations, arcing to high altitude over the equator are now understood to provide these paths.
The classic long distance multi-hop path involve signals bouncing (actually being refracted) via the ionosphere, then bouncing from the earth's surface, before again refracting form the ionosphere (and so on) before reaching the receiving station. The alternative is that the signal reaches the (typically the higher) F-layer, is refracted, then travels some distance, before again refracting from the ionosphere, before returning to the surface for reception. The benefit is that the signal suffers less loss, partly because it is not relying on reflections from earth.
Diagrams online suggest that this tends to happen via paths which are in darkness, even if the stations half a world apart are in daylight; while those involving bounces from the earth's surface are in daylight. Others indicate that the signal becomes trapped in the F-layer for some distance (behaving a little like a tropospheric duct, I suppose).
You can watch this video regarding it here: Chordal Hop Propagation
These are the actual questions from the Extra licence exam pool, as published by the NCVEC.E3A01 (D) What is the approximate maximum separation measured along the surface of the Earth between two stations communicating by Moon bounce?
This is something like 19,000 km, or 12,000 statute miles, answer D.
A friend's first EME contact was from Sydney to Switzerland. The moon was low in the west at this time in Sydney.E3A02
This is fluttery, irregular fading on the signal, answer B.E3A03
The moon is at its perigee, the lowest part of its orbit, answer A.
This is also when people talk about a "super-moon" if it coincides with full moon, as it appears a little larger than normal.E3A04
The probability of tropo propagation, answer D.E3A05
These are warm and cold fronts, answer C.E3A06
The lower the sun, the lower the frequency, so switching to a lower band may well allow resumption of communications, answer B.
60 metres is one of the bands useful for dusk or night-time communications.E3A07
Ducts often form over bodies of water, answer C.E3A08
The is the E layer, answer A.E3A09
28 to 148 MHz are the most suitable bands, answer C, and while it depends on local practice, and bands are available to willing participants, 6 metres (50 MHz) is often the ideal band. 4 metres (70 MHz) also has potential, for those in Region 1.E3A10
Ducts can be created between layers in the atmosphere, created by temperature inversions, answer B.E3A11
Tropo carried microwave signals can travel 150 to 500 km, answer B.E3A12
Fast moving charged particles from the sun interact with the Earth's magnetic field, creating visible emission from the sky, and reflecting radio signals, answer C.E3A13
As the signal is fluttery (distorted) due to the motion of the charged regions, CW works best, answer A.E3A14
CP waves have a rotating electric field, answer B.E3B01
TEP is propagation between two mid-latitude points roughly the same distance either side of the magnetic equator, answer A.E3B02
TEP provides a range of up to 8000 km, or 5000 miles, answer C.E3B03
Afternoon or early evening, answer C.
Presumably, the recently discovered tubular structures evolve in the presence of solar radiation.E3B04
These are elliptically polarised waves generated in the ionosphere, answer B.E3B05
All HF bands, 160 meters to 10 meters, support long path, answer C.E3B06
20 metres is an effective DX band, with signals easily going more than half way around the globe, answer B.E3B07
These signals become elliptically polarised, answer C.E3B08
If you were aboard the ISS you would see this dusk or dawn zone as a broad grey line, and this is the name of this method, answer D.
This is useful for MW and HF propagation. For US listeners, this gives a chance to hear distant MW broadcast stations, and timing it so it is just dark at the station, but they are still at the higher daytime power and antenna configuration helps.E3B09
This is especially early summer, but also early winter, around the solstices, answer A.
This is the timing of the late spring and early-mid summer VHF-UHF Field Days in Australia, and the occasional winter version.E3B10
The successive ionospheric reflections, without intermediate reflection from the ground of typical multi-hop propagation, means the signal experiences less loss, answer A.E3B11
The word "sporadic" suggests randomness, and this can occur at any time, answer D.E3B12
These are successive ionospheric reflections, without the usual intermediate reflection from the ground, meaning less loss, answer A.
On to: Propagation 2 - Radio horizon, propagation prediction & space weather
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, May 2022.
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