Amateur Radio Info & Exams - Propagation 1 - EME, Meteors, Microwaves, TEP, Grey-line

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Amateur Radio Info & Exams - Propagation 1 - EME, Meteors, Microwaves, TEP, Grey-line

A proper 'gator, or a real crocodile?

Moonbounce

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.

Long Path

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.

Ordinary and Extraordinary Waves

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.

Transequatorial Propagation

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.

Chordal Propagation

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

Relevant Questions

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? A. 500 miles, if the Moon is at perigee B. 2000 miles, if the Moon is at apogee C. 5000 miles, if the Moon is at perigee D. 12,000 miles, if the Moon is visible by both stations

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 What characterizes libration fading of an EME signal? A. A slow change in the pitch of the CW signal B. A fluttery irregular fading C. A gradual loss of signal as the Sun rises D. The returning echo is several Hertz lower in frequency than the transmitted signal

This is fluttery, irregular fading on the signal, answer B.

E3A03 When scheduling EME contacts, which of these conditions will generally result in the least path loss? A. When the Moon is at perigee B. When the Moon is full C. When the Moon is at apogee D. When the MUF is above 30 MHz

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 What do Hepburn maps predict? A. Sporadic E propagation B. Locations of auroral reflecting zones C. Likelihood of rain scatter along cold or warm fronts D. Probability of tropospheric propagation

The probability of tropo propagation, answer D.

E3A05 Tropospheric propagation of microwave signals often occurs in association with what phenomenon? B. Lightning discharges C. Warm and cold fronts D. Sprites and jets

These are warm and cold fronts, answer C.

E3A06 What might help to restore contact when DX signals become too weak to copy across an entire HF band a few hours after sunset? A. Switch to a higher frequency HF band B. Switch to a lower frequency HF band C. Wait 90 minutes or so for the signal degradation to pass D. Wait 24 hours before attempting another communication on the band

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 Atmospheric ducts capable of propagating microwave signals often form over what geographic feature? A. Mountain ranges B. Forests C. Bodies of water D. Urban areas

Ducts often form over bodies of water, answer C.

E3A08 When a meteor strikes the Earth's atmosphere, a cylindrical region of free electrons is formed at what layer of the ionosphere? A. The E layer B. The F1 layer C. The F2 layer D. The D layer

The is the E layer, answer A.

E3A09 Which of the following frequency range is most suited for meteor scatter communications? A. 1.8 MHz - 1.9 MHz B. 10 MHz - 14 MHz C. 28 MHz - 148 MHz D. 220 MHz - 450 MHz

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 Which type of atmospheric structure can create a path for microwave propagation? A. The jet stream B. Temperature inversion C. Wind shear D. Dust devil

Ducts can be created between layers in the atmosphere, created by temperature inversions, answer B.

E3A11 What is a typical range for tropospheric propagation of microwave signals? A. 10 miles to 50 miles B. 100 miles to 300 miles C. 1200 miles D. 2500 miles

Tropo carried microwave signals can travel 150 to 500 km, answer B.

E3A12 What is the cause of auroral activity? A. The interaction in the F2 layer between the solar wind and the Van Allen belt B. An extreme low-pressure area in the polar regions C. The interaction in the E layer of charged particles from the Sun with the Earth’s magnetic field D. Meteor showers concentrated in the extreme northern and southern latitudes

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 Which emission mode is best for aurora propagation? A. CW B. SSB C. FM D. RTTY

As the signal is fluttery (distorted) due to the motion of the charged regions, CW works best, answer A.

E3A14 What is meant by circularly polarized electromagnetic waves? A. Waves with an electric field bent into a circular shape B. Waves with a rotating electric field C. Waves that circle the Earth D. Waves produced by a loop antenna

CP waves have a rotating electric field, answer B.

E3B01 What is transequatorial propagation? A. Propagation between two mid-latitude points at approximately the same distance north and south of the magnetic equator B. Propagation between any two points located on the magnetic equator C. Propagation between two continents by way of ducts along the magnetic equator D. Propagation between two stations at the same latitude

TEP is propagation between two mid-latitude points roughly the same distance either side of the magnetic equator, answer A.

E3B02 What is the approximate maximum range for signals using transequatorial propagation? A. 1000 miles B. 2500 miles C. 5000 miles D. 7500 miles

TEP provides a range of up to 8000 km, or 5000 miles, answer C.

E3B03 What is the best time of day for transequatorial propagation? A. Morning B. Noon C. Afternoon or early evening D. Late at night

Afternoon or early evening, answer C.

Presumably, the recently discovered tubular structures evolve in the presence of solar radiation.

E3B04 What is meant by the terms "extraordinary" and "ordinary" waves? A. Extraordinary waves describe rare long-skip propagation compared to ordinary waves, which travel shorter distances B. Independent waves created in the ionosphere that are elliptically polarized C. Long-path and short-path waves D. Refracted rays and reflected waves

These are elliptically polarised waves generated in the ionosphere, answer B.

E3B05 Which amateur bands typically support long-path propagation? A. 160 meters to 40 meters B. 30 meters to 10 meters C. 160 meters to 10 meters D. 6 meters to 2 meters

All HF bands, 160 meters to 10 meters, support long path, answer C.

E3B06 Which of the following amateur bands most frequently provides long-path propagation? A. 80 meters B. 20 meters C. 10 meters D. 6 meters

20 metres is an effective DX band, with signals easily going more than half way around the globe, answer B.

E3B07 What happens to linearly polarized radio waves that split into ordinary and extraordinary waves in the ionosphere? A. They are bent toward the magnetic poles B. They become depolarized C. They become elliptically polarized D. They become phase locked

These signals become elliptically polarised, answer C.

E3B08 What type of HF propagation is probably occurring if radio signals travel along the terminator between daylight and darkness? A. Transequatorial B. Sporadic-E C. Long-path D. Gray-line

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 At what time of year is Sporadic E propagation most likely to occur? A. Around the solstices, especially the summer solstice B. Around the solstices, especially the winter solstice C. Around the equinoxes, especially the spring equinox D. Around the equinoxes, especially the fall equinox

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 Why is chordal hop propagation desirable? A. The signal experiences less loss compared to multi-hop using Earth as a reflector B. The MUF for chordal hop propagation is much lower than for normal skip propagation C. Atmospheric noise is lower in the direction of chordal hop propagation D. Signals travel faster along ionospheric chords

The successive ionospheric reflections, without intermediate reflection from the ground of typical multi-hop propagation, means the signal experiences less loss, answer A.

E3B11 At what time of day can sporadic E propagation occur? A. Only around sunset B. Only around sunset and sunrise C. Only in hours of darkness D. Any time

The word "sporadic" suggests randomness, and this can occur at any time, answer D.

E3B12 What is the primary characteristic of chordal hop propagation? A. Propagation away from the great circle bearing between stations B. Successive ionospheric reflections without an intermediate reflection from the ground C. Propagation across the geomagnetic equator D. Signals reflected back toward the transmitting station

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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