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Subelement E3
RADIO WAVE PROPAGATION
Section E3A
Electromagnetic waves; Earth-Moon-Earth communications; meteor scatter; microwave tropospheric and scatter propagation; aurora propagation; ionospheric propagation changes over the day; circular polarization
What is the approximate maximum separation measured along the surface of the Earth between two stations communicating by EME?
  • 500 miles, if the moon is at perigee
  • 2000 miles, if the moon is at apogee
  • 5000 miles, if the moon is at perigee
  • Correct Answer
    12,000 miles, if the moon is visible by both stations

So long as both stations have a line of sight path to the moon, they can, in principle, communicate. In practice, the enormous path losses mean that high ERP, high gain antennas, low noise receivers and narrow bandwidth signals are required.

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What characterizes libration fading of an EME signal?
  • A slow change in the pitch of the CW signal
  • Correct Answer
    A fluttery irregular fading
  • A gradual loss of signal as the sun rises
  • The returning echo is several hertz lower in frequency than the transmitted signal

This is caused by interference between the multiple path lengths of a moon bounce signal. The path lengths are constantly changing because the moon is “librating”. Although the moon does appear to always present the same face to the earth there is a small apparent “wobble” due to the fact that its orbit is not exactly circular. This apparent movement is called libration.

Because the moon has a highly irregular surface this rhythmic wobble causes irregular RF reflection.

The correct answer is the only one with "fading" in it.

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When scheduling EME contacts, which of these conditions will generally result in the least path loss?
  • Correct Answer
    When the moon is at perigee
  • When the moon is full
  • When the moon is at apogee
  • When the MUF is above 30 MHz

EME means Earth-Moon-Earth, or in other words, bouncing radio waves off of the Moon. Perigee means the point in the Moon's orbit where it is closest to the Earth.

When radio waves leave the antenna, they spread out, so when they travel far and spread out a lot, few waves hit someone else's antenna. This is much the same as a light bulb: when you're close to it, it's bright, and when you're far away, it looks dim.

The Moon is quite far away, so radio waves will spread out a lot before reaching the moon. When the Moon is at its closest point to Earth, the waves don't spread out quite as much as when the Moon is farther away. The difference between the perigee and apogee (farthest point) is about 40000 km, so round-trip is 80000 km or about 50000 miles. That means the trip is 50000 miles shorter when attempting a Moon bounce when the Moon is at perigee compared to when the Moon is at apogee.

This isn't necessarily the greatest cause for path loss for EME, but it is a factor.


Hint: Remember that apogee is farthest away from the earth, so perigee (think p for personal which is close) must mean the closest to earth, and thus should have the least loss in Earth-Moon-Earth (EME) communications.

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What do Hepburn maps predict?
  • Sporadic E propagation
  • Locations of auroral reflecting zones
  • Likelihood of rain scatter along cold or warm fronts
  • Correct Answer
    Probability of tropospheric propagation

http://www.dxinfocentre.com/tropo_wam.html

Tropospheric Ducting Forecast website with world maps.

HINT - you might get a sunBURN in the Tropics.

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Tropospheric propagation of microwave signals often occurs in association with what phenomenon?
  • Grayline
  • Lightning discharges
  • Correct Answer
    Warm and cold fronts
  • Sprites and jets

Tropospheric propagation occurs when a difference in refractive indices in two adjacent pockets of air - for instance, at the intersection between warm and cold fronts - causes a radio wave to curve along the border between the two regions of air. This propagation is also called ducting. Hint: think about a duct between the fronts.

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

As solar flux decreases and the maximum usable frequency (MUF) becomes lower long distance traffic on the higher frequency bands are the first to vanish but conditions may still allow traffic at lower frequencies.


Study tip: Sun gets lower in the sky at sunset....lower frequency.

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Atmospheric ducts capable of propagating microwave signals often form over what geographic feature?
  • Mountain ranges
  • Forests
  • Correct Answer
    Bodies of water
  • Urban areas

Evaporative ducts form over water where the cooling near the surface from evaporation results in cool air below warm air and a temperature inversion.

From http://www.df5ai.net/ArticlesDL/VK3KAQDucts2007V3.5.pdf

Silly hint: of the choices, water is the only substance you might put in your microwave.

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When a meteor strikes the Earth's atmosphere, a cylindrical region of free electrons is formed at what layer of the ionosphere?
  • Correct Answer
    The E layer
  • The F1 layer
  • The F2 layer
  • The D layer

Meteor scatter propagation occurs via the E-Layer.

Briefly, the explanation of the signal - at least in the vicinity of 20 meters is forward scattering from ionization trails left behind by the myriads of tiny meteors which pepper the E region of the ionosphere at all times. Hence the maximum range for this form of transmission is essentially that for normal one-hop E-layer transmission, or 1500 miles.

Source: QST April 1953 (via NASA)

Memory tip: There are a lot of Es in 'mEtEor' and 'frEE electrons'. Pick E-layer!

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Which of the following frequency ranges is most suited for meteor scatter communications?
  • 1.8 MHz - 1.9 MHz
  • 10 MHz - 14 MHz
  • Correct Answer
    28 MHz - 148 MHz
  • 220 MHz - 450 MHz

The best band for meteor scatter is the 50 MHz band where contacts lasting for several seconds or even a minute or so can be made. At higher frequencies the contacts will be of shorter duration.

There is only one range in the answer choices which 50 MHz falls, and that is 28 MHz - 148 MHz.

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Which type of atmospheric structure can create a path for microwave propagation?
  • The jet stream
  • Correct Answer
    Temperature inversion
  • Wind shear
  • Dust devil

A temperature inversion traps a layer of warm air above and below layers of cold air The microwave signal travels in this layer of warm air, similar in concept to how a microwave signal travels in a wave guide.

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What is a typical range for tropospheric propagation of microwave signals?
  • 10 miles to 50 miles
  • Correct Answer
    100 miles to 300 miles
  • 1200 miles
  • 2500 miles

Silly trick to help remember: We heat most meals in a microwave between 1 and 3 mins (1:00 and 3:00) so choose 100 to 300.

Tropospheric scatter (also known as troposcatter) is a method of communicating with microwave radio signals over considerable distances from 100 to 300 miles depending on terrain and climate factors. This method of propagation uses the tropospheric scatter phenomenon, where radio waves at UHF and SHF frequencies are randomly scattered as they pass through the upper layers of the troposphere. Radio signals are transmitted in a narrow beam aimed just above the horizon in the direction of the receiver station. As the signals pass through the troposphere, some of the energy is scattered back toward the Earth, allowing the receiver station to pick up the signal.

-KE0IPR

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

The Aurora Borealis is actually the result of collisions between gaseous particles in the Earth's atmosphere with charged particles released from the sun's atmosphere. Variations in colour are due to the type of gas particles that are colliding.

Source: Northern Lights Centre

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Which of these emission modes is best for auroral propagation?
  • Correct Answer
    CW
  • SSB
  • FM
  • RTTY

The rough tone is caused partly by the fact that the individual ionised particles, which make up the aurora, are moving around at high speeds and so a Doppler effect is produced which can increase the bandwidth of the signal. Also, the auroral stream as a whole is flowing and turbulent which produces further Doppler fluttering effects. The broadening of bandwidth plus the flutter effect precludes most forms of modulation except for CW.

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What is meant by circularly polarized electromagnetic waves?
  • Waves with an electric field bent into a circular shape
  • Correct Answer
    Waves with a rotating electric field
  • Waves that circle the Earth
  • Waves produced by a loop antenna

In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary manner.

https://en.wikipedia.org/wiki/Circular_polarization

-chevdor

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