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Subelement E3
Section E3A
Electromagnetic Waves and Specialized Propagation: Earth-Moon-Earth (EME) communications; meteor scatter; microwave tropospheric and scatter propagation; auroral propagation; daily variation of ionospheric propagation; circular polarization
What is the approximate maximum separation measured along the surface of the Earth between two stations communicating by EME?
  • 2,000 miles, if the moon is at perigee
  • 2,000 miles, if the moon is at apogee
  • 5,000 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. The Earth’s circumference is about 24,000 miles, so half of that (12,000 miles) would have line of sight to the moon. 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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In what direction does an electromagnetic wave travel?
  • It depends on the phase angle of the magnetic field
  • It travels parallel to the electric and magnetic fields
  • It depends on the phase angle of the electric field
  • Correct Answer
    It travels at a right angle to the electric and magnetic fields

Right angle.

To help remember; just think "right is right".

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How are the component fields of an electromagnetic wave oriented?
  • They are parallel
  • They are tangential
  • Correct Answer
    They are at right angles
  • They are 90 degrees out of phase

They are at right angles.

Remember: right is right.

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What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness?
  • Switch to a higher frequency HF band
  • Correct Answer
    Switch to a lower frequency HF band
  • Change to an antenna with a higher takeoff angle
  • Change to an antenna with greater beam width
This question does not yet have an explanation! Register to add one

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Atmospheric ducts capable of propagating microwave signals often form over what geographic feature?
  • Mountain ranges
  • Stratocumulus clouds
  • Correct Answer
    Large bodies of water
  • Nimbus clouds

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. Silly 2- Ducts (ducks) love water.

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When a meteor strikes the Earth’s atmosphere, a linear ionized region is formed at what region of the ionosphere?
  • Correct Answer
    The E region
  • The F1 region
  • The F2 region
  • The D region

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!

Another: Electrons go to the 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 in which 50 MHz falls, and that is 28 MHz - 148 MHz.

Memory Trick: "Meteor" has six letters. The best band for meteor scatter is 6m. The only answer that covers the 6m band is 28-148MHz.

Additional Memory Trick: MSK144 is a meteor scatter mode. 144MHz is only in the correct answer.

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What determines the speed of electromagnetic waves through a medium?
  • Resistance and reactance
  • Evanescence
  • Birefringence
  • Correct Answer
    The index of refraction

The correct answer, "index of refraction" is a term that's more commonly used in optics than RF. It comes from how much light bends or "refracts" when moving between regions with different indices of refraction. It bends because light (and RF) always takes the fastest (not shortest) path between two points, much like a lifeguard running along the beach before diving into the water to save a swimmer. So, refraction (bending) is a function of the propagation speed in a medium.

Birefringence is when the refractive index is different for different polarizations. Evanescence is when a field exponentially decays when passing through a thin reflective barrier. Resistance and reactance are combined properties of devices, and not bulk properties of a medium itself.

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What is a typical range for tropospheric duct propagation of microwave signals?
  • 10 miles to 50 miles
  • Correct Answer
    100 miles to 300 miles
  • 1,200 miles
  • 2,500 miles

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.

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.

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What is most likely to result in auroral propagation?
  • Meteor showers
  • Quiet geomagnetic conditions
  • Correct Answer
    Severe geomagnetic storms
  • Extreme low-pressure areas in polar regions

Auroral propagation is a result of the suns solar wind interacting with the earths geomagnetic field(GMF). The GMF guides the charged particles of the solar winds towards the earths poles causing extreme ionization up to 1000km in height. These vertical curtain like regions reflect signals similar to the horizontal F2 region in the HF band and sometimes up to the UHF band.

A geomagnetic storm is simply a period of severe excitation of the GMF due to an increase in the solar wind, increasing the likelihood of auroras and therefore auroral propagation. Increases in the solar wind are due to either coronal mass ejections or to a lesser extent by fluctuations in the coronal stream.

Hint: Sun=>Storm=>Auroras=>Auroral Propagation

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

The cause of auroral activity—sometimes called the Northern Lights or aurora borealis—is the interaction in the E layer of charged particles from the Sun with the Earth's magnetic field. CW is the emission mode that is best for aurora propagation.

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What are circularly polarized electromagnetic waves?
  • Waves with an electric field bent into a circular shape
  • Correct Answer
    Waves with rotating electric and magnetic fields
  • Waves that circle 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.


Memory aid: circularly: like a wheel rotating

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