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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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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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.

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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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Right angle.

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

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They are at right angles.

Remember: right is right.

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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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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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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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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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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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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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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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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

Memory aid: circularly: like a wheel rotating

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