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.

Memory Aid: Question asks what the “maximum” distance is - the answer is the one with the largest distance.

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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.

Hint 1:* The correct answer is the only one with "fading" in it.

Hint 2: The lib[r]ation (alcohol) results in the moon being "fluttery"

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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.

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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William Hepburn runs a DX propagation website which includes "tropospheric ducting forecasts" and world maps.

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

HINT - you might get a sunBURN in the Tropics.

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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.

Memory tip: a microwave warms cold food.

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Here's a good explanation:

http://www.wa1mba.org/10grain.htm

The essential problem is that radio waves generally travel in a straight line until they're given a reason not to. Also, it turns out the Earth is round and virtually everyone you might want to talk to lives pretty close to the surface of that oblate spheroid.

Luckily, water droplets scatter some frequencies of radio waves in a similar way to how they scatter light. Water droplets collect and move in rain clouds above the surface of the Earth, so if you aim a directional antenna at rain, then you can get a signal to travel further away before it runs into the surface of the Earth. The receiving station just has to be able to see the same rain.

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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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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.

Hint: A microwave heats the temperature of food.

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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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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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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 the northern hemisphere, your best chance to find an aurora is toward the magnetic north pole. If you were in the southern hemisphere, you would seek auroras toward the magnetic south pole.

In the Northern hemisphere the charged particles from the sun are funnelled in the direction of the north pole. This is where any aurora will be located. Aurora do not need to be visible to be useful for radio propagation.

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Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum. The oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. Electromagnetic waves can be characterized by either the frequency or wavelength of their oscillations to form the electromagnetic spectrum.

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

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Remember all the way back to the Technician-level theory, where you learned that radio waves have an electric and magnetic component. And those electric and magnetic fields are changing because the radio signals are alternating current (and alternating pretty fast too!).

The electric and magnetic fields are 90 degrees apart, always, so they don't become aligned.

Refraction doesn't describe traveling in free space.

If the waves are traveling in free space there is nothing that would cause them to return to their source.

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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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