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.

NASA time-lapse video showing libration. Due to libration, we see different parts of the moon over 27 days.
https://m.youtube.com/watch?v=3f_21N3wcX8

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.

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If you understand the etymology of "perigee," the right answer will be easy to remember. Peri is greek for around, about, near, surrounding, close (think perimeter, perimenopause, pericardium) . And ge is the greek root of earth (think geography, geology etc.). So, peri gee… is close to / near the earth. And, obviously, the closest the moon is to the earth, the shorter distance the waves have to travel, and the less opportunity for loss.

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Memory tricks:

• 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.
• Think “pear” for “perigee.” Pears are bulbous or weighted at the bottom. As a pear is hanging in a tree, the bulbous bottom is closer to the earth than the top. The moon being closer to the Earth will result in the least path loss.
• A(pogee) is for A(far) and P(erigee) is for P(roximate).

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This one is easy if you think about how the wave travels, but the wording is a little odd. The two components (E and M) are at right angles to the direction the wave travels, and also at right angles to each other. So you can picture a traveling EM wave as having three axes like XYZ, and the direction of travel is at right angles to the other two.

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

To visualize, use the Right Hand Rule: put your thumb up (electric field), point straight out with your index finger (magnetic field), and put your second finger (direction of propagation) at 90 degrees to your index finger. Try not to get a cramp!

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The electric field and the magnetic field are at right angles to each other. The orientation of the electric field determines the polarization of the wave.

Remember: right is right.

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The MUF stands for the Maximum Usable Frequency and is affected by various atmospheric parameters, including the day/night cycle. An even higher frequency above the Maximum Usable Frequency would thus have even worse performance, and changing the antenna's parameters wouldn't help as it has nothing to do with the propagation in this case. Thus, the best solution is to switch to a lower frequency.

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Memory Hint: Think of the MUF as a movable ceiling that slowly lowers after sunset as the electron density of the F layer decreases. If you're standing high on a ladder (using a high frequency) and don't step down, you'll soon be going through that ceiling. Instead of being refracted back toward Earth, your radio waves pass through the ionosphere and disappear into space. To stay under the ceiling, you have to step down to a lower rung on the ladder, meaning switch to a lower frequency band.

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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.

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Detailed explanation:

Think of an atmospheric duct as an invisible hallway in the atmosphere that guides radio waves horizontally instead of letting them escape upward into space.

The hallway is created by a temperature inversion, where warm air sits above cooler air. And this often occurs over large bodies of water, because water changes temperature much more slowly than land.

Without a duct, microwaves (UHF and above) travel almost perfectly in straight lines and simply shoot off into space. An atmospheric duct acts like an invisible guide, continuously bending the waves toward Earth and enabling them to travel hundreds of miles beyond the normal horizon.

Memory chain: Big body of water → Temperature inversion → Invisible hallway (duct) → Microwaves stay in the hallway instead of escaping into space.

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Both this question and E9C11 involve signal propagation and terrain features. While the physics involved in the questions are different, the correct answer to both questions is the one where your signal goes farther across water.

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Memory tricks:

• of the choices, water is the only substance you might put in your microwave.
• Ducts (ducks) love water.
• Aqua-duct
• Clouds are not 'geographic,' immediately ruling out two of the answers.

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Think of a meteor as a tiny cosmic bullet. As it first enters the atmosphere (F region), the air is too thin to do much. The meteor keeps racing along.

As the meteor descends and reaches the E region (about 60 to 75 miles up), this is where the magic happens: the air becomes just dense enough to strip atoms from the meteor and ionize the surrounding air. The result is a long, thin trail of ionized gas, like a glowing line drawn across the sky. That glowing trail briefly acts like a tiny mirror for VHF radio signals, allowing them to travel far beyond the horizon by meteor scatter.

The meteor does not typically make it to region (D). It burns up in the lower part of region E, and the ionized trail disappears.

So, during the meteor's Goldilocks journey through the ionosphere: F = Too Few air molecules (the air is too thin) E = Everything is just right (a long ionized trail forms) D = Disintegrated, Disappeared

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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)

Meteoroid: A small rocky or metallic body traveling through space.
Meteor: The streak of light produced when a meteoroid enters the Earth's atmosphere and burns up due to friction.
Meteorite: The remaining portion of a meteoroid that survives its passage through the atmosphere and impacts the Earth's surface.

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.

Also: things are SCATTERED far and WIDE. The right answer is the widest frequency range (over 5:1).

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The index of refraction describes how fast an electromagnetic wave travels through a material compared with its speed in a vacuum:

\[ n = \frac{c}{v} \]

Here, \(c\) is the speed of light in a vacuum, \(v\) is the speed through the material, and \(n\) is the index of refraction. Rearranging the formula gives:

\[ v = \frac{c}{n} \]

A higher index of refraction means a lower propagation speed.

Learning metaphor: Think of the index of refraction as a Traffic Slowdown Score. A vacuum has an index of 1, like an empty freeway. A material with a higher index is like heavier traffic: the wave travels more slowly.

The term is most commonly encountered when discussing light, but radio waves are also electromagnetic waves, so the same principle applies. Changes in propagation speed can also cause waves to bend, or refract, as they pass between regions with different indices of refraction.

Resistance and reactance may seem plausible because they affect electrical signals in circuits, antennas, and transmission lines. Together they make up impedance, which describes opposition to current, but they do not apply to the the propagation speed of an electromagnetic wave through a medium.

Birefringence occurs when a material has different indices of refraction for different polarizations. Evanescence describes a field that decays rapidly with distance instead of propagating normally.

Therefore, among these choices, the speed of electromagnetic waves through a medium is determined by the index of refraction.

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Tropospheric ducting is a phenomenon where radio waves are bent and trapped within a layer of the Earth's atmosphere, called a tropospheric duct, due to temperature inversions and changes in atmospheric refractive index.

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. Signals received via auroral propagation are badly distorted, making CW the most effective mode for auroral work.

Memory aid: "Northern Lights" sounds like the name of a scripted drama you'd find on the CW television network.

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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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