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Subelement E3
RADIO WAVE PROPAGATION
Section E3A
Propagation and technique: Earth-Moon-Earth communications (EME), meteor scatter
What is the approximate maximum separation measured along the surface of the Earth between two stations communicating by Moon bounce?
  • 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, as long as both can "see" the Moon

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 Earth-Moon-Earth 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 type of receiving system is desirable for EME communications?
  • Equipment with very wide bandwidth
  • Equipment with very low dynamic range
  • Equipment with very low gain
  • Correct Answer
    Equipment with very low noise figures

In Earth-Moon-Earth communications you are dealing with very weak signals.

Exclude wide bandwidth: it is an irrelevant distractor.

Exclude high dynamic range: it is helpful in dealing with weak signals.

Exclude high gain: it is helpful in receiving weak signals.

low noise is the correct answer because a low noise figure is highly desirable in weak signal communications. A receiver's noise figure is the ratio of its noise to that of an ideal receiver.

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Which of the following describes a method of establishing EME contacts?
  • Correct Answer
    Time synchronous transmissions with each station alternating
  • Storing and forwarding digital messages
  • Judging optimum transmission times by monitoring beacons from the Moon
  • High speed CW identification to avoid fading

JT65 uses time synchronous transmissions alternating and is commonly used in EME communications.

(This method is also used in FT8, which has become very popular for regular non-EME use.)

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What frequency range would you normally tune to find EME signals in the 2 meter band?
  • 144.000 - 144.001 MHz
  • Correct Answer
    144.000 - 144.100 MHz
  • 144.100 - 144.300 MHz
  • 145.000 - 145.100 MHz

At the very bottom of the 2 meter band, 144.000 to 144.100 is the CW portion, which includes Earth-Moon-Earth (EME) operation. EME operators communicate by bouncing their signals off the moon. (from Bob Witte, KØNR)

Memory trick: (for this VHF question and similar UHF question): The EME segment is only 0.1 MHz, since EME is a small niche area of interest (No band plan segments are so tiny as 0.001 Mhz, so "A" is obviously wrong.) Of the two answers left with 0.1 Mhz widths, pick the LOWER answer for the VHF question, and the HIGHER answer for the UHF question, becaise VHF is lower than UHF. Easy!

Then go look it up in a band plan when you actually want to do moon bounce, since you'll have forgotten these freqs by then!

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What frequency range would you normally tune to find EME signals in the 70 cm band?
  • 430.000 - 430.150 MHz
  • 430.100 - 431.100 MHz
  • 431.100 - 431.200 MHz
  • Correct Answer
    432.000 - 432.100 MHz

CW is invariably used for E-M-E contacts and this is a weak signal CW segment of the 70 cm band. (Digital is also often used for EME.)

Memory trick: (for this VHF question and similar UHF question): the segment is only 0.1 MHz, since EME is a small niche area of interest. Of the two answers left with 0.1 Mhz widths, pick the lower answer for the VHF question, and the higher answer for the UHF question, becaise VHF is lower than UHF. Easy!

Then go look it up in a band plan when you actually want to do moon bounce, since you'll have forgotten these freqs by then!

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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!

Another: Electrons go to the E-layer

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Which of the following frequency ranges is well suited for meteor-scatter communications?
  • 1.8 - 1.9 MHz
  • 10 - 14 MHz
  • Correct Answer
    28 - 148 MHz
  • 220 - 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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Which of the following is a good technique for making meteor-scatter contacts?
  • 15 second timed transmission sequences with stations alternating based on location
  • Use of high speed CW or digital modes
  • Short transmission with rapidly repeated call signs and signal reports
  • Correct Answer
    All of these choices are correct
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