E3A

EME, scatter and wave basics

Explain EME, meteor and auroral scatter, microwave ducts, and how electromagnetic waves are built.

14 pool questions1 on the exam

Syllabus: 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

EME: both stations just need to see the Moon

Earth-Moon-Earth bounces your signal off the Moon. Any two stations with the Moon above both horizons can work each other, up to about 12,000 miles apart. The Moon's rocking (libration) gives fluttery, irregular fading.

Not to scale. Both stations need the Moon above their horizon, so they can be up to about 12,000 miles apart.

Moon distance384,000 km

Round-trip delay2.56s

Extra path loss vs perigee+1.3dB

The Moon is nearest at perigee: the shortest trip, so the least path loss.

RememberPerigee, least loss. Both stations must see the Moon.

E, H and travel are mutually perpendicular; the medium sets the speed

The electric and magnetic fields are at right angles to each other, and the wave travels at a right angle to both. How fast it goes in a medium is set by the index of refraction.

Same frequency on both sides. The medium sets how fast the wave travels.

Index of refraction, n1.5

Speed67% of c

In a circularly polarized wave the electric and magnetic fields rotate as the wave travels.

Left: looking into the oncoming wave. Right: oblique side view of E (red) and H (blue).

RememberFields at right angles. Speed = c ÷ index of refraction. Circular: rotating fields.

Meteor scatter: brief trails in the E region, 28 to 148 MHz

A meteor burning up leaves a thin ionized column in the E region. It briefly reflects signals, so contacts arrive in short bursts. The best range is 28 – 148 MHz.

Schematic. The trail fades in moments, so contacts come in short bursts.

RememberMeteor trails: E region. Best 28 – 148 MHz.

Aurora: severe storms, and CW works best

Looking down on the north polar region. Beams are aimed north at the aurora.

Severe geomagnetic storms push the aurora toward the equator, where it can scatter signals. The reflection garbles voice, so CW is the best mode.

RememberAurora needs a severe geomagnetic storm. Use CW.

Microwave ducts form over large bodies of water

Schematic side view. The duct is a thin layer of air that traps the signal.

A layer of air over water traps microwaves. Typical range is 100 to 300 miles.

RememberOver water, 100 – 300 miles.

When darkness lowers the MUF, move to a lower band

Illustrative shape, not real data. MUF is lowest before dawn and peaks in the afternoon.

The maximum usable frequency (MUF) falls after dark. When it drops below your frequency, the signal is no longer refracted back. Switch to a lower frequency band.

RememberMUF falls at night: go lower.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 14 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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