E3B

Special HF paths and ionospheric waves

Explain transequatorial, long-path, chordal-hop, sporadic-E and ground-wave propagation, and what ordinary and extraordinary waves are.

13 pool questions1 on the exam

Syllabus: Transequatorial propagation; long-path propagation; ordinary and extraordinary waves; chordal hop; sporadic-E mechanisms; ground-wave propagation

TEP: across the geomagnetic equator, afternoon and evening

Transequatorial propagation links stations on opposite sides of the geomagnetic equator, on a path perpendicular to it. Typical spans are 2,000 – 3,000 miles, up to about 5,000 miles. It peaks in the afternoon or early evening.

Schematic map. Path across the geomagnetic equator, afternoon to early evening.

RememberNorth-south across the geomagnetic equator, afternoon or early evening.

Long path is the other way round; low bands need darkness

Looking down on the globe. Same great circle, opposite directions.

The beam heading and the opposite heading both reach the target. Long path is most frequent on 40 m and 20 m.

By day the D layer absorbs low bands, so 160 m wants a path entirely in darkness.

Schematic side view. 160 m favors paths in darkness.

RememberLong path: 40 and 20 m. 160 m: an all-dark path.

One signal becomes two waves in the ionosphere

Schematic. Both waves are created in the ionosphere and travel independently.

The ionosphere creates ordinary and extraordinary waves: independently propagating, elliptically polarized waves.

RememberOrdinary and extraordinary: two elliptically polarized waves made in the ionosphere.

Low angles hop farther; chordal hops skip the ground

Lower the takeoff angle and each hop covers more distance, so fewer hops are needed.

Not to scale. Schematic height of the reflecting layer, about 300 km in the numbers.

Takeoff angle10°

One hop covers about1360miles

Hops for 6,000 miles5

Chordal hop: successive ionospheric refractions with no ground reflection in between. Skipping the ground means less loss than multi-hop.

Not to scale. Side view of a long path.

RememberLower angle, longer hop. Chordal: ionosphere to ionosphere, no ground bounce.

Sporadic E: summer solstice, daytime

Schematic, northern hemisphere. Not real data.

It peaks around the solstices, especially the summer solstice, and happens between sunrise and sunset.

RememberSporadic E: summer, daytime.

Ground waves are vertical and die faster at higher frequency

Schematic. Higher frequency, shorter ground-wave range.

Frequency2.4 MHz

Ground-wave rangelongest

A ground wave follows the Earth, and it needs vertical polarization. Raise the frequency and the maximum range decreases.

RememberGround wave: vertical polarization; higher frequency, shorter range.

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

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Practise the 13 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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