E3B: Advanced Propagation Modes

E3B covers ionospheric propagation mechanisms beyond basic skip: transequatorial propagation (TEP), long-path propagation, ordinary and extraordinary ionospheric waves, chordal-hop propagation, sporadic-E, and ground-wave propagation characteristics.

Each of these modes operates through a different physical mechanism with specific geographic, frequency, and time-of-day constraints. The Extra exam tests whether you can identify the correct conditions and effects for each mode.

Transequatorial Propagation (TEP)

Transequatorial propagation (TEP) is a mode of ionospheric propagation that occurs across the geomagnetic equator. TEP is most likely between points separated by 2,000 to 3,000 miles over a path that runs perpendicular to the geomagnetic equator — meaning the path crosses the equator rather than running parallel to it. The geomagnetic equator is slightly offset from the geographic equator.

The approximate maximum range for TEP is 5,000 miles — significantly farther than the 2,000–3,000 mile typical occurrence distance. TEP most commonly occurs in the afternoon or early evening — not in the morning, not at noon, and not late at night. The physical mechanism involves the enhancement of the equatorial ionosphere by the convergence of geomagnetic field lines, creating a layer of elevated ionization that supports refraction of VHF and higher HF signals across long paths.

TEP is valuable for VHF operators because it can support propagation on 6 meters and sometimes higher frequencies across equatorial paths — far beyond what normal skip propagation provides at those frequencies.

Long-Path Propagation

For any two points on Earth, there are two great-circle paths connecting them: the short path (the shorter arc between them) and the long path (the complementary arc going the "long way around"). Long-path propagation uses the longer arc. For this to be useful, the ionosphere must support propagation across that longer route — which generally requires a combination of favorable F2-layer conditions and a path that remains in darkness (since darkness preserves F2 ionization while avoiding D-layer absorption).

Long-path propagation is most frequent on 40 meters and 20 meters. These bands combine moderate critical frequencies with long skip distances and remain useful through periods of darkness. The 160m and 80m bands work in darkness but have shorter skip distances. The 10m and 6m bands require high solar flux for F2 skip and are not typically associated with long-path propagation. Long paths work best where the path remains entirely in darkness.

For 160-meter long-distance propagation, the path most likely to succeed is one entirely in darkness — because at 1.8 MHz, the D-layer daytime absorption is extremely high, and signals on a path that passes through sunlit ionosphere are heavily attenuated.

Ordinary and Extraordinary Waves

When a radio wave enters the ionosphere, the presence of Earth's magnetic field splits it into two independently propagating waves: the ordinary wave and the extraordinary wave. These are independently propagating, elliptically polarized waves created in the ionosphere through the interaction of the radio signal with the geomagnetic field and the ionospheric plasma.

The two waves travel at slightly different velocities and refract at slightly different angles because the geomagnetic field makes the ionosphere anisotropic (different properties in different directions). The ordinary wave propagates roughly as if the magnetic field were absent, while the extraordinary wave is significantly modified by the magnetic field. This splitting is not about long-skip vs. short-skip, not about long-path vs. short-path, and not about reflected vs. refracted rays — it is a fundamental plasma physics phenomenon occurring within the ionosphere.

Elevation Angle and Skip Distance

The elevation angle of an antenna (the angle above the horizon at which signals are transmitted) determines how far each ionospheric skip travels. Lowering the elevation angle — transmitting closer to the horizon — results in signals that enter the ionosphere at a shallower angle. When a signal strikes the ionosphere at a shallower angle, it travels farther through the ionospheric layer before being refracted back toward Earth, and it emerges at a greater distance from the transmitter.

The effect of lowering the transmitted elevation angle is that the distance covered by each hop increases. This is why low-angle antennas are preferred for DX work on HF — each ionospheric bounce covers more distance. Higher elevation angles produce shorter skip distances (closer signals). The MUF is not directly affected by elevation angle, and Faraday rotation and critical frequency are separate phenomena.

Chordal-Hop Propagation

Chordal-hop propagation is a mode in which signals undergo successive ionospheric refractions without an intermediate reflection from the ground between hops. In normal multi-hop propagation, a signal is refracted down to Earth, reflects off the ground, travels back up to the ionosphere, and refracts again — each ground reflection absorbs some signal energy. In chordal-hop propagation, the signal is refracted back upward within the ionospheric layer itself and continues its journey without touching the ground between hops.

The effect of chordal-hop propagation is that the signal experiences less loss compared to multi-hop propagation that uses Earth as a reflector. By avoiding ground reflections, the signal retains more of its energy over a long path. Chordal-hop does not involve propagation away from the great circle bearing, does not occur across the geomagnetic equator (that is TEP), and does not describe signals reflected back toward the transmitter (that describes backscatter).

Sporadic-E Propagation

Sporadic-E (Es) occurs when localized, dense patches of ionization form in the E region of the ionosphere at approximately 90–120 km altitude. These patches are denser than normal E-layer ionization and can support propagation of VHF signals (and sometimes even higher frequencies) over distances of 500–1,500 miles — far beyond the normal range for those frequencies.

Sporadic-E is most likely to occur around the solstices, especially the summer solstice. It does not peak at the equinoxes. Within a day, sporadic-E is most common between sunrise and sunset — daytime hours — not between sunset and sunrise. The precise physical cause of sporadic-E is not fully understood, but it is associated with wind shear in the upper atmosphere creating thin, intense ionization layers.

The summer solstice peak in sporadic-E is a well-documented pattern affecting the 6-meter and 10-meter bands, making those bands unusually active during late spring and early summer when sporadic-E seasons are in full swing.

Ground-Wave Propagation

Ground-wave propagation occurs when a signal travels along the surface of the Earth, guided by the conductivity of the ground. Ground-wave propagation supports only vertically polarized signals — the horizontal component of an electromagnetic wave is quickly attenuated by the ground surface, while the vertical component follows the curvature of the Earth. This is why AM broadcast stations (which rely on ground-wave for local coverage) universally use vertical antennas.

The maximum range of ground-wave propagation decreases as signal frequency increases. At lower frequencies (160m, the MF broadcast band), ground-wave range can extend hundreds of miles. As frequency increases, the ground absorbs more energy per unit distance, and the ground-wave range shrinks. At VHF frequencies, ground-wave range is essentially limited to line-of-sight distances. This inverse relationship between frequency and ground-wave range is the key exam fact for this topic.

E3B Practice Questions