T3C

Propagation beyond the horizon

Explain how signals get past the horizon: refraction, the ionosphere, ducting, sporadic E, meteors and aurora.

11 pool questions1 on the exam

Syllabus: Propagation modes: sporadic E, meteor scatter, auroral propagation, tropospheric ducting; F region skip; Line of sight and radio horizon

VHF and UHF go in a line, a little past the horizon

VHF and UHF travel in nearly straight lines. The atmosphere refracts (bends) them slightly downward, so the radio horizon is farther than the visual one.

Exaggerated side view. The atmosphere refracts (bends) radio waves a little.

A sharp edge, like a ridge top, can also bend some signal around an obstruction: knife-edge diffraction.

Knife-edge diffraction: a sharp edge bends some of the signal around the obstruction.

RememberRadio horizon is farther than the visual horizon, thanks to atmospheric refraction.

HF bounces off the ionosphere; UHF usually doesn't

Schematic side view. The ionosphere is a high layer of electrically charged air.

HF waves are bent back to Earth, so long-distance ionospheric propagation is far more common on HF. VHF and UHF usually pass through, so simplex UHF signals are rarely heard beyond the radio horizon.

RememberNo ionospheric bounce means no signals beyond the horizon, at UHF.

10 m and 6 m skip when sunspots are high

More sunspots and more sunlight make the F region denser, so it can bend higher frequencies back to Earth.

Schematic. More sunlight and more sunspots make a denser F region.

Band	F-region skip at the sunspot peak
10 m, 6 m	Yes
1.25 m, 70 cm, 23 cm	No

RememberBest for 10 m: dawn to shortly after sunset, during high sunspot activity.

Ducting: a temperature inversion traps the signal

Normally air cools with height. In a temperature inversion, warm air sits over cooler air. VHF/UHF is trapped in the cooler layer and follows the ground, regularly reaching about 300 miles.

Schematic side view of the lower atmosphere (the troposphere).

RememberTropospheric ducting is caused by temperature inversions.

Sporadic E, meteor scatter and aurora

Schematic side view. Each mode gets a signal past the horizon.

Mode	Remember
Sporadic E	Occasional strong signals on 10, 6 and 2 m
Meteor scatter	Best on 6 m
Auroral backscatter	Raspy, distorted sound

RememberStrong, occasional signals past the horizon on 10, 6 and 2 m: sporadic E.

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