7. Radio Wave Propagation on The HamStudy Amateur Extra Class HamBook (2024 - 2028)

7.1 Ground and Tropospheric Paths

Mon, 01 Jan 0001 00:00:00 +0000

Section 7.1: Ground and Tropospheric Paths #

Not every contact depends on the ionosphere. A UHF handheld’s signal seldom reaches far past the radio horizon, while an AM broadcast station, at a much lower frequency, carries far beyond the horizon in daytime. Both signals stay near the ground; frequency sets their limits.

Along the Surface #

The AM station’s daytime signal travels mostly as a surface wave, the part of ground-wave propagation that follows Earth’s curvature. The wave drives currents in the ground, and the ground’s resistance turns some of the wave’s energy into heat. That loss grows as frequency increases. Seawater, a good conductor, supports much better paths than poor soil.

7.2 Ionospheric Paths

Mon, 01 Jan 0001 00:00:00 +0000

Section 7.2: Ionospheric Paths #

A conversation with a distant station on 15 meters can sink into the noise around sunset, though neither operator has touched a knob. A different band may save the contact, or a different route may work. Choosing well means looking at conditions along the whole path.

Keeping a Path Open #

After sunset, ionization falls, and so does the path’s maximum usable frequency (MUF). The D region fades too, so the lower bands lose their daytime absorption.

7.3 Space Weather and Prediction

Mon, 01 Jan 0001 00:00:00 +0000

Section 7.3: Space Weather and Prediction #

A rising solar flux index is usually good news for the upper HF bands. A rising K-index is not. Both numbers trace back to the Sun, which affects radio in three ways. Steady radiation builds the ionization that returns signals, flares can add absorption within minutes, and disturbances carried by the solar wind can upset Earth’s magnetic field hours or days later. Space-weather reports track each one.

7.4 Scatter and Moonbounce

Mon, 01 Jan 0001 00:00:00 +0000

Section 7.4: Scatter and Moonbounce #

A meteor ping and a raspy auroral signal call for different operating choices. One may give you less than a second to pass information; the other roughens every word of the contact. Moonbounce adds a third challenge: recovering a faint, fluttering echo from a round trip of nearly half a million miles.

A Trail That Lasts Briefly #

For a meteor-scatter contact, the two stations need a trail in a useful position before it disperses. Friction with the thin upper air vaporizes a meteor roughly 50 to 75 miles up, leaving a long, narrow trail of ionized gas. That trail scatters energy beyond the stations’ normal terrestrial range.