Propagation · Entry 01.3 · the flagship

One transmitter, two coverage areas, and a layer of atmosphere that keeps office hours

An AM station that fades at the edge of town in daylight can be received several states away after dark. Nothing at the site changed. The lowest layer of the ionosphere, which absorbs those frequencies while the sun is up, thinned out.

That single fact is why medium-wave licences carry different day and night powers, and why some transmitters swing their pattern after sunset. Everything in this register follows from it.

Read the entry

Green and purple aurora glows through clouds above a dark shoreline at dusk — Dusk at a transmitter site. Within an hour or two of this light the absorbing layer has recombined away and the sky path is open.Photo: Susanne Jutzeler, suju-foto / Pexels

A licence is not one station. It is a daytime station and a night-time station that happen to share a mast, a log and a call sign.

The register

How a signal gets there, band by band

Each row is a propagation mode: what carries it, what stops it, and whether the hour of the day changes the answer. Country-specific rules are named in the entries, never here.

Mode	What carries it	Band	What limits it	Day / night
Ground wave	Surface conduction along the earth itself	LF · MF	Ground conductivity, distance, frequency	Unchanged
Skywave	Refraction in the E and F layers, returning to earth	MF · HF	D-layer absorption while the sun is up	Night only, at MF
Line of sight	The direct path from antenna to receiver	VHF · UHF	Terrain, structures, antenna height	Unchanged
Multipath	The same signal arriving twice by different routes	All	Reflectors: hills, buildings, water, aircraft	Situational
Tropospheric	Bending and ducting in the lower atmosphere	VHF · UHF	Weather: temperature inversions and moisture	Weather-led

Bands are given by their conventional designations. Figures, powers and regulatory limits appear only inside the entries, with the country and the source named.

A guyed mast against flat sky with its base insulator visible — The mast · Entry 03.1
A guyed mast with its base insulator visible — the joint that separates the radiator from the earth it stands on.Photo: Anthorn Radio Station central mast (geograph 3354038) · Wikimedia Commons

Drawing · the only one in the register

Cutaway: the base of a series-fed mast

Above ground it is steel and guys. The part that decides the pattern is mostly below the grass: a fan of copper radials acting as the other half of the antenna, and an insulator small enough to hold in two hands separating the whole tower from the earth it stands on.

Schematic. Not to scale; radial count and layout vary by site.

The mast is the antenna →
Ground radials →