AM/FM Reception

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Radio Signal

The radio signal is sent from a broadcast station and is then received by an antenna. The strength of the signal received depends on the following:

The power output (wattage) of the broadcasting station
The location of the vehicle (or receiver) relative to the broadcast tower.
Obstacles between the tower and the receiver
Atmospheric conditions
What band (AM or FM) the station is broadcasting
Type of antenna and the ground plane

AM Reception

The AM band has a lower frequency range than the FM band. These longer wavelengths:

Bend around Obstacles
Follow the curvature of the earth
May reflect off the ionosphere (skip)

The AM frequencies have longer range due to the ground wave. The ground wave follows the curvature of the earth and is effected by its conductivity. Greater conductivity equates to less signal loss thus transmission over water is better than over land. The AM band has a range of 80-320 km (50-200 miles).

FM Reception

The shorter wavelengths of the higher frequency FM band:

Reflect off obstacles
Are absorbed by the ground
Penetrate the ionosphere

Broadcasts in the FM band are limited to line of sight reception which is typically 40 km (25 miles). Even when out of a direct line of sight, the signal may be reflected into areas that would be in a shadow otherwise. Factors which affect the line of sight include:

Height of the broadcast antenna
Height of the receiving antenna
Terrain and buildings in the broadcast path