Radio FM is transmitted in the Very High Frequency (VHF, 30 MHz to 300 MHz) radio spectrum. VHF radio waves act more like light, traveling in straight lines; hence the reception range is generally limited to about . During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference. High power is useful
Earth bulge Earth bulge Earth bulge is a term used in telecommunications, meaning the curvature of the Earth's surface, which limits the range of communication methods that require a line-of-sight path, like high frequency radio waves, microwaves, or lasers. Assuming the Earth is a perfect sphere with no terrain irregularity, the line-of-sight distance to the horizon from a transmitter located at a given altitude above the surface can readily be calculated. Let R be the radius of Earth and h be the altitude of the transmitter's antenna. Line of sight distance d of this station is given by the Pythagorean theorem; Since