FMsäteilyllä

FMsäteilyllä (FM radiation) refers to electromagnetic radiation produced by frequency modulation, commonly used in radio communications. In FM, the instantaneous frequency of a carrier wave is varied in proportion to the information signal, while the carrier amplitude remains constant. This modulation creates a spectrum that carries audio or data.

FM radio typically operates in the very-high-frequency (VHF) band, around 88–108 megahertz. The amount by which the carrier frequency deviates from its rest value during modulation, Δf, determines the signal's bandwidth. The required bandwidth is often estimated by Carson's rule: B ≈ 2(Δf + f_m), where f_m is the highest modulating frequency.

Compared with amplitude modulation, FM offers greater resilience to certain sources of noise and interference, allowing higher fidelity sound and, with stereo and data services, additional features. Stereo FM uses a 19 kHz pilot tone and a 38 kHz subcarrier carrying the left-right difference signal; RDS data can be transmitted at 57 kHz.

FM broadcasting is typically line-of-sight; signals propagate over the horizon with reflections from buildings and terrain causing multipath effects. Higher frequencies provide good audio quality but require more transmitters for wide coverage, especially in urban or mountainous areas.

Safety and regulation: FM radio emissions are non-ionizing; exposure standards limit occupational and public exposure and are generally not a concern for typical listening environments. Governments regulate allocated bands, transmitter power, and licensing to manage interference.

Historically, frequency modulation was developed and popularized in the early 20th century, notably by Edwin Howard Armstrong, and became a dominant method for high-fidelity broadcasting from the 1940s onward.