Wed October 24, 2012
TheEC: Height is King for FM
By Aaron Read
Providence, RI – This time on the EC we'll go back to FM propagation and why are those towers are so tall? In short, it's because FM operates in the VHF - very high frequency - band, from 88 to 108 MHz. The VHF band works primarily via line of sight propagation. That means you need to "see" the transmitting antenna in order to receive the signal. If something gets in the way be it trees, a hill, buildings, etc it can block the signal. Therefore, a nice tall tower has enough height to "see" over local terrain and hills. That's also why an exterior antenna, like on the roof of your house, or a car antenna, usually works better than antennas indoors.
By way of analogy, think of an FM transmitter as a big lightbulb on top of a pole. The taller that lightbulb is, the easier it is to see that lightbulb from wherever you are. The lower it is, the more likely it is that a hill or a house or a tree might block your view.
The analogy isn't perfect, of course. You might have a small hill between you and a FM station, but you can still receive the station, even if you couldn't see the tower. That's because of the differences in frequency between FM signals and visible light. The former is in MegaHertz (MHz), the latter in TeraHertz (THz), which is a thousand times a thousand higher in frequency. For example, RIPR transmits on 88.1 MHz or 88,100,000 Hz. Visible light starts around 400 THz or 400,000,000,000,000 Hz! The lower frequency is somewhat more able to bend around, or penetrate, certain materials like hills and houses. That's also why AM radio, which transmits in the KiloHertz range (such as 1290kHz or 1,290,000 Hz) doesn't need height to reach receivers; most of AM radio's coverage comes from ground conductivity and skywave propagation, which we talked about in an earlier Engineer's Corner.
Still, the phrase "height is king" definitely applies to FM. That's why you typically see FM stations on towers at least 500ft tall (or even 1000 or 2000ft). Or if the tower itself is short, it's because it's on top of a tall hill, such as WHOM up in New Hampshire, which has a measly 50ft tower but is over 6,000ft up on top of Mount Washington, and a huge corresponding signal! Lots of other factors can, and do, influence what tower a station is on and how high on the tower it might be. But wherever possible, FM stations usually try to get more height.