What people refer to as "FM Radio" are Frequency Modulated broadcasts in a part of the radio spectrum known as VHF Band II.

FM Radio is a system that isn't exactly new and has been around a lot longer than people think. It was first broadcast in the USA during the 1940's, the BBC made its first public broadcasts in 1955 while RTE rolled out FM in 1966. It took quite a while for it to catch on though, even with its superior sound quality compared to Medium and Long Wave broadcasts. In the USA it was the home to more specialist broadcasters until the 1970's. In the UK the system was designed for fixed home reception initially and it wasn't until the introduction of commercial radio in the 1970's, where these stations used mixed polarisation as opposed to the BBC's horizontal only polarisation, which made the reception of commercial services easier to pick up on portable and car radios, which seen FM listernership increase. In Ireland it is accepted that unlicenced radio stations, which there were dozens across the country, helped push FM in the 1980's. Today though it is very hard to argue that in most of the developed world, the vast majority of radio listening is done through FM Radio.

How it works

The method used for broadcast FM radio (and analogue television) is known as "Wideband" FM. There is also another mode known as Narrowband FM which is used for amateur radio, two-way radio and for other private transmissions.

The current FM broadcasting system can accommodate audio frequencies up to 15kHz, which is adequate for most of the adult population. It handles this by modulating the carrier away from its nominal frequency in relation to the amplitude of the input signal. A Wideband FM signal to be transmitted however requires a wider bandwidth than the equivalent amplitude modulated signal, but it does make it robust to interference and noise better than AM.

Wavebands

The FM Radio Band that is in use in Ireland, Europe and most of the world extends from 87.5 to 108.0 MHz. However legal transmissions, at least in Ireland and the UK, have a centre frequency from 87.6 to 107.9 MHz. At these extremes such transmissions are either at low power or have strict filtering in place (maybe both) to prevent interference to PMR users and aircraft communications.

When the FM Broadcast Band was first developed, the spacing between frequencies in Western Europe was 50kHz or 0.05MHz. However in recent years this has now been set at 100kHz or 0.1MHz except in a few countries e.g. Italy.

In North America the FM Broadcast Band is similar to ours, ranging from 87.8 to 108.0 MHz, though most stations start at 88 MHz or higher. The frequency spacing between channels is set to 200kHz or 0.2MHz with the centre frequency always being an odd digit after the decimal point i.e. 89.5, 94.9, 102.1 etc.

The 525 line analogue TV system has channel 6 close to the FM broadcast band with its audio centred on 87.75MHz, meaning that most radios can pick up the audio if a TV programme transmits locally. Also for this reason there are only a couple of low powered transmissions below 88 MHz, these existing in areas where there are no TV transmission on channel 6.

The original FM broadcast band in the USA was between 42 and 50 MHz which was set by the FCC in 1940, but this was then moved to the current allocation in 1945.

In Japan a separate part of the VHF Band has been developed between 76 to 90 MHz which exists to this day, with the frequencies between 90 and 108 MHz used for TV broadcasting.

In Russia and Eastern Europe (except for the former East Germany and Yugoslavia) and band from 66 to 74 MHz was used. However following the collapse of communism in these countries, the 87.5 to 108 MHz band was adopted. Some of these countries still however broadcast in the 66 to 74 MHz band though the amounts of stations here continue to decrease.

Coverage

The coverage of an FM Radio station depends on quite a few variables, including the location of the transmitting aerial, the radiated power, the height of the receiving aerial and the sensitivity of the receiver among others.

At best a high-powered station transmitting from a high mountain site on a tall mast could serve a distance of 100km line of site quite comfortably. On the other hand a small station with only a handful of watts of power with the aerial 10 metres above ground may only get out a few kilometres.

The important thing to remember is that usually FM Radio is restricted to line of site (LOS) between the transmitter and the receiver, plus a little more due to refraction of radio waves bending slightly around the curve of the earths surface.

If you are on a mountain top on a clear day and can see a town 50km away on the edge of the horizon, placing a transmitter and aerial where you are with say a 5kW transmitter broadcasting, you should reach the town and a little bit beyond it, say 60km.

Jack up the power to say 20kW and you might be able to gain another 5 to 15km (estimate) provided there are no large objects in the way. This is because when the radio waves go over the horizon, refraction "scatters" the radio waves, weakening them in the process.

This area beyond 60km is what is regarded as a "fringe" area and the slight increase in power compensates the scattering losses.

However there is a downward curve regarding the extra over the horizon distance you can obtain by increasing your output power, to the point where increasing your output power does not get your signal out any further. Jacking the power up to 100kW may then gain you up to another 10km over 20kW or maybe none at all, and increasing it up to 1000kW is unlikely again to offer any significant distance from 100kW.

Of course, at the same site if you just use a 10W transmitter into an aerial you are unlikely to reach to town 50km away as by the time the signal reaches it, it'll be too weak to be received having been drowned out by atmospheric noise.

To be added further...

* FM Stereo Radio
* Radio Data System (RDS)
* Atmospheric effects on FM radio
* Using directional aerials to broadcast
* How to improve your reception at home & in the car

Originally written by Northern Correspondant on ICDG Wiki.