The FM broadcast band is the most popular broadcast band. It is the busiest VHF band and one of the busiest of all radio bands. Because it is such a busy band, it is excluded from the VHF spectrum monitored by the LOFAR radio telescope.
In the USA, the band between 42 and 50MHz was used for FM broadcasting in the late 1930s. In 1945 the VHF bands got completely rearranged and the FM broadcast band was relocated to 88-108MHz. In Europe, West Germany was the first country to introduce FM on a large scale. Germany lost the Second World War and as a consequence, they got allocated comparatively few mediumwave and longwave frequencies. VHF was a viable option for Germany to get more radio programmes on the air.
FM was introduced in The Netherlands in 1953, in Germany it was a few years earlier. In the early 1960s, The Netherlands had only 12 FM broadcast transmitters, today it is more than 700 official transmitters. To find out how many (and which) FM transmitters there are near your location, please have a look at www.fmscan.org.
The FM broadcast band is also called Broadcast Band II. The bands I, III, IV and V are TV broadcast bands. Originally the band was allocated in Europe from 87.5 to 100MHz. In some countries it was even illegal to sell radios capable of receiving the entire band up to 108MHz. In 1964 there were serious plans to extend the FM band and at that time, radios with an FM range up to 108MHz became legal in Germany. The band was extended, first to 104MHz in 1968, then to 108MHz in 1979. As late as 1980, radios with an FM range up to just 104MHz were still sold in The Netherlands, even though radios with an FM range up to 108MHz were legal to sell and were also available.
In the second half of the 1980s the Dutch authorities started to authorize licenses for local FM stations, most of which were allocated on frequencies between 105 and 108MHz. Until then, the only legal stations were the public national stations (three networks) and the public regional stations (one per province). Commercial stations were authorized in the 1990s. By that time the FM band had become very crowded.
In 2003 The Netherlands completely reallocated the frequencies of its FM broadcast transmitters to optimize their coverage, to reduce interference and to create room for more stations. This was called "Operation Zerobase". Originally it was planned to shut off all analog FM transmitters on Jan 1 2015 (some even targeted 2012), but given the lack of success of digital radio so far, this seems unlikely. Apparently the commercial FM licenses have been extended to 2017 already.
Some countries have (or had) TV channels in this range. These are the Eastern European countries that use the OIRT FM band and Japan, that has its own FM band. But also Australia used to have TV channels in this range.
In The Netherlands the band between 100 and 108MHz was almost completely unused, until pirate radio came along. Apparently this band was allocated to the military, but they did not use it either. In the early 1970s a transmitter of American Forces Network (AFN) was allocated a frequency of 103.5MHz, but all other official broadcast transmitters remained below 100MHz until the mid-1980s.
Apparently some other European countries used this part of the band for landmobile communication. In the UK one could hear mobile radio communications between 100 and 104MHz around 1990. In fact, the portion of the band between 104 and 108MHz was used for broadcasting earlier than the portion between 100 and 104MHz.
Around 1980, FM pirate stations started to operate in The Netherlands. They mostly used frequencies between 100 and 104MHz, as these were available on most radios and were not otherwise used. Many of the early pirates were hobbyists who wanted to get more Dutch language music on the air. They found that the official public broadcasters devoted too little time to Dutch language music. Many of them played songs on request. They broadcast their own telephone number, that you could call.
Many of those hobby pirates used transmitters built from a kit. The simplest such transmitters contained one single power transistor, which served as the oscillator as well. This power transistor ran very hot and it could be cooled by putting the heatsink in a bath of water. Meanwhile the signal frequency would drift to all edges of the band and beyond. The authorities were not amused by the interference caused by this. They devoted much manpower to pirate hunting and penalties were very stiff. For repeat offenders, all equipment found at the transmitter site, would be confiscated.
Many pirates became commercial stations. When they were caught a few times, they realized that it was better to keep the studio separate from the transmitter. This ruled out live telephone calls to request a song, but it reduced the damage when the transmitter was seized by the authorities. What happened in the studio was not illegal; only a tape was recorded there. The tape was moved to the transmitter site, where only the transmitter and a tape recorder were located. Transmitters were often located on the top of an apartment block, close to the elevator motor. Of course they ran on illegally tapped electricity.
Where I lived, in Brabant in the South of The Netherlands, most pirates had moved across the border. In Belgium it was easy to obtain a broadcast license to transmit legally on FM. These stations were supposed to be noncommercial and they were supposed not to transmit programs directed to The Netherlands, but enforcement of these rules was rather lax.
At the end of the 1980s, legal local radio stations were possible in The Netherlands. At least some former pirates went legal. All those local stations made the band more crowded, leaving less room for pirate stations. Pirate radio hasn't died out though. This summer the Dutch authorities have even intensified their campaign against pirate stations.
Many illegal baby monitors operated in the FM broadcast band. When you sell a baby monitor that works in the FM band, you do not have to provide a receiver, the buyer already has a few. Besides, there were no legal frequencies for baby monitors anyway. More recently, FM micro transmitters appeared on the market, with which you could transmit the output of an MP3 player to your car radio. First they were seized from the shops because they were illegal. A few months later, they were legalized.
In the USA, channels are 200kHz wide and all FM broadcasts have a frequency that is an odd multiple of 100kHz. So they use frequencies of 88.1MHz, 88.3MHz, 88.5MHz... up to 107,9HMz, but not frequencies in between. These channels do have channel numbers, but nobody uses them. Modern (PLL synthesizer controlled) FM radios for the American market can only tune to those channels, so they do not work well in Europe.
Europe used to have channels of 300kHz wide, originally from 2 to 43. Dials on old FM radios tend to be marked with channel numbers as well as frequencies. In the 1970s, frequencies of FM radio stations were still listed along with channel numbers (often with + or -), but this is no longer the case. Not all transmitters used the official channel frequencies. Most European countries use frequencies that can have any multiple of 100kHz, but in Italy, multiples of 50kHz are sometimes used. Most European PLL synthesizer controlled FM radios can tune to any multiple of 50kHz, so they can exactly tune to all American channels as well.
Europe and America do have different deemphasis settings though. After demodulating the FM signal, the high frequencies are attenuated. but how exactly differs between Europe and America. This only matters for Hi-fi reception though, it does not affect the ability to hear the stations.
Broadcast FM uses a much higher bandwidth than either AM or narrowband FM commonly used for landmobile communications. Increasing the bandwidth (deviation) of an FM signal increases the sigal-to-noise ratio, provided that the received signal is sufficiently strong. The bandwidth of an FM signal is roughly twice the maximum deviation plus twice the maximum modulating frequency.
In the 1950s FM was still used exclusively for quality reception inside the home. Depending on the local situation a directional rooftop antenna was either required or recommended for quality reception. For high quality stereo reception it is even more necessary.
For FM reception at home, Yagi-Uda antennas (mostly called just Yagi antennas) are very suitable. Such an antenna consists of a half-wave dipole and a few parasitic radiators parallel to it. These parasitic radiators are just metal rods, isolated from the rest of the antenna. Their lengths are just a bit larger or smaller than half a wave length. These parasitic radiators shape the directional pattern of the antenna. In this situation a horizontally polarized antenna works best. This is, the dipole and all parasitic radiators are laid out horizontally. Therefore FM transmitters in The Netherlands used horizontally polarized transmitting antennas.
For reception of just the local stations, a two or three element antenna was often sufficient, but for reception of foreign stations, a Yagi with more elements was needed. Such an antenna needed to be rotatable, so it could be pointed to the desired (foreign) transmitter.
When cable television was introduced in the late 1970s, most households no longer had outdoor receiving antennas for television. At least in The Netherlands, cable television came with a selection of national and foreign radio stations as well, so an FM antenna was no longer needed. Apparently the quality of the FM signal from the cable provider is less than could be achieved with a good outdoor antenna, but it is better than what can be picked up without an outdoor antenna. These days FM broadcast signals are mainly received by portables and car radios, both of which use vertical antennas. Therefore FM transmitters transmit vertically polarized signals these days.
As with all VHF frequencies, propagation is mostly limited by line of sight. Putting your antenna on a tall mast extends your line of sight. Sporadic E propagation occurs fairly often, but not nearly as often as on 50Mhz. Sporadic E brings in signals from a distance between 1500 and 2000km. which often means Spanish stations in The Netherlands.
Another mode of propagation is tropospheric ducting. Layers of air in the troposphere (the layer of the atmosphere below 10km) can have different densities depending on their temperatures. Especially when a layer of cold air moves over a layer of hot air, radio waves can be refracted by the transition between the two layers and thus can be bounced back to earth. At 50MHz, tropospheric propagation is not yet significant, but in the FM broadcast band it starts to get prominent. Tropospheric propagation covers shorter distances than sporadic E. When your portable radio starts receiving regional stations from remote parts of The Netherlands, from Belgium and from Germany, then you have tropospheric conditions.
Transatlantic FM DX is extremely rare, but it has occurred. Stations from the East Coast of the USA have been heard in Europe. The odds that you witness this are very slim though.
Most FM broadcast receivers are superheterodyne receivers with an IF (Intermediate Frequency) of 10.7MHz. Most FM radios can also receive mediumwave and some also receive longwave and/or shortwave. The FM and AM tuners in most radios share the same IF amplifiers, but switch the filters and the detector. The front ends for AM and FM are mostly completely separate circuits. To understand the schematic of a radio, it is best to study the situation for FM and AM in isolation. The band switches make the schematic cluttered.
Throughout the ages, the block diagram of a radio has remained constant. Tubes have been replaced by transistors and later by ICs and IF transformers have been replaced with ceramic resonators. The analog local oscillator (with a variable capacitor or inductor or potmeter and varicap diode) has been replaced with a PLL synthesizer in most radios. However, the overall block diagram has not changed. It has been in use for home radios, portable radios, car radios and Hi-fi tuners alike. The quality of the components (such as amplifiers and filters) varies a lot across the price range.
It is possible to implement an FM tuner in a single IC. Internally it uses a superheterodyne circuit, but with a much lower IF. Traditional single-chip radios required an external coil and variable capacitor (or varicap), but modern chips have a complete PLL synthesizer built in and can be controlled by a microcontroller. These radios do not perform as well as their full-size counterparts, but they are low cost and can be added to devices like MP3 players and mobile phones. More modern ICs however, such as the Silicon Labs Si473x series, do perform as well as conventional FM tuners, if not better. They include a PLL synthesizer, a quadrature mixer to mix the signal to a low-IF, AD converters, a DSP and DA converters. The DSP does all the IF filtering, demodulation, stereo decoding and RDS. They can be controlled by a microcontroller. The Si4735 contains a full-featured FM tuner (including RDS decoding) as well as a quite capable AM/SW tuner for the broadcast bands from longwave to 13m.
Some newer radios replace part the the IF filtering and demodulation with digital circuits and software. This is especially the case with radios that can also receive digital radios (HD Radio, DAB).
Broadcast FM can be received by most wideband VHF/UHF receivers and the more expensive scanners, but most of them do not perform as well as dedicated receivers for this band. Their wide band FM filters are often too wide.
When broadcast FM was introduced, the FM transmitter was modulated with just the audio signal to be transmitted. The FM demodulator also yielded just the received audio signal. This still works today. The simplest FM receivers just ignore anything above the audio range and pass the demodulated signal directly to the audio amplifier. However, FM transmitters can pass signals with a much higher bandwidth than just audio. Signals with frequencies up to 100kHz can be transmitted.
FM stereo came first. The left and right channels are added together to form the mono signal. This signal is sent directly to the modulator and a mono receiver sees just that. The difference between the left and right channel is modulated onto a 38kHz subcarrier and this is sent to the modulator as well. However, the 38kHz carrier is suppressed and a 19kHz pilot tone is transmitted instead. The 19kHz pilot tone can be transmitted more efficiently than the 38kHz carrier. The receiver has to reconstruct the 38kHz suppressed carrier from the 19kHz pilot tone and then it has to demodulate the difference signal. Early stereo decoders were complex circuits with lots of transformers and coils and they required lots of adjustments. Today we have PLL-controlled stereo decoder ICs that work just as well, require only a few external components and are very cheap. Constructing a stereo encoder, on the other hand, was still a challenge in the 1980s and for a pirate station this was a big achievement.
The high frequency difference signal (around 38kHz) is much more vulnerable to multipath reflections than the low frequency sum signal. These multipath reflections tend to cause quite an annoying type of noise. For weak signals the signal to noise ratio can be improved tremendously by switching the radio back to mono. Modern car radios contain sophisticated circuits for mitigating the noise of weak stereo signals, often by eliminating higher frequencies from the difference signal.
Speaking about FM car radios, one of the more annoying things was the need to retune the car radio on a long trip each time when you moved to an area served by a different transmitter of the same program. In the 1980s Philips had a car radio that could automatically select the best frequency for any given program. All frequencies had to be programmed manually though and if you happened to come across a strong (pirate) station that transmitted on one of the programmed frequencies, you could get the wrong station. Radio Data System (RDS) was introduced in 1992. It is a digital signal, 1187.5 bits per second, modulated onto a 57kHz subcarrier. This signal contains useful information, such as:
It is possible to transmit other digital information, not intended for the general public, on different subcarriers. The use of broadcast transmitters for such purposes is called datacasting. In The Netherlands this is apparently used to transmit time table information to displays at bus stops, so they can show expected arrival times. In the mid 1990s Seiko introduced the MessageWatch on the Dutch market. This watch contained a pager and could also show short news flashes. The necessary information was transmitted via FM broadcast transmitters..
In the USA, many FM stations carry additional audio programs on subcarriers. This system is called SCA. The extra audio subcarriers are FM modulated on frequencies between 67 and 92kHz. These audio programs can be special interest programs for certain groups. audio books for the blind and background music. As these programs are not intended for the general public, listening to them without permission is illegal.
It is also possible to transmit digital audio programs on subcarriers (which are intended for the general public) and this system is called FMeXtra. It is used on some FM stations in The Netherlands, as well as in other European countries and the USA. It is very rarely used and receivers are very rare as well. It is yet another incompatible system in the already fragmented digital radio market.
In the mid 1990s a friend of mine connected a shortwave receiver to the discriminator output of an FM broadcast receiver. This way it was possible to hear the datacasting subcarriers, as well as the RDS subcarrier, the stereo L-R signal on 38kHz and the 19kHz pilot tone. No hidden audio programs were present though.
DRM+ (the same system as DRM on mediumwave and shortwave, but with a higher bandwidth) can be used in VHF broadcast bands and it is primarily intended for use in the FM band, so it can gradually replace the analog stations. The standard is specified, but no receivers are on the market yet. So far only a few test transmissions have been carried out. It is more suitable for local stations than DAB, as a single transmitter can transmit a low-bandwidth (100kHz) signal for a single radio program.
The HD Radio system used in the USA is similar in concept, but it uses different modulation and coding. As opposed to DRM+, the HD radio transmitters transmit an analog FM signal along with the digital signal In the future however, this analog signal could be switched off. HD Radio is already deployed and receivers are already available.
It is unlikely that analog FM will be completely switched off in the near future, as there is an extremely large installed base of FM receivers and acceptance of digital radio is slow in Europe. Without a single standard and a clear European policy for digital radio, it is unlikely that digital radio will be widely accepted in the short term.
See you next time when we discuss the higher VHF bands.
Norway has already decided to stop FM broadcasting in 2017 and the UK is likely to do the same. DAB radio is already well established in both countries. If DAB+ will be successful in the next few years, many countries will likely switch off FM between 2017 and 2023.
It is likely that FM will continue to be used for local radio stations, as there is no suitable digital alternative for them yet. Both Norway and the UK will allow local FM stations past 2017.
The FM broadcast band is the only band where specialized jammers are officially allowed in The Netherlands. The system, called Flister, is used by emergency vehicles (fire, police and ambulance) to alert nearby car drivers of their presence, essentially by jamming selected broadcast frequencies and transmitting the sound of a siren on these frequencies, along with some RDS signals. Cars are soundproofed too well and some drivers tend to listen at high volumes, so they will no longer hear the sirens of these vehicles normally. So far only regional public broadcasters in the North of the country are allowed to be jammed this way, The downside is that residents who live close to the same road will also experience interference, hopefully not too frequently and only briefly if the vehicle is moving fast. The system does not work for DAB (yet).
I added a band diagram. I updated the part about single-chip radios.