This week I'll talk about the longwave band and the portion of the frequency spectrum that leads up to the mediumwave band. Technically the portion of the spectrum above 300kHz is already mediumwave (MF instead of LF), but it is typically included in the longwave band on radios that cover it.
Longwave broadcasting is mainly used in Europe and the countries of the former Soviet Union. A few stations exist in Mongolia, North Africa and the Middle East. This whole region plus the rest of Africa is called ITU Region 1. Region 2 consists of North and South America and Region 3 is the rest of the world (East Asia, Australia, Oceania). The longwave broadcast band is only allocated in ITU Region 1. Longwave broadcasting does not exist in the USA.
Radio waves in this part of the spectrum can still reach long distances via ground wave propagation, typically 1000 kilometers or more. Reception is not affected much by ionospheric conditions. If a station can be received during one part of the day, it can typically be received 24 hours per day and throughout the year. Therefore the longwave band has less DX-appeal than, say, the mediumwave and shortwave bands. However, European and North-African longwave stations can sometimes be heard on the East Coast of the USA. The part of the band above 300kHz is more affected by conditions.
The longwave broadcast band runs from 148.5kHz to 283.5kHz with channel center frequencies from 153kHz to 279kHz. Each channel is 9kHz wide, so the longwave band contains 15 channels.
Frequencies in this band are already way above the audible range. If you would amplify the signal from the antenna and feed it to a speaker, not even bats would be able to hear it. If you want to use a frequency in this band to convey audio information, the transmitter must modulate the carrier frequency with the audio information and the receiver must demodulate the signal again to recover the audio information. Broadcast stations in this band (and also on mediumwave and shortwave) use amplitude modulation. The amplitude of the carrier frequency is varied by the waveform of the audio signal. This modulation causes the radio signal to occupy a certain bandwidth, which is twice the maximum audio frequency. For longwave stations, the maximum audio frequency is 4.5kHz, making the radio signal 9kHz wide.
Demodulation is as simple as rectifying the signal and then filtering it. The simplest receivers consist of little more than a tuned circuit (consisting of a coil and a capacitor), a diode and a pair of headphones. The diode rectifies the signal (pass the positive half of the wave and block the negative half). The average value of this rectified signal is proportional to the amplitude of the carrier wave, so it contains the audio waveform. A receiver like this is called a crystal set. It needs a large antenna, but it does not need any power supply: no batteries or mains power. All the sound energy in the earphone (typically less than 1 microwatt) comes from the radio signal picked up by the antenna. In the early years of radio, these crystal sets contained a crystal detector, a crystal of some mineral and a pointed metal wire just touching that crystal. This crystal detector acted as a diode. However, it took quite a lot of patience to make the pointed metal wire touch the crystal in exactly the right way. The slightest vibration could disturb this primitive diode and the listener had to start over again. A simple schematic of a crystal set is shown below:
The diagrams below show an amplitude modulated signal, the rectified AM signal and the filtered rectified signal respectively (which is the original audio). In this example the audio signal is a sine wave and the carrier is 10 times the frequency of the audio signal.
When radio broadcasting started around 1920, there were no separate longwave and mediumwave broadcast bands. In fact there existed no broadcast bands before broadcasting was recognized as a significant use of radio spectrum that needed its own bands. In 1919, the Dutch radio pioneer Hanso Schotanus à Steringa Idzerda started his broadcasts at a wavelength of 670 meters, so it was in the segment of the spectrum that we discuss today.
During the 1920s you could hear broadcast stations roughly from 200m to 4km (75-1500kHz) with no clear concept of broadcast bands. In the late 1920s and in the 1930s, spectrum use became more tightly regulated. so the longwave and mediumwave bands (and also the dedicated broadcast and amateur bands on shortwave) emerged.
In the 1930s we already had the longwave and mediumwave broadcast bands that we know today, but there was also a small broadcast band between 400 and 433 kHz, I don't know if this was an official broadcast allocation, but broadcast stations did transmit in this range. That extra broadcast band only ceased to exist completely in 1977, when the last station in that band, Oulu Finland, left its frequency of 433kHz.
In the 1930s, The Netherlands had a longwave station in Huizen at 1875m, 160kHz. Since World War 2, The Netherlands has not had a longwave broadcast station and no Dutch language programs can be heard on this band. Since 1975 we do have a frequency allocation (171kHz), but it was never used. Longwave stations from nearby countries (France, Luxembourg, Germany and the UK) can be heard throughout the country. Radios with longwave are not uncommon in this country, but it is not a "required" feature either. In countries like France you sometimes see radios with longwave and without mediumwave, but not in The Netherlands.
After World War 2, longwave stations were allocated with a channel spacing of 9kHz. These channels were all at an even multiple of 9kHz relative to 200kHz. The station in Droitwich England was at exactly 200kHz. Then there were stations at 209kHz, 218kHz, etc. and also at 191kHz, 182kHz etc. In the late 1980s the stations were all moved 2kHz down, so they would be at an even multiple of 9kHz. Even the famous Droitwich station (always exactly 200kHz, 1500m) had to move to 198kHz. Later the Germans crudely disturbed this regular channelization, by putting stations at 177kHz and 183kHz. These two stations are only 6kHz wide (audio modulation limited to 3kHz) and they are crammed between 171kHz and 189kHz, were normally only one channel would fit. A few longwave stations have been active at roughly the same frequency since the 1930s and are still active today (Droitwich. BBC4 being one of them).
Some of the stations you can hear in The Netherlands are Deutschlandfunk (153kHz), France Inter (162kHz), BBC Radio 4 (198kHz) and Radio Luxembourg (234kHz),
As already stated, a diode can be used to demodulate an AM signal. Modern radios still contain such a detection diode, but several amplifiers and filters come before it. A good broadcast receiver should pass as much of the 9kHz bandwidth as possible and block as much of the neighboring channels as possible. Making a such a filter at a fixed frequency is possible, making such a filter that can be tuned (across the entire longwave band or mediumwave band or both) is nearly impossible. Therefore most modern radios convert the incoming signal to a fixed frequency, the Intermediate Frequency or IF. This conversion can be done by mixing the signal with the signal from a local oscillator. The local oscillator and the received station have a frequency difference equal to the IF. Such a radio is called a superheterodyne receiver.
In most radios this Intermediate Frequency is between 450 and 470kHz, 455kHz being a very common value today. While it is not completely impossible to build a superheterodyne receiver that can receive frequencies very close to or equal to the IF, building such a receiver that works well is not easy. Therefore it is no coincidence that the standard IF falls conveniently in the gap between the longwave and mediumwave broadcast bands.
In the early days of radio, most people specified the wavelength (in meters) of a radio transmission instead of its frequency (in kilohertz). If you are a professional making a band plan or designing a receiver, it makes much more sense to specify the frequency. Of course wavelength is relevant for antenna design, but for antenna design only a comparatively rough estimate is needed. However for the average radio listener it made no practical difference whether a station's wavelength or frequency was specified. Radios had analog dials and they could be marked in either meters, kilohertz or both. It depended greatly on the country which convention was more common. America and Germany were "kilohertz" countries, while The Netherlands was a "meter" country. Philips radios from the 1960s all have their dials marked in meters, while contemporary German radios had kilohertz. In the late 1970s, when digital frequency readout appeared, everyone in The Netherlands switched to kilohertz.
Speaking of radio dials. As opposed to American radios, most European radios had dials with station names on them. Of course the longwave band had only two or three rows of station names, where the mediumwave band could have 6 to 12 rows. Indeed, old radios from the late 1940s also had a few station names around 400kHz (700m).
In the late 1940s the longwave band on most radios extended to well over 400kHz, so you could tune to the broadcast stations in that part of the band. When there was only one such station left (Oulu, Finland, 433kHz), an unimportant station, longwave coverage on most radios was reduced, either to the longwave broadcast band (150-285kHz) or to the Swiss telephone line broadcasting band (150-350kHz). In the Alps, radio reception in the valleys is often problematic. In Switzerland they distributed radio programs (modulated on longwave frequencies) over telephone lines. Stick the line (with a suitable series capacitor) into the antenna input of your radio and you have undisturbed reception. Many German radios had coverage till 350kHz.
But during the 1970s it was fairly common for radios to have incomplete coverage of the longwave broadcast band. Our family had three radios (all transistorized) whose longwave coverage stopped at 260kHz. The station at 263kHz could be tuned (with the dial in the extreme position), the channels at 272 and 281kHz could not be tuned (and you could hear stations on them on other radios). In Western Europe the longwave broadcast band officially reached till 259kHz, in Eastern Europe and Russia it reached till 285kHz. Radios with wider longwave coverage were legally available, even in West Germany, so there was no legal requirement to have limited longwave coverage. It's beyond me why radio manufacturers were so stubborn to skip those channels. And those radios had enough other frequencies on which you could hear Soviet propaganda.
A broadcast radio will perform well in the longwave broadcast band, but its coverage beyond this band is often limited. Normal radios never cover the whole band up to 500kHz. Some world receivers (like those made by Sony) do cover the entire band without gaps. The best receiver for this band (especially the non-broadcast portion) would be a communications receiver, but some communications receivers only start at 500kHz or perform poorly below 500kHz.
In the USA the band between 160 and 190kHz is called the LowFER band. Hobbyists (they do not even need an amateur license) are allowed to carry out experiments with homebuilt transmitters in this band. Permitted power output and antenna size are limited though.
A large portion of the band is used for nondirectional aeronautical beacons. These are used for navigation by aircraft. They transmit their own call letters in slow morse. In the 1980s I could hear quite a few of these stations on one of our tube radios. Such beacons are still around in The Netherlands, but fewer than in the 1980s. Most of these beacons are low power and some hobbyists specialize in catching these stations. Useless trivia: the non-directional beacon receiver aboard an airplane can also tune to the mediumwave band, so pilots can navigate on AM broadcast stations (or just listen to them).
The band between the longwave and mediumwave broadcast band used to be the main band for maritime communications in morse telegraphy, 500kHz being the international distress frequency. At least in our part of the world, the use of morse telegraphy for maritime communications stopped in 1999. Our famous maritime station Scheveningen Radio has since been dismantled. Many European countries have allocated an amateur band around 500kHz. The Netherlands has a temporary amateur band between 501 and 504kHz, where only morse telegraphy is allowed.
Not all maritime use of this band has ceased though. The NAVTEX system is a system that sends warning messages to ships. It operates at 490 and 518kHz
See you next time when we discuss the mediumwave broadcast band.
Some countries (the UK, Germany) have decided to stop broadcasting on longwave and mediumwave by 2015, so we will soon miss some of the major station on this band.
We do have an official amateur band now: 472-479kHz, which was harmonized in 2012 at the WRC-12 conference. The temporary 501-504kHz allocation has been terminated in The Netherlands.
I added many diagrams. The Droitwich transmitter at 198kHz is still alive, but I don't expect it to last very long. The German stations at 153 and 177kHz are gone.