Today we will talk about the upper mediumwave band and the very lowest shortwave bands. This part of the spectrum is often called the "marine band", used for communication with ships at sea. In The Netherlands this band was often called the "Visserijband", which means fisherman's band. Although this band is still mediumwave (MF), on many radios it is already called shortwave. In the 1950s and 1960s you could buy radios on which this part of the spectrum was really labeled "Visserijband" or even "Visserijgolf" (Fisherman's wave). This section of the spectrum also contains two tropical broadcast bands and an amateur band.
Signals in this band reach a comparatively short distance during the day, still being mostly absorbed by the D layer. At night, worldwide communication is sometimes possible.
The band below 500kHz was used for morse telegraphy. Communication with ships was the very first application of radio ever. The Dutch coastal station Scheveningen Radio started already in 1904 (morse telegraphy only). Later, the upper mediumwave band was mainly used for radio telephony (voice communication). Scheveningen Radio (call letters PCH) was also active on this band. This band is mainly used for relatively short distances. Higher parts of the shortwave spectrum are also used for maritime communication, but at longer distances.
Before 1920 most experts thought that any wavelengths below 200m were basically useless for long-distance communications. In the USA this band was free for all to use. In the early 1920s, radio amateurs discovered that long distance communication was indeed very well possible on wavelengths below 200m. The first long distance amateur experiments took place between 100m and 200m (1,5 and 3MHz), this is in this part of the spectrum.
When Radio Kootwijk was still busy constructing a large VLF transmitter to send signals to the Netherlands East Indies, radio amateurs had already discovered that shortwave signals could also reach that far, but at a tiny fraction of the cost. A few tens of watts were sufficient to reach distant places instead of a few tens of kilowatts. Transmitting antennas were also much smaller. As an added bonus, shortwave offered much more bandwidth and hence telephony was possible in addition to telegraphy. The downside was that propagation on shortwave is very variable and you need to carefully select the correct band. Later, Radio Kootwijk switched to shortwave transmitters. Radio amateur really pioneered long distance communication on shortwave. As a recognition of this, radio amateurs got their own bands when the valuable shortwave spectrum was divided among all users.
In most European countries, amateurs were not allowed to transmit on shortwave. A few amateurs in The Netherlands still did it and they were prosecuted for it and their equipment was confiscated. However, the equipment was later returned to them and they got eventually recognized for their experiments. In 1929, radio amateurs were allowed to obtain licenses in The Netherlands.
This band is not as suitable for long distance communication as the real shortwave bands. Before VHF became widespread, this band was often used for short range communications. In the 1930s the band just above the mediumwave was used for police communications in the USA. Much later (around 1970) the same band got used for cordless telephones (1600-1800kHz).
Marine radiotelephony was introduced in the 1930s and it used amplitude modulation until around 1980 (SSB afterwards). The international distress frequency for telephony was 2182kHz. Since the late 1990s this band is much less used for marine communications, especially in Europe. Most has been taken over by satellite communication. Nearly all coastal radio stations have disappeared, Scheveningen Radio stopped in 1999 and its building (in IJmuiden) has since been demolished.
When marine radiotelephony (and telegraph) was still widely used, there were elaborate official procedures for radio communication. There were periods of radio silence where all ships had to stop transmitting (except when you had a real emergency) and were required to monitor the distress frequency. Scheveningen Radio transmitted weather bulletins on certain frequencies, they handled distress calls and they also facilitated telephone conversations (from the public telephone network) to ships.
Let's go back to morse telegraphy, once the main mode of sending messages over the radio, nowadays only used by radio amateurs. Before 1920 most transmitters were spark gap transmitters that transmitted a very noisy signal. When you listened to the signal on a crystal set you could easily hear the dashes and dots of the morse signal. When the signal was on, a distinctive noise could be heard.
Machine transmitters and vacuum tube transmitters gave off a much cleaner signal. When the operator pressed the key, the transmitter would send an unmodulated carrier (this was called continuous wave). An unmodulated carrier sounds like silence, as does the absence of a carrier. So on a crystal set you could hear the clicks when the signal went on and off, but no longer the distinctive tone patterns of a morse signal.
Some transmitters modulated the transmitted signal with a tone, so AM detectors would again give off an audible morse signal. But for most professional applications, the receivers had to be adapted. The receiver had to contain an oscillator that gave off a signal whose frequency was close to the received CW signal. When both signals were present and were fed to a detection diode, the difference frequency could be heard. This oscillator was called the Beat Frequency Oscillator (BFO) and in a superheterodyne receiver it worked at the Intermediate Frequency (IF). Since the 1930s, communications receivers have always contained a BFO, while most broadcast receivers do not have it. Old-fashioned regenerative receivers could be made to oscillate close to the received frequency and you could hear CW transmissions with them.
Until around 1980, marine radiotelephony used AM. If you analyze the frequency spectrum of an amplitude modulated signal, you seen a large peak in the center (the carrier) and the spectrum of the modulated audio on each side of the carrier (the side bands). So if you modulate a tone of 1kHz on a carrier frequency of 1850kHz, you see a large peak at 1850kHz and smaller peaks at 1849 and 1851kHz. Now all the audio information is in each of the side bands. The carrier itself contains no audio information, yet it contains over 50% of the transmitted energy. So if it would be possible to transmit just one side band, we would convey all the audio information in half the bandwidth and with less than half the transmitter power.
The amazing thing is that this is indeed possible (and this is called single sideband modulation or SSB). The problem is that a normal receiver can no longer demodulate the signal. Now if the receiver is equipped with a BFO, we can tune the BFO to exactly the frequency of the missing carrier. Then the AM detector gets an intelligible signal out of that. It is not really ideal, but it gets the job done. A product detector works much better and all modern communications receivers have such a detector (or the software equivalent). As we transmit just one sideband, we can choose which one we transmit: either the lower sideband (LSB) or the upper sideband (USB). For historical reasons, radio amateurs on the 160m, 80m and 40m amateur bands use LSB, all others (amateurs on other bands, marine, aeronautical) use USB.
Tuning an SSB signal is very critical. If you tune an AM radio 1kHz off frequency, this is no big deal. To tune to an SSB signal, the local oscillator (BFO) has to be tuned within 50Hz of the missing carrier to get intelligible speech, much closer if you value natural sounding voices or even music.
My first radio with this band (a large transistor portable with mediumwave and four shortwave bands covering all the spectrum from mediumwave to 30MHz) did not have a BFO. It would have been fairly easy to construct a BFO for it (any old junk radio contains all the parts you need), I could still listen to SSB. It was 1981 and the switch to SSB was just about complete. An ordinary mediumwave radio contains a local oscillator to mix with the input signal to obtain the IF. So if you tune the radio to 1008kHz and the IF is 455kHz, the local oscillator has a frequency of 1008 + 455 = 1463kHz. So if you put another radio next to it and tune that to 1463kHz, you get an unmodulated signal on that other radio. Try tuning the other radio to a weak station and then tune the first radio so it interferes with the weak station. You can hear a beat note.
An ordinary mediumwave radio can tune its local oscillator up to about 2050kHz. So by putting it next to my transistor portable, I could indeed demodulate SSB signals between 1600 and 2050kHz. Many tube radios had a local oscillator that was powerful enough to use the second harmonic as a BFO. So if I wanted to listen to an SSB signal on 3700kHz, I tuned the transistor portable to 3700kHz. The local oscillator of the tube radio had to be 3700/2 =1850kHz. If the IF was 455kHz, it had to be tuned to 1395kHz. If the tube radio had a shortwave band, it was possible to listen to amateurs on 40m and 20m as well. This only works if the other radio has continuous (analog) tuning, not with synthesized radios that tune to the exact channels only. Tube radios work OK.
In the 1950s and 1960s some broadcast radios had the marine telephony band. In The Netherlands it was often called "Visserijband" (fisherman's band). In this country it was (and still is) legal to listen to all radio communications. This range often covered 60-187m (1611-5000kHz) or part of it. In other countries the same range was often called the tropical broadcast band. As these radios have no BFO, you cannot hear SSB transmissions on them, at least not without some tricks.
Many world receivers and almost all communications receivers have this range as well. Portable world receivers use a ferrite rod antenna for mediumwave, but switch to the internal telescopic whip antenna for this range, which performs much worse. External (outdoor) antennas are often necessary, especially when trying to receive tropical broadcast stations.
In Italy it was illegal to sell radios covering this band to the general public. This restriction is somewhat contrary to the EU regulations stating the legality of receivers. Mind you, there are two tropical broadcast bands and two amateur bands in the blocked section. World receivers made by Sony had special versions for the Italian market with the band between 1611 and 3850kHz removed (no 80m band either). The Sony ICF-SW7600G, introduced in 1992, still had an Italian version with this restriction. Its successor, the ICF-SW7600GR does not have a restricted version, but there is no Italian language section in its manual. I guess this model is not "officially" available in Italy. Probably the law still exists, but these days nobody obeys it anymore.
The entire radio spectrum is carefully divided into bands, each of which is allocated to a specific service. A large part of this range happens to be allocated to the marine communication service, but other services (such as fixed and mobile services and air traffic) also have their specific bands throughout the spectrum. The military also make use of this band, either channels that are officially allocated to them or frequencies not otherwise used. If the information has to be really secret, they make sure that this communication is encrypted. All non-broadcast non-amateur stations are often called utility stations. In many countries it is officially illegal to listen to these stations, but on shortwave this is hardly a problem. So even in countries where it is officially illegal, you have shortwave listeners specializing in utility stations.
What's certainly illegal is transmitting on frequencies that you are not allowed to use. Pirate stations can be heard mainly between 1611 and 1700MHz (AM, playing music), and between 3400 and 3500kHz (SSB, speech only).
In tropical regions, reception on mediumwave is often plagued by atmospheric noise from thunderstorms. At higher frequencies this is less of a problem. Therefore three tropical bands are allocated, two of which fall into this part of the spectrum.
|120m||2300-2495kHz||Tropical broadcast band|
|90m||3200-3400kHz||Tropical broadcast band|
At night and with a good antenna, it is possible to hear tropical broadcast stations in this part of the world. On 120m it is much harder than on 90m (on 60m it is still easier). These are not (supposed to be) international broadcasts, so they are in the language of the country of origin. Note that in this part of the world, these bands are not allocated to broadcasting and are used by other services (marine, air traffic).
Before World War 2, the band between 1800 and 2000kHz was allocated to amateurs in the USA. After the war, this band was mostly taken away, to be used by LORAN navigation. When it was replaced by LORAN-C (which uses frequencies around 100kHz) in 1980, amateurs regained the full use of the band. In the 1980s this band was also allocated in many European countries. In Europe there is still wide variability between countries. In The Netherlands, only the part between 1810 and 1880kHz can be used. This band has very little activity during the day. In this band it is still very hard to construct an efficient transmitting antenna. A quarter wave vertical would be 40m high, much higher than an amateur is allowed to build in most cases.
See you next time when we discuss the lower shortwave bands.
I added several diagrams.