Today we discuss the upper part of the shortwave spectrum. It contains two amateur bands, one broadcast band and the citizens band.
During solar minima this part of the spectrum behaves mostly like a VHF band, where communication is mainly line of sight, but sporadic E is possible. During solar maxima it is a very interesting shortwave band in which worldwide communication is possible. The end of the HF band is at 30MHz, but there is no sudden change in the way radio waves propagate exactly at this frequency. Indeed, above 30MHz, F2 layer reflection is sometimes possible, but this is the topic of next week's column.
The oldest way to transmit audio over the radio is amplitude modulation. The amplitude of the carrier is modulated with the audio signal. This is still used for broadcasting on longwave, mediumwave and shortwave. Radio amateurs and professional users almost always use SSB instead on shortwave. There is another (analog) way to modulate a carrier with an audio signal and that is varying the frequency slightly with the signal. This is called frequency modulation. When one transmits on 26965kHz the carrier frequency can vary between 26963kHz (-2kHz) and 26967kHz (+2kHz). In this case we say that the deviation is 2kHz. For broadcast FM the deviation is much larger, up to 75kHz. FM occupies more bandwidth than AM, but it is less sensitive to atmospheric and man made noise.
On bands below 25MHz, FM is never used, but in the range 26-28Mhz it is sometimes used (as well as in the 10m amateur band). CB uses FM in Europe and in some countries there are land mobile services using FM in this band.
FM can be demodulated with an AM detector if it follows a selective filter. By tuning just beside the desired frequency, the signal is amplitude modulated by moving into and outside the passband of the filter. This is how you can receive CB conversations on a shortwave receiver without an FM demodulator. This is not ideal. A dedicated FM detector works much better. An FM detector suppresses amplitude modulation, but less so if you tune just off frequency (so you can still hear amplitude modulated signals, like air traffic communication on an FM radio).
An FM detector produces a loud irritating noise when no signal is present. Therefore a squelch circuit blocks the audio output if the signal drops below a certain level. Squelch circuits are sometimes used on AM receivers (air band receivers), but are more or less essential on non-broadcast FM receivers.
Some countries have land mobile services (such as taxis) in this part of the spectrum and this traffic can be monitored with a scanner. Scanning (automatically switching a receiver between a number of fixed frequencies and staying on the frequency as long as a signal is received) is seldom effective on the real shortwave bands, but in this part of the spectrum it can be used. In this respect the band between 26 and 28MHz is already a VHF band. During a solar maximum such signals can sometimes be heard far outside the country of origin, but it is a real challenge to identify these transmissions.
The 11m broadcast band is very little used. Recently however, it is used by some DRM stations, not for international broadcasting, but for local broadcasting. DRM signals in this band can be wider (and hence have a higher bit rate and better audio quality) than in the other shortwave bands, as this band is almost empty, at least for now. If DRM really becomes a success, this band will provide insufficient bandwidth and during solar maxima this band will not be suitable for local broadcasting anyway. Therefore it is unlikely that this band will be the main local broadcast band in the future.
|11m||25600-26100kHz||Shortwave broadcast band|
This part of the spectrum contains two amateur bands.
|12m||24890-24990kHz||Amateur band||10m||28000-29700kHz||Amateur band|
The 12m band is a WARC band. This is probably the least used shortwave amateur band. When it is open, world wide DX is possible with low power and small antennas.
The 10m band is the widest shortwave amateur band (1.7MHz). When it was originally allocated in 1930, it was even 2MHz wide. Different types of amateur users can have their own section of the band. There are separate sections for local FM use, for CW (including DX), for SSB (including DX) and for satellite traffic. At least in the past, many amateurs modified old CB equipment to use on the 10m amateur band. During solar minima, the 10m band is used as a VHF band, but during solar maxima, if offers the best DX opportunities of all shortwave bands, using low power and small antennas. You can sometimes work the other end of the world with an indoor antenna and a few watts of transmitter power.
The band around 27MHz has since long been used for ISM purposes (Industrial, scientific and medical), so this band was not very desirable for professional users. In the USA the 11m amateur band was allocated. In 1958 the Citizens' Band was allocated in the USA, at first using 23 channels. Early CB sets needed two crystals for each channel they could use: one for transmitting and one for receiving. These crystals were 3rd overtone crystals, so their fundamental frequency was around 9MHz, 1/3rd of the desired frequency. Many radio amateurs built 9MHz crystal filters for their equipment using (cheaply available) CB crystals.
Initially, CB operators needed a license (as opposed to real radio amateurs, they did not have to pass an exam) and many restrictions existed as to which channels could be used for which purpose, maximum antenna height and the maximum distance between two communicating stations. In 1977 the CB was extended to 40 channels and the license requirement was dropped in 1980. Most other restrictions have also been dropped. The lowest channel has a frequency of 26965kHz and the highest channel has a frequency of 27405kHz. Allowed modes are AM and SSB. There are five channels in this band (3a, 7a, 11a, 15a and 19a) that are reserved for radiocontrolled models and cannot be used by normal CB radios. These are used for radio controlled cars and boats. not for airplanes.
In most European countries the use of CB was illegal until 1980. Until the mid 1970s it was not illegal to possess a CB transmitter in The Netherlands, only to have an installation (complete with power supply and antenna), ready to transmit. If the authorities knocked on your door, you had enough time to disconnect the power supply and antenna plugs and you were safe. When they did prohibit the possession of such transmitters, it did not help much to stop CB transmissions. There was a real need among the people to communicate via the radio. In 1980 the use of CB was legalized. You needed to purchase a license (which cost 35 guilders, 16 euros). Legal CB sets used FM instead of AM and could use 22 channels and had 0.5 watts of power. Later the use of 40 channels (the same as those in the US) was authorized with a power of 2 watts, later 4 watts. This system of 40 channels, 4 watts and FM would later become the CEPT standard, which can be used throughout Europe.
When CB was legalized, it became tremendously popular, so the bands became overcrowded. Conversations on the band became very rude and uncivilized, so most people quit the CB a few years later. The license requirement was dropped in the late 1980s. In the 1990s the CB got a revival with packet radio, using the same equipment that radio amateurs used on the 2m band. When the Internet became affordable to the masses, this stopped too. CB is not completely dead yet, but its use has declined considerably. It is still popular among truck drivers.
Some other European countries had different standards for CB before they moved to the CEPT standard. Those countries still allow the use of the old standard alongside the CEPT standard. Germany had 12 channels with AM mode and now it supports 80 channels (the 40 CEPT channels plus 40 more). The UK used 40 channels on different frequencies. Poland used channels 5kHz lower than the standard channels (26960 instead of 26965). It is legal to sell or possess equipment that supports the other standards, but of course you must not use channels that are not authorized in your country.
The authorities in most countries do not devote most of their resources to checking whether CB operators obey the rules. Many CB operators use more channels and more power than allowed. In the USA the sale of linear amplifiers that support CB frequencies is restricted. As the 10m amateur band borders the restricted frequency range, the sale of legitimate amplifiers for the 10m band to radio amateurs is still complicated.
Most general coverage shortwave receivers can receive frequencies up to 30MHz. In nearly all countries around the world, the sale and possession of general coverage receivers for this range is permitted without a special license. On the other hand, non-broadcast VHF and UHF receivers, especially scanners, are severely restricted in many countries.
Between 1980 and 1990, West-Germany restricted the coverage of shortwave receivers sold to the general public to 150kHz-26.1MHz. Receivers sold to radio amateurs were exempt. Many popular amateur and shortwave broadcast receivers had special versions for Germany. Even my Lowe HF-225 receiver had a version for Germany. In most cases it was easy to restore the coverage back to 30MHz (e.g. by placing or removing a jumper on the circuit board), but in West-Germany it was strictly prohibited to point a screwdriver, let alone a soldering iron at any radio equipment if you were not a radio amateur or certified repairman. When Germany was unified in 1990, shortwave receivers were allowed to have a coverage up to 30MHz again. A few years later, all restrictions on coverage of receivers were removed. For a few years during the 1990s, scanning was completely legal in Germany, just like The Netherlands.
Italy had even more severe restrictions on the coverage of shortwave receivers, at least for portable receivers. You had to do without the range between longwave and mediumwave and withouh the band between 1.6 and 3.85MHz. This means that the 80m amateur band was prohibited on these radios. What a shame! I wonder when this restriction was lifted, if ever. It has to be, because the Italian firm Microtelecom markets the Perseus SDR receiver.
The Perseus SDR is a general coverage receiver that converts a large part of the spectrum between 0 and 30MHz to digital and then lets a personal computer do the rest. Advantages of this approach.
The disadvantages are:
Many VHF receivers (such as scanners) start their coverage at 25MHz, so their coverage overlaps with this part of the spectrum.
See you next time when we discuss the lowest VHF bands.
I added a band diagram.