Column 14: From VHF to UHF (174-400MHz, 2010-12-15)

Today we discuss the upper part of the VHF spectrum and the lower part of the UHF. Roughly speaking this consist of the high VHF TV band and the military air band.

Band diagram 174-400MHz


Almost every country around the world has or had a TV band around 200MHz. In ITU region 2 (America), this runs from 174 to 216MHz and contains 7 channels (numbered 7-13), each 6MHz wide. In most European countries this runs from 174 to 230MHz and contains 8 TV channels (numbered 5-12), each 7MHz wide. This band is called band III (where band I is the low VHF TV band, band II is the FM band and IV and V are the UHF TV band).

TV DX opportunities are much rarer in this band than in band I. Sporadic E propagation is very rare, but tropospheric propagation can bring in distant stations. DAB DX is also possible. Stations from the UK have been heard in The Netherlands.

In the 1950s and early 1960s, most TV sets could receive the VHF channels only. Some of the very, very first sets could only receive the low VHF channels or just one fixed channel, but these are very rare. Most sets had a large rotary knob to switch between the 11 or 12 available VHF channels. These were fixed frequency channels, but some fine tuning was possible. Later TV sets would come with a band switch (I, III and UHF) plus a conventional rotary tuning knob for continuous tuning within each band. Around 1970 many TVs sold in The Netherlands had an all-mechanical preset mechanism (not unlike the one found on some car radios). You have six push-buttons, each of which could be adjusted to any channel in any band. Adjustments of these presets was cumbersome, but once the channels were pre-set, changing to any of the six channels required just push of a button. This system was soon replaced with an all-electronic version, where adjustment was achieved with tiny switches and potmeters. This was just as cumbersome to use, but expensive precision mechanics could be replaced with cheap electronics.

In the UK this band has not been used for TV since the mid 1980s, when the 405-line system was abolished. Many European countries will not use this band for digital TV, so DAB will not have to compete for spectrum with TV, once the analog TV stations have been shut down.

DAB (Digital Audio Broadcasting) was developed in Europe in the late 1980s and the standard was finalized in 1995. Its signal is 1.75MHz wide and it occupies a quarter of a TV channel. The DAB channels are named after the TV channel that occupies the same frequency, so there are channels 5A, 5B, 5C and 5D for each quarter of TV channel 5, up to 12A, 12B, 12C and 12D for channel 12. Six additional channels (13A-13F) are defined, so the band can be extended to 240MHz.

DAB uses COFDM (Coded Orthogonal Frequency Division Multiplexing), which means that the channel is divided into a large number (typically 1536) of narrow band carriers, each one carrying a low bitrate data signal. The frequencies of the carriers and the bitrates have been chosen such that the signals on the carriers do not interfere with one another. Modulating and demodulating that many carriers at once is done by digital logic, i.e. the DAB receiver digitizes the entire channel and applies techniques like Fast Fourier Transform to extract the bits from all 1536 subcarriers.

DAB was designed for mobile reception and hence it is extremely resistant to multipath interference. The digital signal has error correction codes applied to it and related bits are spread both in time and in frequency, so that short dropouts (the signal disappears for a few milliseconds) and loss of a few subcarriers can be tolerated. A side effect of resistance to multipath interference is that single frequency networks are possible. Multiple transmitters transmit the exact same signal on the same channel. A receiver halfway between two stations will get the signals from both transmitters, it can cope very well with this situation.

Depending on the error correction level chosen, a DAB transmitter can transmit a bit stream of up to 1200kbit/s. This bit stream can be divided into several audio bit streams very flexibly: either a few high bandwidth channels (6 channels of 192kb/s each) or many low bandwidth channels (10 channels of 112kb/s each), or a mix of high bandwidth and low bandwidth channels. It is also possible to transmit non-audio channels, such as data, static pictures or even video. It is possible in principle to transmit encrypted programs (requiring a subscription and a special decoder), but this is not used in Europe.

The audio codec (MPEG layer 2) of DAB is already more than 20 years old. When it was introduced, it provided a spectacular compression rate compared to uncompressed CD-quality audio. To achieve close to CD quality, a bit rate of 192kbit/s is required. Quality reduction is quite noticeable at 128kbit/s and anything less is completely unsuitable for stereo music. Today there exist audio codecs that need just half the bit rate to achieve the same quality. DAB+ is a new standard that adds such a modern audio codec. This way a DAB multiplex can carry twice the number of radio programs with acceptable quality. The problem is that the majority of existing DAB radios cannot decode it. To make confusion complete, there also exists DMB (Digital Multimedia Broadcasting), which also includes an improved audio codec and the capability to transmit video. France is planning to use DMB, some countries have announced they will introduce DAB+ and other countries will stick with the old DAB standard.

Until 2005, Europe had a clear plan to use DAB as the only standard for digital radio in the VHF range. Even though its acceptance was rather slow in most countries, it seemed like a safe investment, both for broadcasters and for consumers. DAB has been moderately successful in a few countries like the UK and Denmark, but it has become a total failure in some other countries. The UK has close to 5 million DAB receivers, while there are less then 10000 in The Netherlands (I own two of them). The Netherlands has a single DAB network, covering only part of the country and carrying the six public radio channels plus a few extra channels. This network was introduced in 2004 and it has not been extended during the past six years.

At the moment there are two improved successors to DAB (DAB+ and DMB) and several other contenders, such as DRM+ and DVBH. As far as countries have made a choice among them, they have chosen one of the DAB varieties. So it seems that a radio that can be used all across Europe will need to support both DAB+ and DMB (as well as traditional DAB). But there is a danger that other European countries will adopt a completely unrelated standard. Europe has one GSM standard for mobile phones, one DVB-T standard for digital television, one FM standard for analog radio, so the last thing we want is a multitude of different radio standards in different countries.

During the past few years the Dutch government did not make any policy for digital radio and intended to leave it to the free market, but now they appear to have made a decision to adopt DAB+. Very recently, the Dutch government extended the analog FM licenses of the commercial broadcasters to 2017 under the condition that they start transmitting in DAB (DAB+) and have their networks ready to cover 80% of the country in 2015. So in the next five years we will see some investment in DAB transmitters, if these plans are not changed meanwhile.

Amateur radio

The 1.25m ham band exists only in Region 2 (America). Originally this band was 5MHz wide and ran from 220 to 225MHz. In the USA, the lower 2MHz have been taken away and allocated to other users. Later however, the band 219-220MHz was given to the amateurs again, so in the USA, the 1.25m band is 219-220MHz and 222-225MHz

Military air band

The band from 225 to 400MHz is used for military aircraft communication. It crosses the boundary between VHF and UHF. Not all countries use the entire band. In many countries the band starts at 230 or 240MHz. The part above 380MHz is used for digital landmobile communication in Europe.

AM modulation is used, like in the civil air band. The distress frequency is 243MHz. This band is used much less intensely than the civil air band and it is much wider. Frequencies are more variable (the same military airfield does not use the same traffic control frequency all the time). Therefore you need much more patience to hear anything in this band. Compared to the civil aviation band, far fewer hobbyists listen to it.

Digital landmobile communication

Throughout the world, police, fire and ambulance services are moving towards digital communications. In Europe the TETRA (Terrestrial Trunked Radio) system is used. The Netherlands was the first country to switch off analog police communications completely around 2005. Scanner listeners were a mixed blessing (they could either help or hinder the police), but the ability to get rid of them was one of the motivations to go entirely digital.

The TETRA network used in The Netherlands by police and other public services is called C2000. While TETRA is an open standard, the encryption algorithm used in C2000 is not and of course the keys are a closely guarded secret. The C2000 network uses the bands 380-385MHz and 390-395MHz, like similar networks in other European countries. In Belgium this network is called ASTRID.

While the mobile phone network facilitates conversations between two persons, the TETRA network facilitates conversations among groups of more than two persons, as was already commonplace with traditional landmobile communication. But in TETRA the operator has a much more fine grained control over which persons are in which groups. This does not depend on physical location. When it is used properly, TETRA can make sure that workers only hear communication from groups that are related to the operation that they work for and that they talk only to the appropriate group or groups. When it is not used properly, the TETRA network can be overloaded, just when it is needed most.

Most of the time, TETRA stations communicate via a network of repeaters, but there is a mode in which mobile (or handheld) radios can talk directly to one another. This mode (DMO or Direct Mode Operation) is mainly used by firemen when they work inside a building, so they can communicate with each other and call for help when they are in trouble. Being a digital system, communication is all or nothing. Inside buildings, conditions for radio communication are often marginal. With analog walkie-talkies it was at least possible to hear that someone was calling for help, even under adverse conditions. With the digital system you would hear nothing at all. This is why many firemen find the new digital system unacceptable.

Private TETRA networks also exist. They can be used by public transportation companies, large businesses and possibly taxi companies. These use UHF frequencies above 400MHz and typically they use no encryption. In theory a hobbyist with suitable hardware and software can monitor these networks.

See you next time when we discuss the lower UHF bands.

Update 2013-07-10

Europe appears to have standardized on DAB+, which is a good thing. Germany started to roll out a nationwide DAB+ network on August 1, 2011. This appears to be a success, as opposed to the old DAB stations in Germany. National radio services like Deutschlandfunk will soon be available on DAB+ throughout the country, so the AM transmissions can be stopped. Even France, which originally planned to use DMB, appears to standardize on DAB+ after all. The UK with its large installed base of old-style DAB receivers is not likely to switch to DAB+ soon, but DAB+ radios will work there anyway.

The Netherlands started a nationwide DAB+ network with the commercial stations on May 31, 2013 (due to be officially launced on Sep 1). This multiplex has 11 stations at the moment. Apparently the public broadcasters will accelerate their move towards DAB+ and nationwide coverage and will already do it by the end of 2013 instead of 2015. Several regional multiplexes are also planned with a mix of public and commercial regional stations on each of them.

Update 2016-05-10

I added a band diagram. The public broadcasters in The Netherlands did move to DAB+ as planned, making old DAB radios obsolete. In 2015 the regional networks were introduced as well. In most parts of The Netherlands you can now receive three DAB multiplexes: the public stations. the commercial stations and the regional stations.