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Improve DAB Reception
The UK DAB digital radio service was introduced in 2005 and DAB coverage now covers 90% of the UK for the BBC Digital One multiplex, which includes all of the popular BBC radio stations. This guide provides tips to improve DAB reception and helps you choose the best aerial for use in fringe or poor signal areas.
Investment in the UK DAB infrastructure has accelerated recently with 13 new local DAB multiplexes recently launched. It is encouraging to see that the much-overdue regional DAB services for Oxfordshire as well as Northamptonshire, Bedfordshire and Hertfordshire (Block 10D) are now operational.
Despite the bad press over DAB audio quality been worse than high quality FM reception, the UK's DAB digital radio platform still offers some advantages when compared to FM transmission, particularly where FM signals are weak. DAB benefits from:
Across England all BBC National DAB services are broadcast in "ensembles" on Channel 12B (225.64MHz) and the National Commercial radio multiplex is on Channel 11D (222.06MHz) in England and 12A in Scotland. Stations broadcast on these multiplexes are available to 90% of the UK population.
Independent local radio stations with regional variations (including BBC local radio stations) are transmitted on separate DAB multiplexes. The frequency allocations are designed to minimise co-channel interference between regions. this means that reception of DAB stations from several different regional transmitters may be possible with a suitable high gain DAB aerial.
Extending DAB to the more rural areas of the UK now looks increasingly uncertain and the widespread complaints over DAB's inferior audio quality compared to traditional FM broadcasts haven't helped DAB's market share to grow.
To reduce transmission costs and increase the number of stations that can be broadcast in each DAB multiplex, transmission bit rates used across the UK are currently far too low to give high quality sound.
DAB audio quality is particularly poor in London where a number of the London DAB stations are transmitted in Mono at a ridiculously low 64Kbp/s data rate, when 192Kbp/s is required to match the sound quality of FM.
There are still rumours that the outdated MPEG2 compression system which the UK DAB system uses, will at some point be replaced by AAC - a much improved digital compression system which allows CD quality sound at much lower bitrates than the present MPEG2 system. This improved version of DAB, known as DAB+, is already in operation in a number of European countries.
The big obstacle to DAB+ is the instant obsolescence of nearly all of the DAB equipment in current use, apart from some Pure DAB Radios, which allow a firmware upgrade for DAB+ and are more future-proof.
The Digital Britain Report cited the possible change in DAB encoding, but also made comments regarding the 2 million DAB radios in current use in the UK which would be rendered obselete by such a change and the need to maintain service to these users.
An up to date list of UK DAB transmitters will help you assess DAB coverage in your particular region. A UK map showing DAB multiplex channel allocations may also help if you are interested in assessing reception of more distant DAB stations and DX'ing.
DAB uses part of the VHF spectrum vacated by the now obselete VHF Band III TV service. The frequency range is 174.928MHz to 239.200MHz and the spectrum is split into 'Levels' in which digital radio multiplexes are transmitted.
UK DAB transmitters operate in the frequency range 9A to 12D, which means DAB aerials can be optimised for a relatively narrow frequency range of 202MHz to 229MHz. The table (left) shows the frequency of each DAB Multiplex.
The BBC Digital One service is transmitted on the same frequency country-wide (12B - 225.648MHz).
The new regional DAB service for Oxfordshire is now operational on Block 10B from the Beckley Transmitter.
Local DAB services for Northamptonshire, Bedfordshire and Hertfordshire launched in 2013 on Block 10D - from the Sandy Heath, Bow Brickhill (Milton Keynes), Chepping Wycombe (High Wycombe) and Zouches Farm (Luton) transmitters.
Directional DAB Aerial
Since the UK DAB radio service incorporates digital error correction technology, good reception of your local transmitter may be possible by using the telescopic aerial provided with many portable DAB radios.
For good DAB reception, aerials should be mounted with vertical polarisation (I.E. with the rods vertical) to match the polarisation of the transmitter.
As with all digital services, reliable DAB reception requires enough signal margin to avoid signal dropout or audio break-up, which generally results in a rather irritating burbling sound.
To improve DAB reception in weak signal areas an external aerial is recommended. Alternatively, installation of a high gain indoor DAB aerial in the loft (attic) may significantly improve reception.
The author uses a DAB 5 Element Aerial which is directional and delivers around 50% more signal than an Omnidirectional DAB Dipole. Signal attenuation from roof tiles at Band III VHF frequencies is somewhat less than at UHF TV, so indoor DAB aerials can work well, especially when they point through tiles alone.
Directional DAB aerials, (also known as 'beam' or 'yagi antennas') are ideal to improve DAB reception in poor signal areas or for out of region reception.
The author gets good reception of London DAB multiplexes in Newbury, West Berkshire using an indoor Blake 5 element yagi mounted in the loft (see picture left) with a Bit Error Rate of around 0.007 maximum, or 94- 96% signal quality according to my Pure DAB Evoke 2S radio.
DAB utilises a single frequency network (SFN) for National Services, but regional UK DAB transmissions currently occupy other channels. This means that long distance DAB reception (DAB DX) is technically feasible for UK regional and local DAB radio stations.
However DAB DX tends to be hampered by co-channel interference problems, especially during high atmospheric pressure.
For example the author's London DAB reception can suffer a high bit error rate under these circumstances and on occasions drop out, probably due to the interference from other DAB transmitters transmitting the London multiplexes, like Crystal Palace. The Guildford DAB transmitter offers the best reception in Newbury.
My first taste of DAB DX came on October 27th 2009, when good reception of Leicester Sound and BBC Leicester from Copt Oak was possible in Newbury on my Pure DAB Radio with its built-in telescopic aerial (at a distance of approximately 100 miles).
Written by Steve Larkins.