Monday 14 November, 2005, 15:19 - BroadcastingLook at any Frequency Allocation Table (FAT) whether it's the ITU's radio regulations or a national one and there are lots of places where the use of the spectrum changes from one type to another (for example from mobile to fixed links). But what actually happens at the place where the transition takes place? The frequency which marks the transition point between two bands (or sub-bands) is unusable by the users who occupy the spectrum either side.
Let's take an example... A stereo FM broadcast transmission occupies a bandwidth of just over 250 kHz. A transmission on a frequency of 108 MHz (the band edge) would therefore have half, i.e. 125 kHz of its bandwidth inside the broadcast allocation and half inside the neighbouring (aeronautical radionavigation) band. Indeed even an FM transmission on 107.9 MHz would still occupy 25 kHz of spectrum above 108 MHz.
So who can or does use these 'no man's land' frequencies? Well in theory, no-one can, without contravening the band plan that has been put in place. But people do! In fact these frequencies (and those in the neighbourhood) are often occupied by some of the more 'interesting' signals you will find scattered across the radio spectrum.
Around the edges of the broadcasting bands is where you will often find pirate and clandestine stations: the frequency range 6200 - 6300 kHz has long been a short-wave pirate favourite. Such stations typically use much less power than traditional broadcast stations and as such could not compete if they had to battle alongside the MegaWatt signals from stations such as the BBC and Deustche Welle. However at the edge of the band where no-one lives and there are no signals, it's much easier to be heard. Take a look at some of the other frequencies used by pirate stations and you'll almost universally find that they sit roughly on the border between broadcast and non-broadcast users (Laser Hot Hits on 4025 and 9385 kHz for example).
However even the 'big boys' recognise the benefit of sitting in the quiet no man's land at the edge of the broadcast bands. Vatican Radio uses 1611 kHz (edge of the medium wave band); Radio Prague sit on 6200 kHz (edge of the 49 metre band); and Radio Pyongyang (North Korea) inhabit both 7100 and 15100 kHz (edge of the 41 and 25 metre bands respectively). Whilst in theory these stations are operating as 'illegally' as the pirates, spewing interference onto other services, they get away with it by the fact that they are 'internationally recognised'.
Take a tune around the band edges (not just the broadcast bands but other ones too) and see what you find...
Thursday 3 November, 2005, 10:36 - Pirate/ClandestineAny illegal radio transmission runs the risk of being tracked down by the relevant authorities who use sophisticated equipment to monitor the spectrum for unlicensed transmissions. Such equipment is capable of pin-pointing the location of a given transmitter to within a few hundred metres in under a second and together with mobile and hand-held equipment can track down an illegal transmitter with no difficulty. This is easier at VHF and UHF frequencies than it is at frequencies below 30 MHz, as the line-of-sight nature of propagation at these higher frequencies simplifies the tracking operation. At frequencies below about 30 MHz, the accuracy of most of the equipment begins to fall and large, fixed stations are needed to produce any level of detail. Such stations do exist though, and given the fact that MF and HF signals travel large distances, a few of these fixed monitoring stations in two or three neighbouring countries can work together to identify the location of an illegal transmitter (which the mobile teams can then home in on).
It amazes me, therefore, that there are so many pirate radio stations who continue to operate 24 hours per day, 7 days per week. A quick tune across the FM dial in London at lunchtime in the middle of the week yielded the following stations happily pumping out pumping tunes:
87.9 Shine 879
91.8 Passion FM
93.8 Vibes FM
95.4 Roots FM
97.9 Bassline FM
101.9 The Beat
Now it's fair to say that many of these stations are regularly closed down by the authorities, but have sufficient funding and expertise to be back on the air in short shrift. They also tend to use a complicated set of links to connect the studio to the transmitter site such that if the transmitter site is identified, the studio location (where the more expensive equipment and the presenter are located) remains secret. A studio hit on a pirate costs much more to recover from and often leads to prosecution.
It's also fair to say that these staitons have, on the whole, managed to settle on frequencies that tend to minimise the amount of interference they cause to the legal stations. Many pirate stations in London operate only 200 kHz away from legal stations which is far too close to prevent interference (the reasons for which are something to talk about on another day perhaps), but all these stations sit sufficiently far enough away from legal stations (typically a minimum of 400 kHz) that the interference they cause is relatively small. The fact that they are all in mono also improves the situation (and has the bonus for the pirate station that their coverage area is increased). Nonetheless, these stations do cause interference, especially close to the transmitters concerned where the sheer power of the illegal signal overwhealms nearby receivers and the authorities tend to act more virulently where a complaint of interference has been received.
However these aren't the only long-term illegal transmissions that can be found. A recent scan of the short-wave broadcast bands brought to my attention 'Laser Hot Hits' - a pirate station that broadcasts 24/7 on a number of short-wave frequencies (try 4025, 6219, 6285, 7460 and 9385 kHz, though the 7460 kHz signal is often wiped out by a strong BBC DRM transmission from Kvitsoy in Norway on 7465 kHz in the afternoons). What makes this more intriguing than the London pirate situation is that many of the frequencies they are using will cause significant interference not, in this case, to other broadcasters, but to maritime, aeronautical and defence spectrum users, yet their transmissions have remained on-air, seemingly unchecked for many months now. In particular, the 6219 kHz transmission will directly interfere with safety-of-life, emergency maritime communications on 6215 kHz.
So how come they are still on-air? One possibility is that their transmitter site location (not needing to be near to the listeners due to the propagation at HF) is somewhere so remote that the authorities either can't, or can't be bothered, to get to it. A persistant pirate of the late 1980's, 'Radio Fax' used to transmit from a rural location in Ireland and the Irish authorities who have always been much more tolerant to pirate transmissions than many other countries, were very sluggish to step into action. Maybe Laser Hot Hits' transmitter site is on one of the mountains of the Netherlands, which the local mountain rescue teams can't reach! Wherever it is (or they are - there is no reason to suspect that all the frequencies are transmitted from the same site) they have obviously escaped scrutiny so far.
So what conclusions can we draw from this? Perhaps those in power accept 'pirate radio' as a problem that will never go away and for stations who act responsibly turn a blind-eye (remember that pirate radio has had a greater impact on the shape of the UK radio industry than possibly any other factor, and even has a major impact today). Perhaps they do not have the resources to track down every station and concentrate on those who cause the biggest sores (e.g. those with outspoken political views or whose transmitting equipment is of low quality and is causing lots of interference). Perhaps they wait for compliants to be raised before tackling the problem. Whatever the reason, one must doff one's cap to those who continue to flout the law for our entertainment. Many are outlets for music which ordinary stations turn their back on but which has a large, cult, following. Some are there just for the fun of it (don't tell me that doing something naughty isn't at least a little bit thrilling...?) Gentlemen (and ladies), I raise my glass to you.
Thursday 27 October, 2005, 09:45 - LicensedAfter many years in a sleepy backwater, Digital Audio Broadcasting (DAB) in Europe is finally becoming more mainstream (with one or two exceptions). This is certainly in part due to the fact that the prices for DAB receivers have now dropped to the level where, when buying a new radio for the kitchen, a DAB radio is in the running; and with the extra stations and 'improved quality' they offer, DAB radios are now becoming a must have item.
Posted by Administrator
Posted by Administrator
However, the gradual move towards DAB brings with it a problem for local FM broadcasters whose coverage is limited to a relatively small geographic area. For you see, DAB works by means of a multiplex (mux), whereby a number (typically 8 or more) stations are combined together and transmitted from one or more transmitter sites. The content of the mux must remain identical on all the sites such that all the stations on the mux achieve exactly the same coverage area. This is fine for national stations and for those who cover a relatively large geographic area, or the same city, such that there are likely to be other broadcasters who wish to achieve the same coverage and could share a multiplex; but who would wish to share a multiplex and duplicate the coverage of a neighbourhood or a small rural enclave? The straighforward answer is no-one! So these small scale stations are left with no option other than to continue using FM and hope that DAB does not become the de facto broadcasting standard.
Salvation, however, may come in another form of digital radio, otherwise known as Digital Radio Mondiale (DRM). DRM is designed to offer a replacement for outmoded AM radio on the long, medium and short wave bands, but it has recently had its frequency range extended to 120 MHz so as to offer a possible replacement for FM broadcasters too. For existing AM broadcasters, a move to DRM offers the chance to replicate their existing coverage but with much improved quality ('near-FM quality' is often quoted as being achieveable in an AM channel using DRM).
For AM broadcasters, this is great news: Using the same spectrum they can improve their transmission quality and many broadcasters, especially in mainland Europe, have already done so.
Using DRM in the FM band is a largely untried quantity and in theory there is no reason why it could not only work but at the same time expand the number of stations that could be broadcast in the FM band, as a DRM transmission takes up only about one tenth the bandwidth of an analogue FM transmission. It may not, though, be an ideal solution for a local FM broadcaster, particularly as at present, the very few receivers that exist for DRM do not cover the FM band (the manufacturers assumed that the standard would only be used on the HF bands below 30 MHz). Also (and I'm sure there are cleverer people than me working on this), the carrier spacing for the OFDM signal of a DRM transmisison varies between 41.66 and 107.5 Hz. At a frequency of 108 MHz, a vehicle travelling at 250 km/h (a train for example, or a BMW on a German motorway) experiences a Doppler shift of 25 Hz. Compared with the 41.66 Hz carrier spacing this is significant, and even at 107.5 Hz carrier spacing it represents a significant difference for the receiver to have to track, and must introduce lots of errors in reception.
There is another cat in a bag which offers a potential solution to the question of a digital replacement for local FM broadcasting in the shape of a little used HF broadcast band. The 11 metre band (25670 - 26100 kHz) is virtually unused by broadcasters as, other than at the peak of the 11 year sunspot cycle, propagation is virtually non-existent except for local ground-wave coverage. During the day at the peak of the cycle, signals travel thousands of miles, especially trans-equatorially (i.e. north to south and vice versa). Experiments have been conducted in Germany which showed that using DRM in the 11 metre band offers good local coverage similar to that which FM enjoys, however there is the potential for occasional long-distance interference during the day. This is, if anything, an improvement on the situation which traditional AM broadcasters enjoy in which there is a virtual guarantee of interference from distant stations every night!
Experiments using the 11 metre band are continuing in Germany and France and in London by WRN (1kW on 26070 kHz from Crystal Palace) and if all goes well, I think we could see a raft of local stations opening up DRM transmissions in this band, complementing their FM coverage and giving them a digital outlet to compete with the growing number of DAB stations. For the sake of small local radio stations, let's hope so.