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UK Local TV - Learning The Lessons Of The Pastsignal strength
Thursday 29 November, 2012, 04:07 - Licensed
Posted by Administrator
The local TV landscape in the UK is slowly taking shape as the decision on the final, and arguably the most commercially viable/advantageous franchises is delayed to give Ofcom more time to make the right decision. Whilst decisions on which companies will succeed in the lucrative markets of London, Leeds and Liverpool are still outstanding, also still to be announced is who will supply the transmission services for the local TV stations. Four bidders emerged when Ofcom invited tenders to deliver the local TV multiplexes:

avanti communications logoAvanti Local TV (backed by broadband satellite company Avanti Communications – plans to use small transmitters to provide coverage tailored to local TV audiences)

comux logoComux UK (backed by Canis Media who run the local TV multiplex in Manchester – a group of consultants who will provide their expertise to local TV companies to help them get going)

bbc local tv logoLMux Ltd (backed by the BBC – will shepherd the roll-out of the network in good old BBC style, and keep it going with any available funds until such time as those funds run out)

cma logoLocal TV Multiplex Ltd (backed by the Community Media Association – will act as a central procurement facility to try and negotiate down prices of transmitters and other services for local TV companies)

The majority of the bidders will rely on existing television transmitter sites to provide the coverage for the local TV multiplexes. This has a number of advantages, not least that viewers’ TV antennas will be pointing in the right direction and (hopefully) will be of the correct antenna group to receive the transmissions. Historically, however, placing local television transmitters on larger transmitter towers has not necessarily offered an ideal solution.

Lanarkshire TV (LTV, later rebranded as Thistle TV) used the main Black Hill transmitter site situated between Glasgow and Edinburgh, both of which it covers. The old analogue TV transmissions on Black Hill used a power of 500 kW whereas LTV only had around 10kW of power. At 500 kW coverage of the region is excellent, at 10 kW (50 times or 17 dB less), coverage is marginal at best. Even close to the mast where the lower power signal is notionally strong enough to provide good reception, viewers receivers will be set up to receive the stronger signals (ie antennas will be of lower gain, or even indoor) and the large disparity in signal strength will render the lower power station largely unwatchable.

A similar (but worse) situation occurred on the Isle of Wight where not only was the local TV station Solent TV (and its predecessor TV12) using only 2 kW compared to the power of the main station of 250 kW but it was also on an ‘out of group’ channel. solent tv isle of wightThe main transmitter at Rowridge uses channels at the lower end of the UHF TV band (channels 21 to 31 were used for analogue services), whereas Solent TV was on channel 54, meaning that not only was it 21 dB weaker leaving the transmitter due to its lower power (worse even than Lanarkshire TV) but the TV antennas of viewers (which are ‘grouped’ in order to focus their gain on the frequencies being used in the area) would add another 6 to 10 dB differential making the signals from Solent TV around 27-30 dB (500 to 1000 times) weaker than the main TV channels. The Solent TV transmitting aerials were also not as high up the mast as those of the main services, further reducing coverage. It is any wonder they went bust?

Arguably one of the more successful local TV stations (in terms of coverage) was Oxford TV (later known as Six TV). six tv oxfordThe station was transmitted from the Oxford transmitter site which is, you might have guessed, relatively close to Oxford itself. Though the power was lower, it was ‘in-group’ and though the picture was not as good as the main services, most people in Oxford (and the surrounding area) could watch the programmes relatively happily.

Under Ofcom’s proposals, local TV is once again being planned from sites such as Black Hill, this time using digital terrestrial (DTT) multiplexes. One of the advantages of DTT is that the modulation and error correction can be varied in order to allow weaker signals to have coverage that is on a par with stronger ones, at the expense of the amount of data they carry. It is proposed that the local TV multiplexes will use QPSK and 2/3 rate FEC giving a capacity of around 8 Mbit/s, enough for 3 standard definition (SD) pictures. Current multiplexes (ignoring the DVB-T2 multiplex used for HD services) use 64QAM and either 2/3 or 3/4 FEC and provide up to 24 Mbit/s. The difference in signal strength needed to receive a QPSK signal compared to a 64 QAM signal is around 11dB, meaning that if the local TV transmitter power is 11 dB (about a factor of 12 times) or so less than the main station, coverage parity is maintained. This was not the case for analogue broadcasts of local TV, and makes the case for local TV using DTT much improved. It still requires sensible transmitter powers from stations to provide coverage and not the 20dB or less that the original analogue local TV stations enjoyed (if enjoyed is the right word).

There are, however, two distinct problems with using main stations for local TV:
  1. The main stations are often well outside the areas where the audiences are located (you wouldn’t put a 300 metre tall mast in the middle of a housing estate). The strongest coverage area of such towers is therefore outside of the area where the audience is located thus exacerbating the lower power, lower height, out of group issues that the old analogue local TV stations faced. Fundamentally they don’t put the signal where it is needed if all you’re interested in doing is serving a local community. You almost never see local radio stations on the same masts as the main national services for exactly this reason. Even in London, local (capital wide!) services were transmitted on FM from Crystal Palace whereas the national services were transmitted from Wrotham which is around 20 miles south east of London in the county of Kent. Only in relatively recent years (compared to the age of the FM network) did the BBC add the national stations to the Crystal Palace site as (guess what...) the coverage of Wrotham in central London was not ideal.
  2. Secondly, the main station masts are expensive to use both because higher power transmitters are needed to reach the desired service areas, but also because the masts themselves are expensive to operate and maintain and this has to be passed on to any organisations using the sites. Similarly, given their relatively remote location, power, access and other services can be difficult to provide increasing the cost of using the site.
Contrast the situation of Solent TV to the way in which analogue transmissions for Channel 5 were dealt with in the same area of the country. No high power frequencies were available from Rowridge due to its location being virtually line-of-sight to France and thus the need to share frequencies with the UK’s Gallic neighbours. Instead, a transmitter was added to a chimney at the Fawley power station, providing 10kW on channel 34 (an in-band frequency for Rowridge). Being closer to the target area the lower power was less of a problem, and being in-band meant that TV signals were not further attenuated by viewers’ TV antennas. Fawley is also roughly in the same direction as Rowridge if you live in the Southampton area and as such there was no need to re-point antennas or install additional ones even though the signals were coming from different sites.

rowridge fawley southampton

Although the coverage of the Fawley transmitter was not as widespread as that from Rowridge, in Southampton (the area of greatest economic interest) the Channel 5 analogue signal was good enough for most people to watch. Local TV channel Six TV also had a transmitter on the Fawley mast on channel 29 (also in-group) but with the slightly lower 4kW.

As with Six TV above, not all of the original analogue local TV stations used the main station masts. channel m manchesterFor example, Channel M in Manchester used a site on a water tower in Bolton. Like the Fawley solution for Southampton, this put the transmitter in roughly the right direction for viewers in Manchester whose antennas were pointed at the main station at Winter Hill. The frequency (channel 39) was, however, out of group and the transmitter pattern severely restricted to avoid interference with other transmitters and with the radioastronomy users at Jodrell Bank who used channel 38. rsl 39 manchesterThe upshot was that Channel M's signal was strong enough for good reception to a large number of viewers who were nearest to the water tower, but, conversely, coverage of Channel M in downtown Manchester was relatively poor (click on the map on the left to see it in full). Despite that, the station did better than many local TV companies and lasted for the best part of 12 years before finally closing down just before analogue services ended in the UK.

What would have been great would to have been able to put more than one site on-air to provide additional coverage to fill-in coverage not-spots. In analogue terms this is difficult to do but for digital services such a solution is inherent to DTT in the form of a single frequency network (SFN). In an SFN multiple transmitters are put on the same frequency and as long as certain technical criteria are maintained (eg the distance between sites is small enough, ensuring that the transmitters are time and frequency synchronised, and that they carry the same content), they do not cause each other interference. In fact, the signals from multiple sites can even add together to improve coverage. If this sounds too good to be true, it is exactly how the digital audio broadcasting (DAB) multiplexes work.

So... Wouldn’t it be great if local TV in the digital age could take advantage of the use of SFNs to put a number of lower power (and thus much cheaper) transmitters right where their audiences are located, in line with existing masts (so that antennas don’t need re-pointing) and providing good coverage where the viewers are, but not wasting power or money on areas where few viewers are located? That is just the solution that Avanti’s bid to run the local TV multiplexes proposes. Whilst it might appear to be a ‘whacky, out-of-the-box’ type of solution, on the contrary it does what needs to be done in an efficient and effective way that has been proven to work well for local TV, even in the days of analogue transmission.

The jury (Ofcom) is still out on which of the bids to operate the multiplexes will succeed, but it is to be hoped that those making the decision are aware of the chequered history of local TV in the UK and don’t fall into the same traps that led to the commercial failure of the original analogue services in the 1990s and 2000s.

ltv lanarkshire
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Freakquencies in Dhakasignal strength
Tuesday 23 October, 2012, 07:48 - Licensed
Posted by Administrator
Have you ever read the book 'Freakonomics'? It tries to demonstrate that sometimes cause and effect are far, far removed from each other. Rather like Sherlock Holmes adage:
When you have eliminated the impossible, whatever remains, however improbable, must be the truth.

Take a look at the list of FM frequencies for radio stations in Dhaka, Bangladesh. It may not seem odd at first, but look closely:

radio foorti logo88.0 MHz Radio Foorti
88.4 MHz Radio Aamar
88.8 MHz Bangladesh Betar (Traffic Channel)
89.2 MHz ABC Radio
89.6 MHz Radio Today
90.0 MHz Capital FM
dhaka fm logo90.4 MHz Dhaka FM
91.6 MHz Peoples Radio
92.4 MHz Radio Shadhin
97.6 MHz Bangladesh Betar
100.0 MHz BBC World Service
103.2 MHz Bangladesh Betar (Home Service)

Taken from a variety of sources such as: Asiawaves

Notice anything odd? What about the fact that there are 7 stations spread out every 400 kHz between 88.0 and 90.4, two more below 93 MHz and then the rest of the whole FM band up to 108 MHz contains only 3 more stations.

The 400 kHz spacing is sensible (see the previous Wireless Waffle article on the bandwidth required to transmit a stereo FM programme), but why are they crammed down at the bottom end of the band? Here's a quick Wireless Waffle quiz. See if you can get the right answer.

Is it because:
  1. Propagation at lower frequencies is better than at higher frequencies and thus the lower end of the FM band will yield marginally better coverage than the top of the band for the same power/antenna.
  2. Like many countries (including the UK which only had access to 88.0 to 97.6 for a long time), the bottom of the band was opened up for broadcasting first, and the upper frequencies have only recently become available.
  3. Buildings in Dhaka are built to a Government controlled specification which, ironically, has a resonant frequency at the top of the FM band, causing signals at this end of the band not to be able to penetrate inside them.
  4. The majority of cars in Dhaka are imported from Japan which has an odd FM broadcasting band that runs from 76 to 90 MHz and thus the radios in those cars don't extend much above 90 MHz.
  5. The transmitters used by FM stations in Dhaka are very old and work best at the lowest possible frequency, giving the highest output power and greatest efficiency.
So, which do you think it is?

banglatransmitterWell (a) is certainly true, though the difference in propagation is less than 20% between 88 and 108 MHz and is offset to some extent by the slightly better performance of receiving antennas at 108 compared to 88 MHz. As for (b), this is not true as the BBC service on 100 MHz has been on air for over 15 years and was one of the first FM stations in Dhaka. Answer (c) is a joke – have you seen the state of buildings in Dhaka?! Answer (d) could certainly be true as the Japanese FM band does run from 76 to 90 MHz. And finally (e) would be true of old transmitters were used, but most are modern and therefore don't suffer from this problem, which, as with (a) is pretty marginal anyhow.

dhakajamThe real reason that stations in Dhaka are clustered on frequencies around and below 90 MHz is (d). Think about it... when you listen to FM radio the most? Yes, perhaps you listen at home, especially on your alarm clock and in the morning, but most listening is done whilst in the car. There’s not much point being on a frequency that car radios can’t tune into and so there is the highest demand for frequencies on or below 90 MHz. Some older analogue radios will tune slightly above this so 90.4 MHz and thereabouts is not bad either. At the time of launch of the BBC service on 100 MHz there were very few cars (or FM radios for that matter) in Dhaka and so the issue didn't manifest itself.

Did you guess right? Would you have guessed that the choice of FM frequencies was driven by car imports if it hadn't been suggested to you? We doubt it! A case of 'Freakonomics' that is perhaps best labelled 'Freakquencies'.
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Pirate Radio disposal squad called into action in Surreysignal strength
Sunday 27 May, 2012, 17:50 - Pirate/Clandestine
Posted by Administrator
A report recently arrived on the Wireless Waffle newsdesk of the discovery of some kind of (probably illegal) transmitter discovered on land in Surrey next to a military establishment. According to the report:
Army bomb disposal experts were called to Camberley on Monday morning to deal with a "suspicious" object that turned out to be a radio device... Shortly before 3pm, Surrey Police said the unit had been confirmed to be a "transmitter or repeater for a citizen or pirate band radio station", and that roads had been reopened and cordons lifted.

Of course this piqued the interest of the Wireless Waffle team. Let's look at the possible options:

CB Repeater
Such things do exist but require a great deal of technical skill to install and operate. As CB radio operates at 27 MHz, they generally require large antennas, and as transmitter powers are usually at least 4 Watts, need power supplies the kind of size that wouldn't easily be left lying around in a piece of woodland.
Probability rating: ***** (2 out of 5)

Pirate Repeater
Not something that is often heard of, in fact, do a 'google' search and nothing of any substance comes up. One possibility is that it could have been a PMR446 repeater which, whilst not strictly legal, is not fully pirate as it operates on an unlicensed basis. Such devices would (sensibly) be located on high ground, though they could be operated from batteries for some time. Ideally, they would be high off the ground too (the news report does not indicate whether the so-called repeater was on the floor or up a tree).
Probability rating: ***** (1 out of 5)

FM Pirate Transmitter
piratelistenerMost FM pirate stations in the UK use transmitters on top of tower-blocks. The area in question is clearly not one bedecked with such skyscrapers, it is woodland. It is not unheard of for pirates to put transmitters up trees but there are better places, especially those with power and somewhere dry to put the transmitter. There aren't that many pirates in this area, Point Blank FM was one which used to broadcast specifically to the area but it hasn't been on air in that part of the world for many months. Pressure FM (also known as 'Presha') is another station in that part of the world. It is possible they would use a site such as that, but if they were locals, presumably they would know the area was largely military and steer clear of it?
Probability rating: ***** (4 out of 5)

Short-Wave Pirate Transmitter
Short-wave pirates are almost uniquely confined to woodlands in order to string up the large antennas necessary given the frequencies they use. They often use hidden battery powered equipment. On the face of it then, this is exactly the kind of place you would expect to find such a transmitter. The bigger question is whether any such pirates operate from that part of the country, the answer to which is, of course, 'who knows?' Certainly there is some history in the area including Radio Fax which used to operate from Surrey in the late 1980s, and of course, the infamous Radio Jackie which used to operate from the London end of Surrey. There's also a vague recollection of a maildrop for short-wave pirates that was in that part of the world (though my memory is a bit hazy on that). Anyhow, it's not impossible but the number of UK-based short-wave pirates seems very much in decline.
Probability rating: **** (3 out of 5)

Our conclusion then - probably an FM pirate - but a bit of a stupid one to go so near to ministry of defence land!
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Russia is the Tsar of Piratessignal strength
Thursday 19 January, 2012, 14:30 - Amateur Radio, Pirate/Clandestine
Posted by Administrator
Wireless Waffle has talked extensively about pirate radio in the past, from short-wave music stations, to Brazilian sat-jackers. But it seems that, of all the nations on the planet, the Russians hold the baton for being the biggest pirates of them all.

This story begins when reading the latest intruder report from the IARU Region 1 Monitoring System. The report indicated that there had been an intrusion into the 80 metre amateur band between 3.5 and 3.6 MHz by Russian pirate stations running AM. Now historically the Voice of Korea (the North Korean broadcaster) has been transmitting in the 80 metre band (or the 75 metre band as it's called in in North America) on 3560 kHz in AM and the immediate assumption was that these new signals couldn't possibly be Russian pirates, but must be the Voice of Korea and perhaps a few other stations trying to jam it. The IARU report, however, says that the carriers are very unstable and that the modulation is voices in Russian.

blue soldier red squareSo the only thing to do to verify this story is to turn on a receiver and have a listen. Having done this, there were no obvious signals in the 80 metre amateur band. Having previous heard pirates just below the band at around 3450 kHz, the tuning dial was slowly rotated to ever lower frequencies. Nothing. And then, at 3175 kHz, something. A weak carrier... no, two carriers alternating... both rather unstable in frequency. Switching the receiver to AM yielded weak modulation. A bit more tuning, to 3125 kHz and a much stronger AM signal with a Russian voice and a wobbly carrier. Hey presto!

But what are these odd signals? Are they military operators in a private net (if so, why AM and why unencrypted)? Are they some kind of harmonics or intermodulation? Googling didn't bring much until a page on Sparky's Web Blog was found. It seems that these are effectively the equivalent of Russian CBers but presumably using much lower frequencies given the large distances between Russian cities. The band is known as the тройка band ('troika' in English which has several meanings from 'three of a kind' to a sledge or fairground ride). The band runs from approximately 2900 to 3200 kHz which are internationally allocated to the Aeronautical Mobile and Mobile services.

red square blue squareThere are aeronautical frequency assignments in the band (2872, 2899, 2921, 2962 and 3016 are frequencies assigned to North Atlantic traffic for example), but these lower frequencies are less often used unless propagation makes it totally necessary. Oddly, the various frequency lists for the band show very little aeronautical use in Russia (other than Irkutsk on 3016 kHz) - a coincidence? Probably the pirates know this and therefore feel free to mess about in the aviation bands, knowing that the Russian authorities are likely to be little interested in their activities.

If you're in Europe, when it gets dark (and lower frequency propagation opens up over the continent), why not give them a listen. It's fun to chase the carriers up and down in frequency. If you speak Russian, perhaps you could provide some translation as to what on earth they are talking about!

P.S. You might also want to take a listen to 2920 kHz USB as this seems to be a common calling channel for the more technically adept Russian pirates.
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