Digital Terrestrial TV and 4G

At the beginning of 2011 I used OfCom data to generate maps showing the locations and powers (ERPs or Effective Radiated Powers) of most of the UK’s DAB and VHF transmitters. This was so that people interested in radio could assess the coverage in their area. I had wanted to do an equivalent map for Digital Terrestrial TV (often called ‘FreeView’) transmitters because many people use this to listen to BBC Radio stations. Unfortunately at that time I was unable to obtain the data I needed. However during an investigation into the possible impact of ‘4G’ upon DTTV reception I was able to get transmitter data. So I’ve now done some maps showing the DTTV TXs...

Figure 1 - Digital TV Transmitters

Figure 1 is a map that shows most of the DTTV transmitters (TXs). I’ve omitted those that radiate with effective powers of 100 Watts or less. So there are some low-power ‘filler’ transmitters I have left off the map as an aid to clarity. Each TX is represented by a blue-filled circle whose center is at the location of the TX. Please note that the diameters of the circles represents how powerful the transmissions (for the BBCA multiplex) are from the TX. They do NOT show the geographical areas covered. You can use the map to see where you live relative to the nearest and most powerful transmitters. As with the VHF and DAB maps I produced, you can see that some areas are much better served than others. In particular, quite a lot of the North West of Scotland is poorly covered. However the decision of the BBC to remove their radio stations from the Scottish DTTV TXs in what is sometimes described as ‘the evening’ means that in Scotland coverage for Radio listeners is even poorer than shown. (And it is worth pointing out that ‘the evening’ seems to mean from 4pm onwards at the weekend!) So millions of people lose the option to use DTTV for BBC radio every ‘evening’, and they may not be well covered by DAB or VHF as an alternative! Instead they are offered Alba TV. This is a Gaelic-language service introduced as a result of political lobbying. It seems to spend a lot of time showing football games and programs for small children. Possibly because they expect this to attract more viewers who either just want to watch the football and can ignore the commentary, or are kiddies dumped in front of the set. Unfortunately, the situation in Scotland for those who want to view or listen to DTTV may be about to get worse. And this time, the effects may also be felt across other parts of the UK...

Between the end of 2012 and the end of 2013 a large chunk of the established UHF TV band will be ‘cleared’ to make space for 4G mobile services. This means many TXs will have their transmission frequencies changed. As a result many millions of people will have to re-scan their DTTV boxes/sets again. This tediously familiar irritant is just a prelude to what may become more of a problem, though. The 4G base stations that come into service will have the following characteristics

They will radiate with ERPs up to more than 5kW. This is rather more than earlier generations of mobile-phone base station. The higher power is to allow them to convey much higher data rates.
They will transmit at frequencies which existing DTTV boxes/sets are designed to receive. This is because until the end of 2012 these frequencies were dedicated to UHF TV.
They will transmit at frequencies that are very close to the ‘reshuffled’ frequencies which will continue to be used for DTTV in many areas after the ‘clearance’.

As a result, many people will find that their TV antenna picks up an unwanted 4G signal that is hundreds, or thousands, or even tens of thousands more powerful than they get for DTTV! The result may be a blank TV screen. Losing TV reception is particularly likely in areas where the reshuffled DTTV uses TV channels 59 or 60 because these are close to the 4G base station channels. The dividing line in the UHF spectrum between DTTV and 4G will be at 790MHz. DTTV channel 60 ends at 790MHz, and 4G starts just 1MHz above that. The ‘gap’ allocated to keep them apart is a difference of only 0·13% in frequency. The DTTV channels are 8MHz wide, so ch59 comes to about 9MHz from 4G – i.e. a 1·1% difference.

Figure 2 - 4G clearance and possible trouble hot-spots for DTTV

Figure 2 shows two maps. The one on the left shows the DTTV transmitters whose frequencies will be changed by the end of 2013. The one on the right shows the transmitters which even after this change will continue to use ch59 and/or ch60. If you rely on one of the transmitters shown on the right-hand map you may be particularly likely to experience loss of DTTV reception. The timetable for the changes to DTTV transmitters is now decided. What won’t be clear until later is when the companies that ‘win’ the new UHF 4G allocations will start operation in each locality. So the occurrence of problems may not follow the same geographical timetable as the clearance.

At the time I write this (November 2012) it is hard to know how many people will lose DTTV reception. The official estimates from OfCom and others vary wildly, and depend on the assumptions they make. But in general the raw results come out as showing it likely that millions of households will find their DTTV reception is seriously affected. Despite this, the plans for the clearance of DTTV and introduction of 4G have recently be re-timetabled to have the process completed quicker than originally planned! This is apparently because the telephone companies have been demanding 4G as soon as possible. And OfCom / Government are apparently hypnotised by the cash being waved in their faces for this sell-off of spectrum. There is, however, a cunning plan...

OfCom have arranged with the mobile phone companies that they well set up and run a ‘Mitigation Company’. This will provide help to those affected. The promise is mainly based on the approach of handing out ‘4G filters’ to those who find that 4G base stations act as jammers, blocking their DTTV reception. The theory is that these filters will pass DTTV, but reject 4G, so protecting DTTV viewers from the jamming. The belief is that these filters will be cheap enough to hand out like sweeties to those affected amongst the 40% of the UK population who use DTTV as their only way to watch TV. Alas, fine as this theory may be, experienced RF engineers may find the practical details less convincing...

The real practical problem is that an unknown number of people may require a spectacularly good filter to allow them to go on watching DTTV. The jamming potential of 4G is alarmingly high. Hence the filter may have to pass UHF frequencies up to 790MHz with little loss, yet reduce the powers of signals from 791MHz upwards by a factor of thousands or more! Filters with such impressive performance can certainly be made. But they would be unlikely to be cheap. And if they also need to be compact and durable, become even more expensive. Feasible for military or serious commercial applications. But - with the changes already under way in November 2012 – it remains far from clear if it will be feasible to hand out what may turn out to be millions of filters that will be up to the task.

The continued use of ch60 makes the challenge to protect DTTV reception from 4G jamming more demanding, more costly, and possibly also more confusing for viewers. This is because cheaper alternative filters with lower performance may be fine in some areas, but not in others. Places where channels 59 and 60 are unused will tend to require a much less demanding filter design because the filters could have a 16MHz ‘gap’ to allow smooth change from their passband to their rejection band. Whereas in areas where ch60 continues to be used the gap for this will only be about 1MHz. Again, the outcome is hard to predict in advance. But the handing out of ‘free’ filters has restrictions. For example, it is planned that it will be “one filter per household”, and this is only for individual households. Not individual apartments in a blocks of flats, etc. You may also only be given a free filter if you already have an external TV antenna. So any sets/boxs using an internal (or loft?) antenna won’t get a free filter. And in some places there may be no intent to hand out such filters because it is assumed that they won’t be needed.

One result may be that a variety of filters will be on sale. These may perform in different ways. So some will work OK in some cases, but be inadequate elsewhere. Some may be cheaper than others. The result being that people may have to guess what they might need, and end up wasting time and money.

Overall, I wonder how many people will simply switch to satellite TV because their set stops working and the repair man tells them this is the solution. Maybe not even knowing a filter or re-directing their antenna would help. However if you’ve read this page before mid-2013 you have been warned. If your TV set goes blank during 2013 or 2014, wonder if it is being jammed by a new local 4G service, and see if you can claim a ‘free’ filter. In theory the promise is that if 4G is killing your reception, and a filter can’t fix it, the companies will pay for you to install a satellite dish. But as with the other theories here, I wonder how it will turn out in reality...

Jim Lesurf
22nd Nov 2012
1600 Words