"FIVE" ANALOGUE RECEPTION ISSUES

Please note that this page is no longer relevant as analogue television has ended in the UK.

With the introduction of a fifth terrestrial television network in the UK comes a whole new series of unexpected reception problems. In order to understand these, a history lesson is in order.

The introduction of the Marconi 625-line television system

When television was first officially launched by the BBC, the system in use was the Marconi 405-line system, invented by Alan Dower Blumlein in 1934. The official launch took place in 1937. Transmission was in VHF band I. The London transmitter was at Alexandra Palace. The roll-out to the rest of the country started in 1949, with the addition of masts at Sutton Coldfield, Holme Moss and Kirk O'Shotts, among others. With the exception of Alexandra Palace, the system used later became known as A.

In 1955, the ITA launched ITV, on the Marconi 405-line system A, using VHF band III. The ITA built their masts in different locations, for instance Croydon near London, Lichfield in the Midlands, Winter Hill and Emley Moor in the north and Black Hill in mid-Scotland. The BBC moved their London transmitter to Crystal Palace in 1956, as the area was better situated for television masts. The BBC also added some band III transmitters in the 1960s, mainly relays at ITA sites, but also an additional frequency from Wenvoe in order to provide English and Welsh programmes simultaneously.

VHF had the advantage that it could easily cover a very wide area. It had the disadvantage that the frequency spectrum consisted of only 14 channels, with no more than 13 of these allocated for broadcasting. In addition, the band I transmissions (channels 1 to 5) were notoriously prone to co-channel interference, owing to the ability of the ionosphere to reflect the signals considerable distances. In the case of the BBC Crystal Palace transmitter, reflection to South Africa in 1957 was detected, and a few people managed to receive the signal occasionally in Australia. The system was designed for black and white transmission only, although the BBC did experiment with NTSC colour in the late 1950s.

With the launch of BBC2 in 1964 came a new system, the Marconi 625-line system I, broadcast in UHF bands IV and V, but originally broadcast in Ireland on bands I and III. This system, allowing 8MHz per channel, gave 49 possible channels, numbered 21 to 69. Channel 69 was considered unusable, and channels 35 to 38 were reserved for non-broadcast purposes, leaving 44 channels. Although more transmitters would be needed, it was agreed to make this a four-channel capable system. Before BBC1 and ITV joined the system in 1969, it had been converted to PAL colour, the first official transmission of colour being in 1967. It was agreed that the BBC and the ITA would amalgamate their sites.

In all, 50 main transmitters with four-channel capability were built. A further main transmitter carrying only BBC channels to those parts of Scotland receiving ITV from the Caldbeck transmitter in Cumberland was also built. Many relays were built, the vast majority of these carrying four channels.

As much as possible, it was planned to put all four channels within a small part of the bands from each transmitter, avoiding duplication and close-channel usage in neighbouring areas. Aerials were built with optimised bandpass characteristics for these small parts. They were:

Group A (red) 21 to 34
Group B (yellow) 39 to 53
Group C/D (green) 48 to 68

It was not possible to squeeze the channels into these groups in all areas. To this end, three further types of aerial were made:

Group E (brown) 39 to 68
Group K (grey) 21 to 48
Group W (black) 21 to 68

Group K aerials were never specified in the UK before the introduction of FIVE, but have been available.

The narrow-grouped aerials have higher gain and better directivity than wide-band types.

The system coped fully with the introduction of Channel 4/S4C, with only a few minor hiccups.

System A was gradually withdrawn from 1980, the last transmitter closing on 6 January 1985.

Introduction of FIVE

The ITC (successor to the ITA/IBA) was instructed by the government to create a fifth terrestrial TV channel, fitting it in where possible within the existing four-channel system. With the crowding of the spectrum, it was decided to make channels 35 and 37 available, in addition to some spare frequencies in other areas. To coincide with this, manufacturers began re-grouping their aerials. The groups were amended to the following:

Group A (red) 21 to 37
Group B (yellow) 35 to 53
Group E (brown) 35 to 68
An additional group covering 34 to 40 also exists, as do some very narrow band aerials designed to receive just a few channels, with good gain and good directivity.

Other aerials remained unchanged. Old aerials of groups A, B and E may just manage to receive channels 35 and 37. I have even seen one old Group B aerial that performs adequately on channels 37 to 55.

NTL (successor to the ITA/IBA technical division) won the transmitter contract. In order to save money, some transmitters previously used only for radio and the discontinued band III TV service were used for the transmission of FIVE.

FIVE became available on satellite a few weeks after it was launched, and cable operators were generally quick on the uptake.

Channels 35 and 37 were within the area allocated for non-broadcast purposes, including video recorders and satellite TV decoders. These units had to be adjusted (retuned) to avoid the interference problems which would have resulted. In areas where interference was a possible problem, FIVE themselves commissioned a retuning service, which started in late 1996 and remained available in affected areas for three months after the local transmitter entered programme service. There are, of course, problems with retuning:

• In some areas, there is congestion in the band. This is a particular problem in Lichfield's overlap areas with the Wrekin and Waltham, among others. It can be difficult to find a clear channel, especially if there is more than one unit in the aerial lead, and even more so if the retuning range is limited.

• If your equipment has been retuned, whether by Channel 5, your dealer or yourself, and you move to a new area, you may find a channel (not necessarily Channel 5) stuck in the way of your equipment, and you will have to retune again.

• Many older pieces of equipment have a limited range of retuning, usually covering only channels between 30 and 40. This limits the number of channels available to you for the equipment.

However, there is good news:

• Recent video recorders and satellite receivers have an unlimited range of retuning. This is usually done by front panel or remote controls, and a clear channel can usually be found quickly.

• Some cable receivers also block signals. These remove the need for retuning.

• Some cable companies supply receivers set to a guaranteed clear channel, and all such units are rented, so there is never a problem with retuning.

• FIVE themselves at least used to supply signal blockers for any channel on which they transmitted, though in my experience they were usually ineffective.

• If you connect your equipment using SCART cables, with the use of some aerial splitters, you need not use the RF output at all, but the insertion of a splitter can cause a signal reduction of 3dB or more. Alternatively, some equipment allows you to turn off the modulator.

OK, what's the best way of retuning?

It all depends on the type of equipment you have. I would recommend this method, if your television is suitable and the equipment is retuned by a screw:

1. Ascertain the range of retuning available on the equipment. This is often 32 to 40, sometimes 21 or 22 to 68.

2. If your television has a mechanical tuner, tune the selector to a clear channel which appears to be within the range of available retuning. If your television has a direct electronic tuner, tune the selector to a clear channel within the range. Some direct electronic tuners work in MHz, not channels. In such instances, channel 32 is 559, channel 40 is 623, and channels go in 8MHz steps throughout the band. (Note: if your television has neither of these tuners, the procedure is more complicated. See below.)

3. Slowly rotate the equipment retuning screw, with the equipment on and set to either a colour channel or a setup menu, until a clear picture with good sound appears. If necessary, fine tune the television. (Note: if no picture appears, check that the equipment is on. If there is still no picture, adjust the tuner on the television so that it is closer to the centre of the retuning range and try again.)

4. Check that all channels (including all satellite and cable units) are free from interference. If not, retune again, to a different channel.

5. Retune any other receivers (including video recorders) that use the equipment as a source to the new setting.

If your television has only a scanning electronic tuner, the procedure must be altered as follows:

1. Adjust the equipment retuning screw slightly.

2. Retune the selector on the television.

3. Check all channels including the equipment for interference. If there is any, repeat the process. (Note: This can be very time-consuming.)

4. Retune any other receivers (including video recorders) that use the equipment as a source to the new setting.

If your equipment is retuned electronically, proceed as follows:

1. Ascertain the channels used in your area. Sometimes, your television will show these. Otherwise, you may need to examine engineering information.

2. Choose a channel for the equipment. It must be within the range of the equipment's retuning, and not:

   • Identical to a channel already in use by either a broadcast station or another piece of equipment.

   • Adjacent (i.e. one channel apart) to a channel already in use.

   • Separated from a channel in use by five channels (e.g. 37 and 42).

   • Separated from a channel in use by nine channels (e.g. 37 and 46).

   Allowable separations are therefore 2, 3, 4, 6, 7, 8, 10 or more channels. If no suitable channel is available, separations of 5 and 9 channels may be tried, but are not recommended.

3. Follow the manufacturer's instructions to adjust the output channel.

4. Tune the television to the new channel.

5. Examine all channels, including those used by other pieces of equipment, for interference. If there is any, repeat the above steps.

6. Tune all other receivers to the new setting.

You can always get help with retuning, although not from FIVE, and it may be chargeable.

What if I can't find a clear channel within the retuning range?

If this happens, you may have to have the modulator on the equipment changed to one with a different range of available outputs. This is a job for your dealer. If, however, you have multiple aerials, resulting in a large number of channels available, and you can do without one of them, you may be able to clear a channel by having an aerial removed from the system. Another way would be to have a channel changer fitted, but this is only worth it if you really cannot do without one of the available signals.

I should also point out that you will need to consider the digital TV signals when choosing your channel.

Phil's retuning story

A few years ago I moved half a mile down the road, to an area with no cable television service, and a shared aerial system set up for Waltham. The general reception conditions were quite poor, only Channel 4 and BBC1 being grade 5, the other three channels tending to be rather grainy. To make matters worse, the aerial was also picking up fringe signals from Nottingham and Sutton Coldfield/Lichfield. No problem, I thought, on finding BBC2 from Sutton Coldfield interfering with my video. "I'll put the channel 37 blocker in and retune." Did it work? No. The blocker did not effectively block the signal from Lichfield here. After several attempts, I managed to get the video onto channel 32... but there was interference on FIVE (channel 35) when I did that. I obtained blockers for channels 30, 31 and 39, but could not get anywhere with just these. I then moved my spare TV into the lounge and tuned a series of spare selectors individually to all the channels from 30 to 43. The video's specified retuning range was 32 to 40, but I managed to get it down to 31. This did not prove to be any better than 32, so I gradually worked my way up the band, eventually finishing on channel 42, without using any blockers at all! Neither TV was equipped with a SCART connection.

PROBLEMS WITH ANALOGUE TERRESTRIAL RECEPTION

A number of factors may affect the terrestrial reception of FIVE. In no particular order, they are:

Standing Wave Patterns: These can cause problems whenever a new channel starts. Some aerial systems optimised for BBC2 before the other two channels were available on UHF needed adjusting for BBC1 and ITV, and some needed adjusting when Channel 4/S4C started. This is because the standing wave patterns change sharply over small changes of frequency, especially towards the top end of band V. The cure is usually to have the aerial adjusted by an aerial fitter, although sometimes this may not work and it may be necessary to use a more distant transmitter.

Non-co-location: The use of some radio masts and band III masts can lead to problems if the transmitter site for FIVE is in a direction beyond the acceptance angle of the aerial used for the other four channels. The cure could be to adjust the aerial such that it does not point directly at either mast but both are within the acceptance angle, but this will not work in locations where the direction is significantly different, or where very low ghosting is required, as well as technically speaking it being a bit of a bodge. In these instances, a new aerial, and filtering equipment, should be fitted, with the aerial being of the group most appropriate for the FIVE signal. In many cases, however, reception of all five channels will be OK with just one aerial.

Low transmission power: For once, it is true that FIVE is not necessarily being broadcast with the same power. In many cases, FIVE is actually using a significantly lower power level than the other four channels. In some cases, the power level is identical. Unusually, the transmitters at Hastings, Middleton, Perth, Tay Bridge and Cambret Hill are actually using higher power levels. If the power level is lower than the other channels but there is enough FIVE field strength in an area, the signal, possibly via a dedicated filtered aerial, can be amplified. There is no point in amplifying a signal of poor quality, but if the quality is there, amplification is possible.
Why do FIVE use lower power?
FIVE's transmitters are operated at the maximum power permitted in each instance under international agreements, intended to prevent interference to other stations. The only exceptions to this rule were during a test transmission at Black Mountain, during which a fault was discovered, during a test transmission at Emley Moor, when a fault-reaction test was carried out, and during maintenance work at some of their transmitters more recently.

A power difference of 10dB is not usually significant for people in a good reception area.

Out of aerial group: The use of frequencies simply on the basis of their availability has resulted in frequencies outside the aerial group previously suggested being used. In a tiny number of cases, this even affected some people when Channel 4/S4C started. The cure is either to install a new aerial for FIVE or to replace the existing aerial with one having a wider bandpass. Replacing a grouped aerial with a similar wideband aerial is not an upgrade - in fact, a better quality wideband aerial, possibly larger, may be needed in some circumstances in order to maintain the quality of the signal. Older group A, B and E aerials are sometimes more affected by this problem than newer ones.

Directional transmission: As different radiating aerials are used, the directional properties of the FIVE signal can differ quite significantly from those of the other channels. Nothing can be done by the viewer to improve reception marred by this effect. This is another consequence of the congested bands.

Lower aerial height: Once again, as a consequence of different radiating aerials being used, some of them are lower than the ones for the other four channels. This causes a reduction in coverage area. Some aerials do not differ in height, however, and a small number are higher. If this does cause a problem, adjustment of the receiving aerial, or replacement with a high gain type, may help, but often nothing can be done. Information from FIVE themselves seems to suggest that there is some misinformation going around here, so watch this space.

Opposite polarisation: This happens in Northern Ireland, and also partially at Cambret Hill. Black Mountain has two television transmitters, the FIVE mast, formerly Ulster's band III main transmitter, and a relay owned by the BBC. The polarisations are opposed. Londonderry uses opposite polarisation from nearby Limavady, although Londonderry also carries the other four channels, vertically polarised. Croydon Old Town is a vertically polarised transmitter, except FIVE which is horizontal. Some of the beams for the other four channels from Cambret Hill are vertically polarised. The only cure in instances of this problem is to install a new aerial or a remotely-controlled adjuster if you use an outdoor aerial.

Co-channel interference: Does not occur very often, but occasionally, signals may overlap in some weather conditions, or the FIVE signal may interfere with a relay. Nothing can be done about the weather-related problems, but the others generally need the relay adjusting to a different output frequency. The transmitter at Winter Hill uses channel 48 for FIVE, which was also the channel used for Channel 4 from the Middleton relay. Middleton now uses channel 37 for Channel 4. Sadly, it seems similar problems may be affecting some viewers in Biddulph. The viewer cannot usually do anything to cure these problems. In some cases, they even lead to the need for retuning in areas where it is not the local transmitter causing the problem.

No FIVE on your transmitter: FIVE have a much smaller network of transmitters. Very few relays will carry it, and some main transmitters will be left out. In some locations where relays are generally used, FIVE can be received from another transmitter, usually the main transmitter for the area or one very nearby, but this will nearly always necessitate a new aerial.

A FEW NOTES ON AERIAL ISSUES

If, for any reason, you appear to need aerial work, it is worth getting it done by a quality installer. Especially when dealing with poorer signals, avoiding bodges could prove to be vital. Impedance mismatches should be avoided as much as possible - a good quality aerial with a balun will help, as will avoiding any unnecessary joints in cable, and making the necessary ones without using unnecessary Belling-Lee connectors. Unfortunately virtually all TV equipment in Britain uses Belling-Lee connectors, so they are not completely avoidable, but if you do need to join two lengths of cable, F connectors will generally do the job better.

Bodges such as multiple downleads on the same aerial or "twist and tape" splitters can cause standing waves to develop in your aerial system. These can lead to signals cancelling each other out, if, for example, the signal returning from the end of such a bodge is exactly out of phase with respect to a wanted signal. All such bodges should be avoided. Proper splitters or distribution amplifiers should be used.

It should be noted that many of the points about aerials are equally applicable to digital terrestrial television, although this is not the subject of this page.

Two highly respectable aerial installers have in-depth information about some of these issues, including some pictures of bodges. Please take a look at the web sites of Wright's Aerials and A.T.V. so that you know what I mean by quality.

TABLE OF "FIVE" TRANSMITTERS AND LIKELY PROBLEMS

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About the author

Phil Reynolds is widely accepted as an amateur expert in television reception. From his first home in Nottingham, he could receive signals from three FIVE transmitters (Lichfield, just about watchable, Waltham, perfect, and Nottingham, extremely poor owing to terrain blockage). In his second home, Nottingham was very slightly improved, Lichfield was better and Waltham was worse, but still very watchable. He moved to Edmonton, Middlesex, where reception of Croydon was difficult, so he got cable. Since then, he has moved to Colindale near Hendon, where reception of Croydon is acceptable, and has in fact improved further since the replacement of the old aerial which was destroyed in a storm. He has no formal qualifications in the field but has set up UHF aerials at campsites and on set-tops across Britain. He cannot take up aerial work professionally, as he has a disability which prevents him climbing ladders. He plans to experiment with VHF if he visits Ireland, although the closure of VHF relays and the Maghera band I transmitter may make this less interesting. He has also had interesting experiences in Kent receiving French analogue and Belgian analogue and digital television.