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Mounting A Terrestrial TV Aerial In The UK

1  General Introduction

Terrestrial TV

Terrestrial TV is what we used to call simply TV, in the UK the standard five analogue channels  BBC1, BBC2, ITV1, Channel 4, & Five.  The complicated modern name simply distinguishes it from other options that have recently become available.  In Terrestrial TV, content is encoded into electrical signals, which land-based transmitters convert into signals superimposed onto electro-magnetic (e-m) waves (universally aka 'radio' waves, though strictly this is incorrect, as generally the signals used for radio lie in a different part of the electro-magnetic spectrum).  Reception by domestic aerials converts these back into electrical signals in the downlead, which feed into the aerial socket of a TV or STB to be converted back into content.

Transmitter Groups

UK Terrestrial TV is broadcast from a surprisingly large number of transmitters, about 1130, with the greatest distance between neighbouring ones being only 32 miles (51km).  Consequently, most locations are within about 18 miles (30km) of a transmitter.  Nevertheless, this doesn't guarantee that a signal can be received anywhere in the UK, as, for example, 'holes' or 'blind spots' can be caused by local topography, such as hills obstructing a transmitter's signals into neighbouring valleys.

Transmitters are grouped with a main transmitter, usually atop a tall mast on high ground broadcasting a circular 'radiation pattern', that is equally in all directions, to cover a general area, and sub-transmitters (aka 'relays'), again often atop a mast but sometimes tall buildings, filling holes in the main's coverage, often by broadcasting a pattern directed into the hole with signals differing from the main to prevent mutual interference.  Transmitters near the coast or the edge of a region may also broadcast a directed pattern inwards to minimise interference in neighbouring countries or regions.

Digital Switch Over

The UK is currently undergoing a nationwide conversion from analogue transmissions to digital, known as DSO, sometimes ASO.  Very approximately, DSO can be regarded as having started at the extremities of the nation and to be working inwards towards the capital.  In areas already converted to the west and to the north, all analogue transmissions have ceased.  In areas yet to be converted, main and a few other important transmitters broadcast digital transmissions interleaved between the existing analogue channels, albeit at a significantly lower power than their analogue transmissions, in order to protect the latter from degradation by the additional transmissions.  Thus in any given area, most or all of the population can already receive Digital Terrestrial Television (DTT, aka 'Freeview').

There are many common misunderstandings related to DSO, but an important point to grasp is that there is no such thing as a digital electro-magnetic wave, it is the signals superimposed on the wave that are being converted to digital, and consequently, contrary to common claims, there is no such thing as a digital aerial  all aerials are analogue in operation, and, in principle at least, digital signals can be received by the same aerial as existing analogue signals.

Therefore, a claim that a particular aerial is 'digital' is at best an erroneous simplification for the non-technical, at worst the claim of the unscrupulous.  While encountering this claim may not in itself be necessarily a reason for outright rejection, at very least it should put any potential customer on their guard.  There are a number of other common misunderstandings about DSO which also might allow the unscrupulous to take advantage of the unwary.

Having said that, for the following reasons DSO is a good reason to ensure the household aerial system is adequate and in good condition.  However, it should be remembered that any upgrade made solely on the basis of pre-DSO Freeview reception will often prove to have been unnecessary after DSO, as signal levels will be increased and channels better grouped.

There is another aspect of digital TV which should be mentioned, the controversial one of compression.  Digital TV is usually not just a straight swap from analogue to digital representation of the content, but digital compression is also employed.  There are two types of compression techniques, 'lossless' where the original data can be reconstructed exactly, and 'lossy' where information is actually thrown away  to use audio as a more familiar analogy, an example of the former would be the Flac and Ape compression algorithms, while of the latter would be MP3.  Unfortunately, lossy algorithms are used in TV.  Worse still, they are abused  in the UK, approximately 1-3% by bitrate of the original data captured by the cameras is all that ever arrives at our TVs!  Symptoms of over-compression that are commonly noticed are 'fly-swarms' around moving objects, breaking up of the pictures into squares, and excessive softening of the picture by the broadcasters in an attempt to ameliorate the first two.  What is really needed is a regulator that has teeth and is prepared to use them to lay down minimum broadcasting standards which all channels should meet, including bitrate.  You can't squeeze a quart into a pint pot, so if that means throwing out some or all of the largely useless shopping and +1 channels and the like, so be it.

High Definition

Besides DSO, there is now also a simultaneous upgrade of one digital mux  BBC B, aka PSB 3 and Mux B  to HD (DVB-T2), which cannot be decoded by SD (DVB-T) tuners.  In most cases, this will occur at DSO, but five transmitters due for late DSO will broadcast an additional HD mux until DSO, while those transmitters already DSO-ed had BBC B retro-converted during 2010.  In the Calculator on the following page, information on such cases is included in the transmissions table for each affected transmitter.

DIY

At any given location, there are four characteristics of broadcast signals important for reception, and to install an aerial, directly or indirectly you will need to know, ideally, all of the following, the first three of which can be obtained easily from the following Calculator page, which also gives a guesstimate of the fourth:

Azimuth

The site & transmitter locations completely determine the direction in which to point the aerial, known as 'azimuth', which is the horizontal angle measured clockwise from True North.  This should be calculated before work begins, and is best set by using one of the methods below (although none are entirely without complication) to find a distant landmark in the required direction, and then pointing the aerial at the landmark:

Signal Aerial Group

UK broadcasting divides the UHF TV band into slices of bandwidth each capable of carrying a single named analogue channel such as BBC1, or now a single digital mux such as BBCA.  Perhaps confusingly, the slices themselves are also called channels and are numbered 21 - 60, familiarly to those who remember the tuning knobs of old TVs.  The actual frequency in MHz of each channel can be determined from the formula: f = 8*c + 306, where c is the channel number.

Aerial gain (signal output level) and bandwidth (the range of frequencies captured) mutually conflict.  Hence, to allow aerials to be of useful gain, the UHF band is also more coarsely divided into aerial groups.  The current system originally had three, one of which would have contained all the transmissions of any given transmitter.  This enabled:

However, Five and DTT have led to wider groupings, so compromising these advantages where they're in use.  Here are the current groups:

GroupChannel RangeCap colourSchematic showing aerial group overlap
A21 - 37RedGroup A Cap ColourSchematic showing aerial group overlap
B35 - 53YellowGroup B Cap Colour
C/D48 - 60GreenGroup C/D Cap Colour
E35 - 60BrownGroup E Cap Colour
K21 - 48GreyGroup K Cap Colour
W21 - 60Black or noneGroup W Cap Colour
PolarityAlignment Of Elements
HHorizontal
VVertical|

Note that where a transmitter uses a semi-wideband group (E or K), Ofcom often suggest wideband (W) as an alternative, resulting in E/W or K/W.  However, for performance reasons, I suspect that in such cases most professional installers would recommend the narrowest group aerial that can accommodate the signals required, and not a wideband.

Signal Polarity

Signals are usually horizontally polarised (H), requiring the elements in the receiving aerial to be horizontal, but some relays broadcast vertically polarised signals (V), while a few transmitters transmit both, near which you may actually be able to receive signals of either polarity.

Signal Level

Signal level (more correctly, not just absolute signal level, which must be above a minimum threshold to drive the receiver, but also the CN Ratio, a measure of the wanted signal's level above other unwanted signals) is determined by:

  1. The transmission broadcast power, known as Effective Radiated Power (ERP).
  2. The transmission radiation pattern (how the broadcast varies with direction).  Unfortunately, Ofcom do not supply radiation patterns for analogue and pre-Digital Switch Over (DSO) digital broadcasts, and for post-DSO will do so only some time after DSO completes.
  3. Distance to the transmitter.
  4. Intervening topography, known as 'terrain', including curvature of Earth's surface.
  5. Intervening natural or artificial features in the landscape such as trees or buildings, known as 'clutter'.
  6. Sources of electro-magnetic interference, particularly other broadcasts in the same part of the spectrum.

Given the complexity of the above, how can anyone find their signal level?  The only truthful answer is to measure it, but only reputable aerial installers or official agencies are likely to have the required equipment.

There are a number of mathematical models used for predicting signal levels given the above information, but even the one that uses the most complete data and is therefore arguably the best of them, the official Digital UK Postcode Checker, probably gets it wrong too often, occasionally surprisingly badly.  It is considered 'pessimistic' because of the strictness of its correction for availability over time, and because the interpretation of its results on a post code wide basis leads to a post code with mixed reception being marked out of coverage even though individual addresses within it may be within coverage.  By contrast, the one on the following Calculator page works from the actual location of the receiving aerial, but is probably too optimistic because it lacks transmitter radiation patterns and a correction for availability over time  hopefully the truth will at least lie somewhere in between!

Aerials

Signals are received by aerials, which can range from, in areas of high signal strength, a simple loop atop a portable TV, to, in areas of low signal strength, complicated arrays of elements atop a mast mounted on the outside of the house.  Ideally, aerials will match the signal group of the transmission as this maximises performance, but the majority of aerials in DIY stores are wideband.  These eliminate returns due to unknowledgeable customers buying the wrong aerial group and finding that they don't work, but at the expense of performance.

If this is your first time, there are cautions about DIY aerial rigging.  Try to get a sense of the forces involved  aerials are usually made of aluminium and so seem comparatively light to us, but the weight alone of a big aerial is not usually the problem, it's the combination of its weight and wind resistance magnified by leverage over the lengths of both the aerial and, particularly, the mast.  Wright's Aerials Rogues Gallery is a good place to browse to see the sort of things that can go wrong!  Jim's Aerials - Aerials & Coax has some systematic data which may be useful for calculating the required gauge of mast.

Installation Requirements

To complete installation successfully, you will need the following:

  1. Beforehand:
    • Aerial details  -  Azimuth (and a method of setting it), Group, Polarity;
    • Aerial of the correct signal group and suitable gain;
    • A site for the aerial that, preferably:
      • Is solid enough to prevent the mast being levered off the wall or chimney in a gale;
      • Has line of sight to the transmitter, free of local obstruction by trees, buildings, passing high vehicles, washing lines, etc;
      • Doesn't look out over nearby sources of interference such as other houses;
      • Is out of reach of vandals or burglars;
    • Fixing bracket & mast, may be part of a kit  -  avoid flimsy single-piece pressed-steel brackets, use industry standard aerial brackets such as T&K brackets for a wall, or chimney brackets for a chimney, a mast of sufficient strength;
    • Fixing bolts, may be part of a kit  -  avoid the plastic-sleeved coach bolts often supplied, use something like anchor bolts having a wedge action to grip deep in solid masonry.
    • xx100 CAI Benchmarked double-screened downlead cable, may be part of a kit;
    • Self-amalgamating tape if there are to be outdoor cable join(s);
    • Sealant for where the cable enters the house;
    • Cable clips for the downlead;
    • Optionally a double-screened 'Belling' Aerial wall-socket and pattress;
    • Good quality 'Belling' plugs for the cable ends;
  2. Installation:
    • Ladders;
    • Electric hammer drill and masonry bits (10mm?) for the fixing bolts and cable entry hole, the latter long enough (400mm?) to drill through into the house, perhaps an extension cable, an RCD is always a good idea.
    • Ring spanners for the anchor bolts, U-bolts, aerial mountings;
    • Screwdriver for the aerial connection cover holder;
    • Sharp knife or wire stripping tool;
    • Wire clippers or pliers;
    • Hammer;
    • Electrical screwdriver for the wall-socket;
  3. May be useful:
    • Plumb line, or a spirit level with vertical as well as horizontal scales;
    • High scale map  -  Ordnance Survey (OS) Landranger 1:50000 or Explorer 1:25000 (Public Library);