Terrestrial TV is what we used to call simply TV, in the UK formerly the standard five analogue channels - BBC1, BBC2, ITV1, Channel 4, & Five. The more complicated modern name simply distinguishes it from other means of reception that are now 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.
UK Terrestrial TV is broadcast from a surprisingly large number of transmitters, about 1120, 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 broadcast a directed pattern inwards to minimise interference in neighbouring countries or regions.
Between 2008 & 2012 the UK completed a nationwide conversion from analogue to digital transmissions, known as DSO or ASO. In DTTV, in the UK aka 'Freeview', a mux carrying many digital channels occupies the same spectrum space as a single former analogue channel. There are three PSB muxes, carrying amongst others the five former analogue channels, available from almost every transmitter, and other muxes only available from main transmitters and some major relays. PSB coverage is broadly comparable to analogue coverage at 98.5% of households, while other muxes reach up to about 90.5%.
One digital mux - PSB 3, aka BBC B or Mux B - was upgraded to HD (DVB-T2). In most cases, this occurred at DSO, but five late DSO main transmitters broadcast an additional HD mux until DSO, while transmitters already past DSO were retro-converted during 2010. Other HD muxes such as COM7 & COM8 have since begun broadcasting.
By EU agreement, largely coincident with DSO, the UK has already released UHF channels 61 to 68 from TV broadcasting to 4G mobile services. As the start of DSO preceded this agreement, some transmitters, mostly from those DSO-ed early, required additional changes to release these channels, but this is now accomplished and of historical interest only.
Between 2017 and 2020 progressively over the entire UK, further channels above 50 @ 700MHz, most of the C/D group, will be released for mobile use. As with DSO, these ongoing changes will cause 'retune events' requiring affected residents to retune equipment to pick up changes, some may even need new aerials. From 14/2/2017, just two weeks before the first channel release at Selkirk, further information should be available from the Freeview Advice Line on Freephone 0808 1000288.
It should be noted that there is great uncertainty as to whether and by how much 4G & 5G services will affect TV reception on immediately neighbouring channels, see the note about cross-modulation below in Reception.
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.
There are many common misunderstandings related to reception, the most important points to grasp being …
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:
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:
Common Errors Using The Sun:
UK broadcasting divides the UHF TV band into slices of bandwidth each capable of carrying formerly a single 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, DTT, and the ongoing clearance of higher frequencies for mobile use have led to wider groupings, so compromising these advantages where they're in use. Here are the current groups:
|Schematic showing aerial group overlap
|21 - 37
|35 - 53
|48 - 60
|35 - 60
|21 - 48
|21 - 60
|Black or none
|Alignment Of Elements
Although, for performance reasons, I suspect that in most cases most professional installers would formerly have recommended the narrowest group aerial that could accommodate the signals required, with the increasing number of additional out-of-band transmissions in populated areas and the release of high numbered TV channels for mobile use, authorities, and therefore probably at least some professional installers, are now recommending that those needing to replace an aerial should fit a wideband replacement.
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 polarisation.
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:
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 equipment of useful accuracy.
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!
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.
To complete installation successfully, you will need the following:
Useful links (no endorsement of external sites intended nor responsibility taken for their content):