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 complicated modern name simply distinguishes it
from other means of reception 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
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 also broadcast a directed pattern inwards to
minimise interference in neighbouring countries or regions.
Digital Switch Over
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 the five former analogue channels and others, available from almost every transmitter, and
others only available from main transmitters and a couple of major relays. Thus PSB coverage is
broadly comparable to analogue coverage at 98.5% of households, while other muxes reach up to about 90.5%.
Besides DSO, there was also a simultaneous upgrade of one digital mux -
PSB 3, aka BBC B or Mux B - to
HD (DVB-T2). In most cases, this occurred at DSO, but
five late DSO transmitters broadcast an additional HD mux until DSO, while transmitters already
past DSO were retro-converted during 2010. Other HD muxes have since begun rolling out.
Largely coincident with DSO, by EU agreement, the UK has released UHF
channels 61 to 68 from TV broadcasting to other purposes, and in the UK these channels will supply 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.
However, it should be noted also that there is great uncertainty as to whether and by how much 4G services
will affect reception of TV broadcasting on neighbouring channels 58-60, see the note below in
Reception about cross-modulation.
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 …
There are no such things as digital or SD or HD
electro-magnetic waves, it is the signals superimposed on the waves that have these properties, and
consequently, contrary to common claims, there is no such thing as a digital or HD aerial
- all aerials are analogue in operation, and, in
principle at least, digital HD signals can be received by the same aerial as former analogue signals.
It is the receiver equipment connected to the aerial that needs to be appropriately matched to the type of
signals to be received, particularly:
Old analogue receivers cannot decode digital signals and vice versa.
UK SD (DVB-T) receivers cannot decode HD (DVB-T2), but HD (DVB-T2) ones are
backwardly compatible and can decode SD (DVB-T).
While analogue reception degrades 'gracefully' with falling signal level, leaving the viewer to judge
when the result becomes unacceptable, digital reception has a threshold level, aka as the 'digital cliff'
- above it reception will be good, beneath it nothing at all, and
around it possible alternation between these two extremes.
Digital reception is more sensitive than analogue to electro-magnetic interference, a particular problem
being 'impulse' interference, such as is caused by 'dirty' central-heating switching, unsuppressed
motor-vehicle ignition, etc. While with analogue this might cause snow and buzzing for a few
seconds, with digital it may result in dramatic picture break up and 'gunfire' on audio, even temporary
complete loss of signal. Such interference is best cured at source, but this may not be possible
if it doesn't originate from within your property, hence it is desirable to have an aerial system that
as far as possible rejects such interference. Key to such a system are a highly directional aerial,
a balun, and double-screened downlead cable, wall-sockets, and aerial flyleads.
Cross-modulation is another type of interference that is worth mentioning at this point, as it often
first became a problem when aerial signals were amplified to pick up the weaker, pre-DSO 'Freeview'
signals, and now may be caused by the latest mobile phone equipment. This type of interference
is when neighbouring band signals - such as 4G, or
used in the UK by the emergency services - are picked up by an
aerial system, where their closeness in frequency causes them to interfere with UHF TV. In theory,
the cure for this is a simple filter in the aerial download between the aerial and any receiving
equipment, including any amplifier. With TETRA this is established practice, however with 4G the
situation is much less clear. There is much talk of homes being provided with filters to be
fitted to the aerial downlead, but there remains great technical doubt as to whether filters will
really be able to remove 4G adequately from the closest of these TV channels, as well as to what
arrangements will be made to distribute and fit these filters and whether and in what circumstances
they will be chargeable to homeowners.
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:
Direction in which to point the aerial, known as 'azimuth';
Signal aerial group (not to be confused with the transmitter groups discussed above);
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
Using A Compass
A compass is simple and sufficiently accurate. Nearby metal can deflect the needle, in which
case walk off in the direction of your sighting until it steadies. Allow for Magnetic Variance,
the difference between Magnetic and True North, which varies significantly over place and time.
The following Calculator page can guesstimate it.
Using A Map (including an internet map)
For printed maps, note that their gridlines are usually laid out as if we lived on a flat rather
than a spherical surface. In the UK, this means that Grid North varies significantly between
about 3°W of True North in the West and 3°E
in the East. Lesser maps such as A-Zs may not give grid data,
but the following Calculator page can work it out mathematically and will use the value to display
a grid-corrected azimuth alongside the true azimuth.
On a suitable local map, measure the Calculator's Grid Azimuth with a protractor and follow its
line outwards from the site looking for a landmark.
Alternatively, there is internet mapping technology, such as on the following Calculator page,
which allows you to position a pointer over a map and/or satellite image of the aerial site.
Although in principle a neat idea, appealing in its simplicity, in practice note the following
A system can only be as accurate as its underlying data (for which often
no figures are published);
Important parts of the map may not print properly in all browsers.
Using The Sun
By the following method, find out, for the location and date, what time the sun is over the given
azimuth, and at this time and place, mark the direction of the shadow of something known to be
Using a spirit level with both vertical and horizontal levels, perhaps less accurately a
plumb-line, measure at least two points at right-angles to each other …
Forgetting about Daylight Savings Time. British Winter Time
is the same as GMT
while British Summer Time is 1 hour ahead of it. Potential
error = 1 hour, or 15°.
Forgetting about the 'Equation Of Time'. Earth's orbital
speed and axial tilt with respect to the sun vary seasonally.
Consequently, our clocks do not keep time with the sun on a daily
basis, only on a yearly average. Thus on any given day at
Greenwich the sun will usually not be directly overhead at
12noon GMT. Potential error = up to 16 minutes, or 4°.
Forgetting to allow for longitude, which affects when the sun will be overhead. Potential
error: 4 minutes earlier/later for every degree of the site's longitude East/West.
Except at sunrise and sunset, the colours of which can change radically within a minute, we tend to
underestimate how fast the sun is moving - the sighting must be done promptly at the
designated transit time. Potential error = 1° for every 4 minutes error in timing.
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:
Aerials receiving from that transmitter to be optimised for the best gain;
Neighbouring transmitters to use different groups to avoid mutual interference in areas of signal overlap.
However, Five and DTT 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
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.
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.
(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:
The transmission broadcast power, known as Effective Radiated Power (ERP).
The transmission radiation pattern (how the broadcast varies with direction). Unfortunately,
despite repeated requests, as yet
do not supply radiation patterns.
Distance to the transmitter.
Intervening topography, known as 'terrain', including curvature of Earth's surface.
Intervening natural or artificial features in the landscape such as trees or buildings, known as 'clutter'.
Sources of electro-magnetic interference, particularly other broadcasts in the same part of
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
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.
The signal group of the aerial should match that of the transmission, and it should be mounted with the
correct polarity. The performance in terms of gain/sensitivity of the aerial should match that of
the (expected) signal level. If in doubt about this, try to get professional advice. In the
UK, you could try a newsgroup such as uk.tech.digital-tv, as linked below.
The mast, downlead, or other metal should not interpose between the aerial elements.
A long aerial should be mounted on the mast seated atop a cradle in the middle rather than from
a bracket at the end, as the latter may snap in high winds.
However a cradle may interpose undesirably between the elements of a vertically polarised aerial.
Ensure that the mast is vertical.
Masts should be substantial enough to resist buckling in high winds -
the taller the mast, the thicker gauge it must be, and the more substantial and widely spaced the
mountings. Steel masts are stronger than aluminium, but may rust in the long term.
T&K wall brackets, and top and bottom chimney brackets, should be separated by at least 1/5th of the
total length of the mast, to withstand leverage in high winds.
The downlead connection must be waterproof, normally it's made in a sealed junction box on the aerial.
Except log periodic aerials, the downlead connection should have a
A small circuit board inside the junction box forming a passive electronic device that matches
the electrical characteristics of the aerial to those of the cable -
this maximises signal, and minimises noise, transfer into the downlead.
Only one downlead should be connected to the aerial, otherwise signal transfer into the downlead will be
compromised. If several rooms are to be supplied, use a distribution amplifier.
To complete installation successfully, you will need the following:
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;
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;
Electrical screwdriver for the wall-socket;
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);
Useful links (no endorsement of external sites intended nor responsibility taken for their content):