All forms of broadcast reception depend upon the use of some type of receiving aerial for the purpose of producing an electrical signal from the waves radiated by the transmitter. The receiver then uses this signal to reconstruct the programme material. Receiving aerial performance is paramount and the receiver cannot produce good results from a poor input signal. This principle holds true for all versions of broadcast reception.
In the United Kingdom the broadcasters currently provide a service to over 99% of the population from a network of 51 high power main station transmitters supplemented by a large number of smaller relay stations. Each station radiates four services at equal power: ITV, Channel 4 (S4C in Wales), BBC-1 and BBC-2 (including Teletext) normally through a common aerial, thus providing similar coverage areas. This also avoids a proliferation of transmitting aerials and usually provides the viewer with all four television channels from a single receiving aerial. (Note: The new Channel 5 is limited to 33 transmitters covering about 74% of the UK population using differing power levels and may require a separate aerial).
Four or five transmission frequencies or channels are required at each transmitting station, one for each programme, out of the 44 channels available by international agreement for UHF broadcasting. This means that the network has been planned in a manner which avoids mutual interference between stations using the same transmission channels. The radiated power of the transmitters must therefore be strictly controlled. In general the powers are chosen so that at the average home good results are achievable by use of a modest aerial mounted externally at roof height and clear of obstructions. Most viewers will be within the service area of a transmitter and in some overlap areas may have the choice of an alternative service. The first step to achieving good reception is to find out which transmitter provides the best signals at your home.
Viewers who are situated less advantageously can still achieve good results by using a larger aerial possibly with a preamplifier. In areas where field strengths are somewhat lower and the construction of a relay station cannot be justified, the use of a high gain receiving aerial and a masthead preamplifier may be required.
UHF aerials are made in a number of types and are also divided into groups for different ranges of transmission channels. It is important that the receiving aerial corresponds to the transmission group serving your area and is of a type suitable for use with the signal levels available at the location where the aerial is to be installed. The two important characteristics are the gain of the aerial which determines how low a level of signal can be used and its directional characteristics which enables an aerial to possibly reject interfering signals. A competent aerial erector will be able to advise on the best aerial to use.
Installation needs to be of a high standard, both electrically and
mechanically as the aerial is exposed to the weather and can be subjected to
considerable forces due to the wind. Electrical performance tends to fall over
a period of time, particularly in industrial and coastal areas. Should
reception deteriorate it is often due to the aerial developing a fault after a
number of years.
Of equal importance is the down lead or feeder cable which connects the aerial to the receiver. This needs to be of high quality and well installed with no sharp bends or damage. Feeders deteriorate with time and may eventually need to be replaced. Water penetration at the aerial connector can also cause a marked loss of performance.
The use of indoor or set top aerials is not recommended because they invariably give inferior results to those obtainable by even the simplest external aerial. In particular loft mounted aerials, apart from suffering a considerable loss in signal due to the roof material, can frequently give rise to impaired pictures caused by the presence of reflected signals from objects within the loft. This can be particularly troublesome for Teletext reception. Loft aerials can also pick up mains borne interference from domestic wiring.
In principle planning permission is required for the erection of a
television aerial but in practice most planning authorities do not enforce
this, provided that the installation is not too obtrusive. A more formal view
is usually taken for the erection of aerials in National Parks, Areas of
Outstanding Natural Beauty and on buildings of Historic interest. If in doubt
you should consult your local planning officer for advice. Some householders
are also barred from putting up external aerials by a restrictive covenant but
in many cases this can be relaxed. Any restriction on the use of external
aerials means that viewers are likely to achieve inferior results unless
provided with an alternative means of reception such as a communal aerial and
wired distribution system.
This fact is accepted by the Department of Trade and Industry's Radio Investigation Service who will not investigate complaints of interference to reception unless an external aerial is being used.
ęCopyright 1996 National Transcommunications Ltd.