trees_factsheet

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The Effect of Trees on Television Reception =

Written by The BBC and ITC Engineering Information = Department (now part of OFCOM) and the Confederation of Aerial=20 Industries.

Originally issued as Arboriculture Research Note, = ref=20 146/98/TV, by the Arboricultural Advisory & Information Service and = this=20 edition was produced by BBC Reception Advice for use on-line on their = web site=20 and is a word for word version of the original note.=20

Abstract

Trees can have a serious effect on television = reception. The=20 effect can be reduced or avoided by careful species selection and siting = of new=20 trees. Where established trees cause a problem the choice and siting of = aerials=20 may provide an acceptable solution.

Introduction

1.Many Local = Authorities, local=20 amenity and conservation societies and householders are planting trees = which, if=20 carefully positioned, can enhance an area. However, few people are aware = of the=20 effect trees can have on the reception of television broadcasts = -particularly=20 those using analogue systems. All UK terrestrial television services are = transmitted in the UHF (Ultra High Frequency) band, on frequencies = between 470=20 and 854 Megahertz (MHz) on Channels 21-68. The greatest effect of trees = tends to=20 be on the higher frequencies, that is on channels 39-68. Television = broadcasts=20 via space satellites are increasing. Reception from satellites depends = even more=20 on a clear line-of-sight.

Effects of tree types

2.Evergreen trees have = more effect=20 on television signals than deciduous trees, and experience suggests that = coniferous trees are worse than broad-leaved evergreens. In winter, when = deciduous trees are bare, they have very little effect compared with = summer when=20 the same trees are in full leaf. During wet weather, when covered in = moisture,=20 all trees can have an appreciable effect on signals. As trees sway in = windy=20 weather the screening effect varies, leading to fluctuations in the = quality of=20 reception.

'Shadowing'

3.UHF transmissions = closely resemble=20 those of light waves. As with light, objects such as hills, buildings, = or tall=20 trees can obstruct or deflect the UHF signals, creating deep shadow = patterns in=20 which reception is difficult. The screening effect of trees has a = further=20 disadvantage in that it can vary greatly with the season and weather = conditions,=20 and is especially important in areas where television signals are weak. = In=20 strong signal areas, it is often possible to accept some attenuation = from trees=20 and still obtain a satisfactory picture.

'Ghosting'

4.If the television = signal is=20 reflected from buildings or trees in such a way that the reflected = signal=20 arrives

appreciably later than the direct signal, the = effect is to=20 produce a delayed image or 'ghost' on the screen. If the reflection = comes from a=20 wooded hillside, it can produce multiple images, which will be = continually=20 changing as the trees move in the wind. The use of specialist aerials, = such as=20 Log Periodic aerials, may help to resolve some problems.

Amelioration of existing problems

5.When the surrounding = trees are=20 below the level of the rooftop, there should be no problem, provided = that an=20 adequate root-mounted UHF aerial of the correct type is used. Where = trees are=20 above the level of the rooftop it may be necessary to install aerials so = that=20 they are above the trees. This can sometimes be achieved by mounting the = aerial=20 on a pole attached to the chimney stack or by using a remote aerial = clear of the=20 trees. The problem is more difficult for people who live in = single-storey=20 houses. However, it is sometimes possible to site a receiving aerial so = that it=20 is below the main branches of the trees. If there are no other = obstructions at=20 low level, then satisfactory reception may result. When deciding the = height of=20 an aerial, allowance should be made for growth of the tree (note 1) =

6.If the aerial cannot = be mounted=20 above the tops of the trees, the signal will inevitably be weakened in = passing=20 through the trees. This is not necessarily disastrous but it may involve = the use=20 of a high-gain aerial, possibly with a masthead amplifier. In any case = it is=20 desirable to use a good quality low-loss feeder to connect the aerial to = the=20 receiver, and it will often be advantageous to try an alternative = position for=20 the aerial - on a different chimney stack for instance.

7.If the trouble is due = to ghosting=20 rather than attenuation of the signal, the directivity of the aerial = becomes=20 important. A high-gain aerial is usually also highly directional but = some types=20 of aerial, such as the Log Periodic type, have very good directional = properties=20 but are not high-gain.

8.Viewers should contact = their local=20 television dealer, or a reputable aerial contractor, and take care that = the=20 aerial is adequately supported. If the obstruction is caused by trees on = a hill=20 which is so far from the viewer's house that the above suggestions are=20 impracticable, the only way of solving the problem, although = undesirable, may be=20 to remove those parts of the trees which are obstructing the signal, or = even to=20 fell the trees. Where trees are subject to statutory control (Note 2) = permission=20 should be obtained or the appropriate notice given. Common law = precedents should=20 also be observed (see Dobson and Patch 1998)

Satellite Broadcasting

9.Satellite broadcasting = of=20 television and radio, directly to private homes, has become common = place. A=20 General Planning Consent (Note 3) has been given for small dish aerials = to be=20 located on private dwellings, subject to constraints on size and = siting.=20

10.Satellite = transmissions use=20 frequencies in the 11 and

12 Gigahertz (GHz) bands. At these high = frequencies (up to=20 20 times greater than UHF broadcasts) the power of the received signals = is low.=20 The signals are completely blocked by any obstruction such as trees. In = siting=20 the receiving "dish" an allowance must be made for movement of trees in = a wind=20 and for future growth of the trees. Loss of signal caused by trees will = require=20 relocation of the "dish", pruning or in extremis removal of the tree. = However=20 permission should be obtained or the appropriate notice given where = trees are=20 the subject of statutory control (Note 2).

11.Several satellites in = geostationary orbit are 'visible' from the UK; of these the ASTRA = satellites=20 attract the biggest audience. "Dishes" need to be aimed at about 30=B0 = East of=20 South and between 17 and 30=B0 above the horizon, depending on the = latitude of the=20 receiving site. Other satellites will be on different bearings but at a = similar=20 angle above the horizon.

Digital Television

12.Any problem with = Digital=20 Television, both satellite and 'through the aerial' are likely to be = almost=20 impossible to diagnose from inspection of the pictures. This is because = Digital=20 Television either gives perfect pictures or none at all. There is a very = narrow=20 area between these states where intermittent frozen pictures or a square = 'blocking' effect may be observed. The cause may be a weak signal or = conditions=20 that lead to ghosting on analogue television.

13.However on the whole = Digital=20 Television is more immune to the conditions that cause ghosting but it = is not=20 more immune to weak signals. Professionals installing Digital Television = receiving equipment or aerials for Digital Television should use special = instruments, which will enable them to measure that the signal is above = the=20 minimum required to get pictures.

14.In the case of = 'through the=20 aerial' problems, it will be worthwhile to inspect the analogue pictures = for the=20 problems described earlier. There is no such guide for picture problems = with=20 Digital Satellite Broadcasts.

TV reception and the Law

15.The TV licence is a = permit to=20 operate a television receiver, it does not guarantee any reception and = it=20 therefore follows that there is no legal right to reception. There are = no court=20 precedents in respect of trees interfering with TV reception. It seems = likely=20 that existing trees on neighbouring land which interfere with television = reception, especially with satellite transmissions, are unlikely to be = regarded=20 as a "nuisance" in law. Any remedial work, therefore, should be by = agreement=20 with the tree owner and within the statutory controls (Note 2). =

Note 1

Arbouriculteral Research Note 84

Note 2

Tree Preservation Orders, Felling Licences and = Conservation=20 Areas

Note 3

Town and Country General Development Order 1998 = Parts 1 and=20 Schedule 2

Avoidance of problems

16.When planning to = plant trees, it=20 is advisable to consider the effects of spacing and the anticipated = mature=20 heights of the trees to be planted (Note 1). However, the problem is not = always=20 easy to identify in the short-term. The effects will change not only = over the=20 season but also gradually over a period of years as growth occurs and = trees=20 mature. The signal path from a transmitting station is not readily = apparent from=20 a receiving site, and, to be accurate, detailed profiles may have to be=20 produced. Arboriculturists and landscape-architects wanting specialist = advice=20 about the possible effect of trees on television reception at a = particular location, should write with the fullest possible = details=20 to one of:

BBC Reception Advice PO Box 1922 Glasgow G2 3WT =

Tel: 0870 0100 123 (national call rate) Office = open 09:00 -=20 17:00 Monday - Friday

Ofcom

03-64 Riverside House 2a Southwark Bridge = Road London=20 SE1 9HA tel: 020 7783 4325 fax: 020 7783 4303 = broadcast.technical@ofcom.org.uk=20 www.ofcom.org.uk=20

Broadcast Technical Policy = (Information)