Radiofrequency Electromagnetic Fields in the Cookridge Area of Leeds, England
Total Page：16
File Type：pdf, Size：1020Kb
Radiofrequency Electromagnetic Fields in the Cookridge Area of Leeds, England S Y Ely, K Fuller, A D Gulson, P M Judd, A J Lowe and J Shaw Occupational Services Department, National Radiological Protection Board, Hospital Lane, Cookridge, Leeds, LS16 6RW, England. E-mail: [email protected] Abstract: The National Radiological Protection Board (NRPB) is an independent body that has responsibility for advising UK government departments and others on the protection of people from ionising and non-ionising radiation (NIR) hazards. NRPB undertakes research into NIR sources and effects, and provides a comprehensive measurement and advisory service. In addition, NRPB receives many thousands of enquiries each year from individuals who are concerned about their exposure to radio waves, particularly those emitted by mobile phone base stations at around 900 and 1800 MHz. This paper describes the response to one such enquiry from a Member of Parliament (MP) representing local community groups and residents, who were concerned about emissions from three large telecommunications installations close to their homes and local schools. A survey of the radio wave exposure in the area around the three installations was carried out, measuring the total exposures due to all radio signals from 30 MHz to 18 GHz, using a variety of antennas connected to a spectrum analyser. The measurement range was chosen to include all the emissions from the local transmitters, including mobile phone base stations, emergency services' radiocommunications, pagers and microwave dishes. Emissions from transmitters further away, such as broadcast television and radio signals, were also detected. The results are compared against the guidelines published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The total exposure from radio signals between 30 MHz and 18 GHz, at the schools and residential areas, ranged from 0.00016% to 0.041% of the ICNIRP reference level for exposure of the general public. The highest total exposure, 0.14% of the ICNIRP public reference level, was measured at the top of a practice tower at a nearby fire station. The paper concludes by describing the way in which the findings have been publicised and the feedback received from the various interested parties. 1. Introduction There are eleven different organisations that have radiocommunications antennas installed on a water tower, a lattice tower and a radio station tower in the Cookridge area of Leeds. Due to concern from some of his constituents living in this area, the local Member of Parliament asked NRPB to carry out an assessment and provide a report of potential hazards arising from exposure to radio waves. The full report of this work can be downloaded from the NRPB website [1]. 2. Description of sources There are antennas installed on three different structures at two separate sites adjacent to Otley Old Road in the Cookridge area of Leeds. An overview of the various radiocommunications antennas is given below and there is also a brief description of other radio transmitters commonly found in the environment. The beam shapes of the two main types of transmitter installed at the sites are also described. The technical terms that are used are explained in more detail in the report NRPB-R321 [2]. Both sites are surrounded by secure perimeter fences that prevent unauthorised access to areas close to the towers. It is possible that power densities in the first few metres directly in front of some of the antennas could exceed guideline levels. It would be normal practice for the operators to have assessed the extent of these regions and then to have controlled access through administrative procedures or physical barriers. As this survey was in response to concerns regarding exposures at public locations no further consideration is given to the potential exposure of workers and all of the measurements were made outside the site boundaries at locations where the general public have access, with the exception of the Fire Station. 2.1. Yorkshire Water site This site accommodates two structures in the form of the water tower and a steel lattice tower. These are described in more detail below. 1 2.1.1. Water tower The water tower supports three mobile phone sector antennas operated by Orange. These are mounted on poles above the top outside edge of the tower. The tower also bears a 0.6 metre diameter microwave dish antenna also operated by Orange. 2.1.2. Lattice Tower The lattice tower supports a variety of different antennas operated by West Yorkshire emergency services and also by the landline telephony provider, NTL. The Police have several different antennas installed on the tower, details of which are confidential. West Yorkshire Metropolitan Ambulance Service also have antennas mounted on the tower. These include three folded dipole antennas and a shrouded yagi antenna. West Yorkshire Fire Service operate two antennas mounted on the tower: a folded dipole and a three-element yagi. NTL operates three microwave dish antennas - a 1.2 metre diameter dish at a height of 12 metres and two 1.8 metre diameter dishes at heights of 12 and 24 metres. 2.2. Tinshill BT Radio Station The radio station is built around a 53 metre tower comprising of a steel lattice tower mounted on a tubular concrete tower. The mid-section of the concrete tower supports approximately 16 large microwave dish antennas. The majority of these are operated by British Telecom (BT) and provide point-to-point microwave telecommunications links to other distant sites. Approximately 50 other smaller microwave dish antennas are mounted on the tower (mainly on the upper lattice tower), along with other mobile phone and TETRA (terrestrial trunked radio) sector antennas. The mobile phone transmitters at this site include three sector antennas operated by O2 at a height of approximately 38 metres, six sector antennas operated by T-Mobile at a height of approximately 46 metres and three sector antennas operated by Vodafone (no information regarding height was supplied by Vodafone, although the OFCOM Sitefinder website [3] indicates a height of 17 metres for the Vodafone antennas). One TETRA sector antenna operated by Dolphin Telecommunications is mounted on the tower, and PageOne operates a stack of two dipole antennas (2-stack), mounted at a height of approximately 35 metres above ground level. O2 also operates two dipole paging antennas that are mounted on the tower at heights of 50 and 55 metres above ground level. 2.3. Antenna beam shapes In terms of their numbers, there are two main types of antenna installed at the local transmitter sites. These are mobile phone sector antennas and microwave dish antennas. These give rise to significantly different radio wave beam shapes, which are required for their different applications. The beam shape also has a significant influence on exposure at ground level. Mobile phone sector antennas produce beams that are transmitted into the area around the base station. These beams are narrow in the plane of elevation and are tilted slightly downwards so the top edge of the main beam is approximately horizontal and the lower edge is directed 5 to 10° below horizontal. The main beam from sector antennas would be expected to reach ground level typically between 50 and 300 metres from the foot of a mast. The general principle of beam formation is illustrated in Figure 1. 2 FIG. 1. Elevation showing the shape of the beam formed by a typical sector antenna used with a macrocellular base station. Microwave dish antennas produce narrow conical beams that are 1 to 2° wide and allow data to be transmitted between distant sites. They are utilised by mobile phone network operators so that base stations can communicate with other neighbouring base stations and connect calls into other networks. They are also used for more general long distance data transmission by these and other operators. The general principle of beam formation is illustrated in Figure 2. The nature of point-to-point communication involving microwave dish antennas is such that the beams are not directed towards the ground or at people’s homes as they must have a clear line of sight to the receiving antenna at the distant site. As a consequence the exposures at ground level produced by signals from microwave dish antennas will be negligible in comparison to those from sector antennas. Dish antenna Conical beam less than 2 wide FIG. 2. Pair of dish antennas used for a point-to-point microwave link 2.4. Other environmental radio transmitters In addition to the signals transmitted from the antennas installed at the Cookridge sites, radio signals were measured from a variety of other environmental radio transmitters. The measured frequencies of these signals gave an indication of their likely purpose through reference to the allocation tables on the Office of Communiciations (OFCOM) website. [4]. A graph showing the frequencies and strengths of all the radio signals measured at one of the locations considered during this work is shown in section 3, below. Broadcast radio and television signals can be measured wherever receiving equipment can be used. VHF FM radio signals are broadcast in the 87.5 to 108 MHz frequency band and UHF television signals are broadcast in the 470 to 590 MHz and 598 to 854 MHz bands. GSM base stations for mobile telephones have been allocated two frequency bands: GSM900 base stations transmit in the 935 to 960 MHz band and GSM1800 base stations transmit in the 1805 to 1880 MHz band. Other signals often measured are those used for paging at approximately 138 and 153 MHz and a variety of analogue professional radio systems operating around 150 to 200 MHz and 425 to 470 MHz. Base stations for the TETRA digital professional radio system operate between 390 and 393 MHz and 420 to 425 MHz.