Modeling Electromagnetic Radiation on Lookout Mountain, Colorado

Transcription

1 Modeling Electromagnetic Radiation on Lookout Mountain, Colorado 1. Introduction 1.1. Goal of Research This Capstone project has been initiated in an attempt to model the Electromagnetic Radiation (EMR) environment on Lookout Mountain in Colorado using the PlaNET software model. This is part of on-going bio-electromagnetic research being conducted by Colorado State University. Currently, the Federal Communications Commission (FCC) maintains on-line databases for transmitter sites in the United States containing basic antenna parameter information. A model was created for the Lookout Mountain region using PlaNET in conjunction with information obtained from the FCC, Jefferson County, and antenna manufacturers. Ultimately, the model's ability to demonstrate accurate EMR readings on Lookout Mountain was challenged and analyzed in comparison to actual field measurements Motive of Research The FCC has characterized Lookout Mountain in Golden, Colorado as "the most intense and complex electromagnetic environment in a residential area in the nation." [1] Lookout Mountain is under scrutiny by its local government (Jefferson County, CO) and residents (through the Canyon Area Residents for the Environment: C.A.R.E.) due to the electromagnetic emissions of over 600 radio frequency transmitters. Three hundred and fifty families live within a one-mile radius of antenna farms that serve the Denver Metropolitan area and its vicinity. [2] These 1

2 telecommunications antennas are currently under investigation by the FCC for not abiding by electromagnetic radiation (EMR) standards Standards Electromagnetic radiation can affect the human body in many ways. Frequencies under investigation on Lookout Mountain lie within the RF spectrum (frequencies up to 300GHz). RF waves are a form of non-ionizing radiation with the most detectable effect being thermal. 1 Biological tissue in an electromagnetic field can absorb power. Friction caused by the movement of induced dipoles, permanent dipoles, and drifting conductive charges results in an increased temperature. [3] In 1954, it was agreed by both the military and industry that exposure to a power density of 0.1 W/cm 2 could cause injury. [4] Varying factors have been introduced over time to achieve a reasonable safety level. Today, the American National Standards Institute (ANSI), the FCC, and the National Council on Radiation Protection and Measurements (NCRP) all have different recommendations for EMR exposure. However, all three agree that 200 µw/cm 2 should be the maximum permissible exposure (MPE) for the 3kHz to 300MHz frequency band in an uncontrolled area. Jefferson County has adopted the ANSI (ANSI C ) recommendation for electromagnetic radiation exposure Lookout Mountain Situation 1 Non-ionizing radiation means there is insufficient photonic energy to ionize an atom or molecule with one quantum event (NCRP Report #119, P. 3). Electromagnetic waves at frequencies 300GHz and above constitute ionizing radiation. 2

3 There are 411 FCC licenses that include over 600 transmitters on top of Lookout Mountain. Transmitting antennas are used for microwave links, TV broadcast, FM and AM radio broadcast, weather radar, PCS, cellular, private channel two way radios, and other functions. Virtually all of the electromagnetic radiation on Lookout Mountain comes from only 18 transmitters. These include 9 television, 8 FM radio, and one weather radar. The effective radiated power for these transmitters range from 22.5kW (KUVO FM radio) to 5MW (KDVR Channel 31). The broadcast frequencies range from 55.25MHz (KWGN Channel 2) to 5.55 GHz (Channel 9 weather radar). [5] 1.5. Relevance to Prior Work Two studies have previously been done on the RF conditions at Lookout Mountain: An Investigation of Radiofrequency Radiation Levels on Lookout Mt ( 86) and An Examination of Radiofrequency Fields and 60Hz Magnetic Fields at the Lookout Mt. Conference and Nature Center ( 95). The first study showed compliance, however in 1986 the MPE standard for an uncontrolled area was 1000 µw/cm 2. Spots were found in public areas that reached 580 µw/cm 2 (2.9 times the current standard). The second study, centered around the Lookout Mountain Nature Center located nearly a mile away from the main antenna farms, showed compliance with the current standard. 2. Analysis 3

4 The data obtained for the software model was primarily provided by two sources: the Federal Communications Committee and Jefferson County Government. Following is the analysis of the information obtained from the two sources and the difficulty in its retrieval Jefferson County Data Jefferson County provided the longitudinal and latitudinal coordinates of the transmitters (degrees, minutes, and seconds) as well as the frequency, the type of service (Radio or TV), the call sign of the owner, and the make/model of the antenna. In addition, the center of radiation and effective radiated power (ERP) were also provided. The center of radiation is the point at which the power density is centered and the ERP constitutes the amount of power used to broadcast the signal in watts FCC Data The FCC has on-line databases providing the same basic information obtained from Jefferson County with more details. [6] For instance, the ERP is sectioned into horizontal and vertical. Also included are the polarization antenna patterns for some transmitters (every 10 degrees as well as any special azimuths) and if the antenna has beamtilt Difficulties with obtaining the data The data obtained from the two sources tend to conflict with one another. For instance, the antennas make/model are not the same from the two. This is due to the nature of broadcast licenses. After a broadcaster is issued a license, the owner may change the original equipment 4

5 the license was granted for. This impedes keeping track of the EMR environment because the documentation may not be accurate and it may change at any time. In addition, the towers are located on private property so the owner's of the towers are neither obligated nor cooperative to release any information pertaining to the details of the towers. Attempts to alert the public to the problem of EMR has been suppressed by both radio and television stations due to conflicts of interest. If an accurate assessment of the electromagnetic radiation environment is to be achieved, any change of data should be documented by the owner of the transmitter, and filed with the regulatory body (either the FCC or Jefferson County.). Only then will studies on the EMR effects of such wireless broadcast become feasible. In the meantime, laws must be passed to allow agencies to record all the specifications in use. 3. PlaNET PlaNET is a comprehensive, high performance and interactive design software tool used for network planning by cellular operators, PCS operators, and equipment manufacturers. It combines high-resolution terrain and land-use databases with sophisticated propagation modeling techniques. The PlaNET software was selected simply because it was the only readily available package to the project. From the FCC and Jefferson County data, 18 transmitters atop Lookout Mountain were identified as the significant contributors of EMR for census tract These transmitters consist of broadcast television and radio stations. This project set out to create a model for the radiation patterns using the PlaNET software package. 5

6 3.1. Strengths PlaNET is robust with respect to modeling radio frequency patterns. Using elevation data, this software constructs three-dimensional models for radiation patterns. Due to the propagation characteristics of electromagnetic waves, elevation data is crucial for developing an accurate model. The following fields were inputted into PlaNET using information obtained from the FCC, Jefferson County, and antenna manufacturers: vertical and horizontal antenna radiation patterns, ERP, center of radiation, longitude and latitude. Figure 1 shows a typical antenna radiation pattern. The example is for K43BK s antenna manufactured by Scala. Figure 1: Antenna Radiation Pattern This graph was created in PlaNET by inputting 36 horizontal azimuths (obtained from the FCC) and 30 vertical azimuths (obtained from Scala) Weaknesses 6

7 When concerned with exploring bio-electromagnetic effects (as well as EMR regulatory compliance), an EMR model should show cumulative power density output for all transmitters. This is the primary deficiency of choosing the PlaNET software package to model EMR environments. PlaNET is designed to give graphical representations of adjacent and co-channel interference patterns, not cumulative power density output. The software is limited to providing power density output (in dbm or dbµv/m) for each transmitter separately. This data can be exported, however, only in dbm format. Since dbm is not distance-related (as compared to dbµv/m), summing for each transmitter becomes a non-trivial task. Complex conversions must be performed to take into consideration elevation effects from distant points to transmitters Output The following section looks into the results achieved using the PlaNET software. Due to inconsistencies between measurements and PlaNET results, the following will look into the farfield and near-field cases separately since the far-field proved more consistent with measurements 2. Figure 2 below is the key for all the following graphs. Figure 2: Signal Strength Key 2 The far-field begins at a distance away from the antenna equal to 2D 2 /λ (where D is the largest dimension of the radiating portion of the antenna and λ is the wavelength). [3] 7

8 Near-field Figure 3 shows a composite coverage graph of the power density output over census tract Note that 200 µw/cm 2 is equal to 74.4 dbµv/m. Points 1, 2, and 3 are located within the nearfield of the transmitters and 2 Figure 3: PlaNET Composite Coverage of Census tract

9 Figure 4 is a close-up of figure 3 showing the Lookout Mountain area. Golden Figure 4: PlaNET Composite Coverage of Lookout Mountain Close-up Measurements at the three areas were performed on July 22 nd 1998 using a Holaday HI-3012 broadband isotropic field strength meter. In compliance with NCRP measurement guidelines, only electric fields were measured within the near-field. [7] The measurement values together with the results from PlaNET are summarized in table 1. 9

10 Table 1: Comparison between PlaNET and Field measurements Point Field measurements PlaNET Location µw/cm W/cm 2 (105 dbµv/m) µw/cm 2 1 GW/cm 2 (138 dbµv/m) µw/cm MW/cm 2 (133 dbµv/m) 72 ft SE of antenna structure on Cedar Lake Road. Private drive on Hill containing towers next to a green metal shed. Colorow Road next to channel 6 fence. The table shows that the simulated data is magnitudes higher than the field measurements. This inaccuracy must come from the propagation model used by the PlaNET software, which blows up within short distances from the transmitters. The figures in the "Field measurements" column represent spatially averaged values. However, slight movements of the electric field probe produced very different readings. This is a characteristic of the near-field. [3] For the Lookout Mountain environment, accurate modeling near the base of transmitting towers is needed since some homes lie within 100 meters of a tower. Therefore, since the PlaNET software is inaccurate in the near-field, it cannot be used to model the Lookout Mountain environment. Far-field Far-field line-of-sight measurements were taken at the Mount Vernon Country Club (point 4). A maximum power density of 26 µw/cm 2 was observed on the north deck of the country club. Spatially averaged values were less than 15 µw/cm 2. Using the PlaNET software, the same area 10

11 was found to reach power density levels greater than 3 times the uncontrolled MPE (i.e. >600 µw/cm 2 ; see figure 3). This triggered the project to look into contributions from each transmitter. It was found that channel 31 KDVR contributed virtually 100% of the EMR. KDVR is transmitting a 5 MW television signal. Figure 5 shows the region without the KDVR transmitter and 2 Figure 5: Census tract without KDVR 11

12 Removing the KDVR transmitter produced numbers in the 10 µw/cm 2 area, which are consistent with the field measurements. The reason for the great contribution from KDVR is due to the improper modeling of the radiation pattern. The FCC databases do not contain radiation pattern information for this transmitter. Thus, an omni-directional pattern was used. Only one area was available for the far-field analysis since measurements were conducted before the software model was completed. After the completion of the model, measurement equipment was no longer available. From this limited analysis though, PlaNET accurately models the far-field EMR. 4. Other models Since the PlaNET model proved to be inadequate, the project looked into other available software for modeling RF emissions. The only other readily available package was the FCC model, which the FCC uses to determine non-compliance with standards. This is covered in more detail below The FCC model The FCC model is solely designed to calculate power density in µw/cm 2 that an FM tower radiates two meters off the ground. [8] For the above reason it is much simpler than the PlaNET software (see Figure 6). The parameters required for each antenna are as follows: distance from the antenna the program should calculate over, vertical and horizontal ERP, antenna height, number of elements in the antenna array, antenna element spacing, and antenna type. The weaknesses of the model include the lack of capability to calculate the total density for a series of antennas, and it does not consider terrain and direction. Failing to consider terrain and antenna 12

13 direction factors will likely result in inaccurate calculations, especially in an area such as Lookout Mountain with its mountainous terrain. Due to this inadequacy, this model was not further pursued. Figure 6: The FCC Model 5. Conclusion A model of the EMR environment on Lookout Mountain was developed using the PlaNET software package based on data obtained from the FCC, Jefferson County, and antenna manufacturers. The obtained data was found to be inconsistent due to the ambiguous nature of broadcast licenses, therefore a significant amount of inaccuracy was introduced into the model. Based on the achieved results, PlaNET cannot be recommended for modeling near-field EMR 13

14 environments since the propagation model breaks down close to the transmitter. However, the PlaNET software proved adequate in the far-field. The FCC model was also pursued, but can not be recommended because of its simplicity. It does not consider terrain variations and antenna directions, nor does it provide a composite output for multiple transmitters. 14

15 References [1] Cleveland, Robert (FCC). Personal interview with Carole Lomond, Editor of City and Mountain Views. [2] CARE, Petition to the FCC, Fall [3] NCRP Report #119, P. 10, 25, 48. [4] Science, Vol. 208, P. 1231,13 June [5] Clark, Russ (Jefferson County Office of Planning and Zoning), July [6] FCC Mass Media Bureau Databases, July 14, [7] Holaday HI-3000 Series Broadband Isotropic FSM Owner's Manual, p.9, [8] FCC Office of Engineering and Technology, FM Model for Windows, October 10,