Minimum Antenna Elevation for Specific Fraction of SC6 Limits Version 1.0 Standard May 7, 2009

Transcription

1 Specific Fraction of SC6 Limits Version 1.0 Standard May 7, 2009 RF engineering team Ian Hung DRAFT

2 TABLE OF CONTENTS 1 INTRODUCTION Purpose Scope Intended Audience ANALYSIS Calculation of Power Densities and Exposure Limits Calculation Constants and Assumptions CONCLUSION... 7 APPENDIX A... 9 A.1 Hotspot (for 6deg Downtilt)... 9 A.2 Hotspot (for 12deg downtilt) DRAFT 11/12/ /11

3 1 INTRODUCTION 1.1 Purpose The purpose of this study was to determine the minimum height of the cellular antenna radiation centre above the rooftop surface where the resulting RF exposure is within a specific fraction of the limits set by Industry Canada s SC6 guidelines. 1.2 Scope This study serves to rapidly identify the antenna height where the resulting RF power density is within some fraction of the SC6 limits. This document is not a safety guideline but serve merely to state those findings based on calculations outlined in the Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3kHz to 300 GHz as published by authority of the Minister of Health, Canada. 1.3 Intended Audience This document is intended for those individuals that direct cellular antenna placement and configuration particularly upon building rooftops in Canada. This document was written under the context of XXXXX thus antenna selection and operating frequency are unique to this study. DRAFT 11/12/ /11

4 2 ANALYSIS 2.1 Calculation of Power Densities and Exposure Limits The approach to determine the power density output by the antenna at a certain point upon the rooftop is to first find the distance of which the far-field begins. For areas that lie within the near-field, the near-field calculations are used which assume the cylindrical modified HPBW Method 1. For areas that lie within the far-field, the far-field calculations are used according to the Free Space Method. In both the near and far-field regions, the horizontal and vertical antenna pattern discrimination is summed and asserted against the isotropic radiated power of the antenna. The cumulative power density due to all transmitting elements on the rooftop at each position on the rooftop is summed. The determination of when the far field begins (for antennas that are physically large compared to the wavelength at which they operate) is defined by the equation in Figure 1. Figure 1: Equation to calculate the distance to the far field The near field equation to determine RF power density is Figure 2 and is based upon the Modified Half Power Beamwidth (HPBW) model in Figure 3. 1 Outlined under Section 4.2 of the RaPD-Calc: Radio Frequency Power Density Calculation Tool with reference to Industry Canada s report on A Study on Electric Field Intensity Distribution in the Near-Field Region of PCS Base Station Antenna Installations. DRAFT 11/12/ /11

5 Figure 2: Equation to calculate the power density within the near field Figure 3: Modified Half Power Beamwidth (HPBW) model Conversely, the far field calculation to determine RF power density is shown in Figure 4. Figure 4: Equation to calculate the power density within the far field DRAFT 11/12/ /11

6 An error window is included in the calculation to account for differences between the theoretical model and reality such as exact building orientation, antenna azimuths, etc. A reflection factor of 60% (where RF=1.6) is included to account for such effect of radio waves reflecting off penthouse walls, etc 2. For additional information and detailed description of the formulas, refer to the Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3kHz to 300 GHz and RaPD-Calc: Radio Frequency Power Density Calculation Tool. 2.2 Calculation Constants and Assumptions The exposure limits for persons not classed as RF and Microwave Exposed Workers (Including the General Public) is chosen to be the upper limit and are outlined under Section 2.2 of the Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3kHz to 300 GHz. The exposure to the public is potentially 24 hours a day for 7 days a week, compared with 8 hours a day, 5 days a week for RF and microwave exposed workers. For the operating frequency of XXXXX which corresponds to the downlink frequency range of XXXXMHz the maximum power density for non-rf workers (general public) is defined as 10 W/m^2. Other assumptions are as follows: The primary antenna pattern used throughout the study is the XXXXXX (Dual Polarized, XXXXXX, 65deg, 18.4dBi, 1.3m, VET, 0-10deg) along with its electrical downtilt patterns as supplied by XXXXXX. One (1) antenna (transmitting element) is used for calculation. The antenna radiation centre is chosen to be the reference point for all relevant calculations. Three (3) variations of the output power from the BTS are used for calculation that are: 20W, 40W, and 80W. The maximum EiRP is respectively 1230W, 2460W, and 4920W from the antenna (transmitting element). The maximum power is assumed to be always transmitted and not simply the pilot power for WCDMA technology. Two (2) downtilt variations are used for calculation that are 6 deg (electrical) and 12 deg (10 deg electrical + 2 deg mechanical). 2 Consistent with measurements performed by Industry Canada and FCC as specified under Section 4.5 of RaPD- Calc: Radio Frequency Power Density Calculation Tool DRAFT 11/12/ /11

7 3 CONCLUSION The minimum cellular antenna radiation centre [d] above the roof surface is as follows: Table 1: Minimum Cellular Antenna Radiation Centre [d] above Rooftop Surface 6deg Downtilt) Percentage of Maximum Allowable Power Density EiRP 90% 50% 10% 1230W BTS) d = Xm d = Xm d = Xm 2460W BTS) d = Xm d = Xm d = Xm 4920W BTS) d = Xm d = Xm d = Xm Table 2: Minimum Cellular Antenna Radiation Centre [d] above Rooftop Surface (@ 12deg Downtilt) Percentage of Maximum Allowable Power Density EiRP 90% 50% 10% 1230W BTS) d = Xm d = Xm d = Xm 2460W BTS) d = Xm d = Xm d = Xm 4920W BTS) d = Xm d = Xm d = Xm For instance, for one (1) antenna with EiRP 2460W with downtilt of 12deg, the minimum radiation centre above the rooftop surface to achieve less than 50% of the maximum allowable power density throughout the rooftop is Xm. The figure below is a visual representation of this scenario. Figure 5: Antenna Radiation Centre [d] above the Rooftop Surface Please note the following conditions in the interpretation of the results: DRAFT 11/12/ /11

8 The calculations assume the maximum exposure levels of the general public and are referenced Xm above the rooftop surface. The power density levels are subject to a single antenna (transmitting element) and thus does not account for any pre-existing power densities due to other antennas from the surrounding RF environment; thus, prudence should be exercised in judgement. Although the minimum antenna radiation centre [d] should ensure that the resulting power density is some percentage of the maximum allowable limits throughout an infinite sized rooftop surface, the actual hotspot may occur beyond the dimensions of the actual rooftop (please refer to Appendix A.1 and A.2 accordingly). Please refer to Appendix A.3 to see the implications towards the XXXXX Greenfield Project. DRAFT 11/12/ /11

9 Appendix A A.1 Hotspot (for 6deg Downtilt) Table 3: Corresponding Hotspot Location for 6deg D/T Percentage of Maximum Allowable Power Density EiRP 90% 50% 10% 1230W BTS) d=3.3m, r=16m to 17m d=3.4m, r=12m to 26m d=4.4m, r=14m to 15m 2460W BTS) d=3.4m, r=around 16m d=3.5m, r=12m to 32m d=5.2m, r=around 15m 4920W BTS) d=3.5m, r=13m to 24m d=4.0m, r=11m to 40m d=6.4m, r=37m to 38m Legend: 90% > Maximum Power Density > 50% 50% > Maximum Power Density > 10% 10% > Maximum Power Density Ex) For one (1) antenna with downtilt of 6deg and EiRP of 4920W and d=3.5m, the hotspot (less than 90% and greater than 50% of the maximum allowable power density = orange area) extends 13m to 24m away from the antenna. DRAFT 11/12/ /11

10 A.2 Hotspot (for 12deg downtilt) Table 4: Corresponding Hotspot Location for 12deg D/T Percentage of Maximum Allowable Power Density EiRP 90% 50% 10% 1230W BTS) d=3.9m, r=15m to 16m d=4.0m, r=14m to 23m d=5.9m, r=24m to 25m 2460W BTS) d=4.0m, r=15m to 20m d=4.5m, r=13m to 27m d=7.5m, r=30m to 32m 4920W BTS) d=4.6m, r=18m to 24m d=5.5m, r=14m to 33m d=9.7m, r=38m to 41m Legend: 90% > Maximum Power Density > 50% 50% > Maximum Power Density > 10% 10% > Maximum Power Density Ex) For one (1) antenna with downtilt of 12deg and EiRP of 2460W and d=4.5m, the hotspot (less than 50% and greater than 10% of the maximum allowable power density = yellow area) extends 13m to 27m away from the antenna. DRAFT 11/12/ /11

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