ARCHIVED—Evaluation of Electromagnetic Field Intensity in the City of Toronto
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To assist in the analysis of the data, a software program was written. The intent was to interpret the collected data and produce the received signal strengths in dBm for each frequency channel, as well as noting the carrier-to-noise (C/N) ratio. Essentially the program used the received band-power of each frequency channel to calculate the E-field intensity, normalized them to their respective SC 6 limits, and squared the values. Both a maximum and average percentage of SC 6 for each site was obtained by summing the maximum and average figures respectively.
The data collected consisted of the received signal strength, the frequency deviation, and C/N ratio. A sample of this information is outlined in Table No. 04.
|Channel Number||Received Level (dBm)||C/N (dB)||Frequency Deviation (Hz)|
If the C/N ratio was zero or negative, the channel was considered inactive for that particular sweep, and was not counted in the average. The electric field intensity for each active channel was derived using the following formula:
Ec = Pr + AF + Ca + At - 13 dBV/m
is the channel electric field intensity in dBV/m
Pr is the receiving signal level in dBm
AF is the antenna factor in dB/m
Ca is cable loss in dB
At is external attenuator in dB, if used
The overall SC 6 value for a monitored site is the sum of the squares of the normalized electric field intensity of all active frequency channels, with respect to the Safety Code 6 limit. This is expressed in the mathematical formula below.
SC 6 = Σ ( Ec / Lf )2 (No unit)
SC 6 is the total Safety Code 6 percentage
Ec is the channel electric field intensity in V/m
Lf is the Safety Code 6 electric field limit for the operating frequency.
The SC 6 figures based on the maximum received signal level of each frequency channel for each site are listed in Appendix A. These values represent a worst case scenario, i.e. all active channels are simultaneously transmitting during the monitoring period.
A "pass" method was used to show a more likely scenario. The SC 6 value for each site was calculated by summing the squares of the normalized electric field intensity values of each active channel in the sub-band, per sweep. Each monitoring period consisted of at least 50 sweeps per sub-band. The maximum and average SC 6 based on "pass" method for each site is shown in Appendix A. It should be noted that longer monitoring periods would have increased the accuracy of the maximum level of non-ionizing radiation and provided a better time average figure for the site. However, due to the numerous sites and the project's time frame, each site was monitored for approximately three hours.
Several assumptions were made throughout the course of the measurements and the project itself. One fundamental premise was that the length of time necessary to complete one pass of the frequency band was equal to, or shorter than, the transmitting time of the signals. This meant that all transmitted signals were logged by the system, and none were missed due to short duty cycles. Another crucial assumption was that at the time of measurement, all transmitters were fully operational and at usual power. Lastly, if the addition of attenuation was necessary to counteract overload, the lower level signals that were no longer recorded would not significantly impact the overall results.
The highest RF level calculated, 1.75 m above ground level, without cross-polarization compensation was 5.63% or 17 times less than SC 6 at the Metro Hall site. The monitoring location was bound by the CBC English program headquarters to the South, Royal Alexandra Theater to the North, Metro Hall building to the West and Roy Thompson Hall to the East. The site is located less than 400 meters from the base of the CN Tower and First Canadian Place, which house most of the television and FM radio transmitters within the city. It was noted that the measured level varied drastically when the position of the receiving antenna was moved. Therefore, the location was monitored on four separate occasions to account for this fluctuation.
The Spadina Parkette registered the second highest measured level. The receive antenna was positioned 1.75 m above ground, in the direct line-of-sight of the CN Tower and First Canadian Place. There was minimal reflection at this site due to the vast open nature of this locale. Photographs, locations and observations were recorded for each site, and a sample is shown in Appendix B.
The third highest measured level recorded 1.75 m above the ground level, was at the Harbour Front, next to the Metro Police station. A significant contributor to the SC 6 level at this site was the AM broadcasting service, due to the proximity of AM radio transmitters located on Toronto Island, directly across Lake Ontario. Other FM and TV broadcasting signals were partially blocked by the highrise buildings surrounding the measuring position.
The average and maximum Safety Code 6 percentages of the locations surveyed 1.75 m above ground level, are summarized in Appendix A. The locations measured throughout the City are graphically displayed using the maximum SC 6 percentages, in Figures No. 04 and No. 05.
Each location's RF field intensity was analyzed to determine the contributions from various radiocommunication services, namely broadcast, cellular, PCS, LMCS, paging, land mobile and aeronautical. All SC 6 levels at the surveyed sites were significantly lower than the guideline exposure limitsFootnote 3, 1.75 m above the ground level. Generally, the major contributors to the SC 6 values at each of the surveyed sites were broadcasting services (AM, FM, and television) ranging from 44% to 71% of the measured level, depending on surrounding area of the test sites. Land-mobile services including two-way, paging, trunking, etc. contributed approximately 10% to 26% of the measured level. Wireless telephone services (PCS & Cellular) contributed about 9% to 24%of the measured level. Aeronautical services contributed 12% of the measured level near Pearson International Airport, but very little elsewhere.
However, depending on the proximity of the radio-communication transmitters to the receiving test antenna, their contribution to the measured level may be significantly higher. For example, at the Lawrence Avenue East & Don Mills Road (site No.29) the major contributor (65% of the total measured level) was a mobile signal operating on 157.830 MHz with a level of 0.09588% or 1043 times less than the SC 6 limit.
The survey, measuring the electromagnetic field intensity levels from 150 kHz to 3 GHz at selected locations, identified that public, accessible locations on ground level (1.75 m above the ground) are well below the recommended public exposure to non-ionization radiation limits set forth by Health Canada. The maximum SC 6 percentage, based on the maximum measured levels of each frequency, was less than 6% or 16 times less than SC 6 limit, found at ground level at the Metro Hall site.
Based on the sites surveyed, broadcasting services were often the major contributors to the total energy, 1.75 m above ground. Depending on the location and surrounding area of the surveyed sites, broadcast services contributed from 44% to 71% of the total measured level. The combined SC 6 values for land-mobile services, trunking and paging (traditional land mobile frequency bands) services ranged from 10% to 26% of the total measured level. Wireless telephone services (PCS & Cellular) contributed 9% to 24% of the total measured level. Aeronautical navigation aids did not contribute significantly to the overall total at the locations surveyed, except for the locations close to the airport, where their measured level was approximately 12%. Since present RF levels were quite low, the total SC 6 percentage figure of a location was easily influenced by nearby mobile transmitters as witnessed at sites No. 29 and No. 25.
The Spectrum Explorer, although mainly a laboratory instrument, was able to perform the necessary measurements. Its limitations were its physical characteristics which resulted in a lack of portability that prevented measurements from being conducted at locations unaccessible to vehicles. Like all equipment there is always room for improvement to gain efficiency: incorporating receivers, ADC boards and controlled PC on the PCI common bus as a single unit having an industrial frame with an external hard drive to increase speed, robustness and data collection. A wider bandwidth receiver and ADC would speed up the measurements. There is a need to develop additional Spectrum Explorer software to include the following features: GPS positioning recording; an antenna factor and cable loss tables; Safety Code 6 limits; calculation and automatic setting of the necessary monitoring time based on the width of the frequency band and channel bandwidth; an overload handling mechanism to avoid a loss of measuring time and data accuracy; and an automatic data recording threshold based on the highest signal level and desired accuracy.
As antennas do not usually have perfect omni patterns in either the horizontal or vertical planes, it is recommended that the testing antenna be rotated in both axes, when dealing with a known source. A slow continuous rotation or stepped rotation of the receive antenna in the horizontal plane will ensure that maximum signals are received from all directions when measuring unknown sources. A non-metallic platform and remote controlled rotator would help to peak the receiving signal level without concern of human body reflection. The receiving antenna should be as wide-band as possible, robust and finely calibrated.
The connecting cable between the receiver and antenna should be low loss, flexible, double shielded, wide-band and at least fifty feet long to avoid the receiving antenna pattern being influenced by the measuring vehicle or human body. The cable connector must be free of dirt and water before it is attached to the antenna and the receiver. Plastic connector covers may be necessary, especially when measurements are conducted in the winter. A set of external attenuators operating in the frequency range from DC to 50 GHz would be useful when entering the high field environment.
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