Dear Mr. Boyts, I appreciate your interest in the technical details around radio frequency (RF) emissions. As I mentioned in earlier correspondence, there are a number of factors to take into account when reviewing radio frequency exposure levels. My analysis was accomplished using the approaches defined in the Institute of Electrical and Electronics Engineers (IEEE) C95.1 as well as guidelines from the Federal Communications Commission’s (FCC) Office of Engineering and Technology (OET). The first major consideration is distance of the receiving surface from the RF source. As the distance between the measurement point and the RF source increases, the power density decreases at a rate of 1/r4 (where r is the radius from the source). This means that the power density of the signal attenuates very quickly as you move away from the source. At the frequencies that the radios use, objects such as the leaves on trees and the walls of your home attenuate the signal significantly. With both of those factors figuring into the equation, a signal inside your home is not likely to be very strong at all. The FCC has very conservative limits for the RF exposure levels for radios used in the United States. “For general population/uncontrolled exposures, the lack of sufficient information or control over how long people are exposed requires the use of much lower public exposure Maximum Permissible Exposure (MPE) limits that allow for continuous exposure. In other words, as long as the absolute limits are not exceeded, indefinite exposure is allowed.”1 Those absolute limits differ depending on the operating frequency, and for the radios we are using, the safe exposure level is 1 milliwatt (1 mW) for uncontrolled exposures. I have included a chart below for your reference that depicts the FCC Maximum Permissible Exposure (MPE) limits.
Power Density (mW/cm^2
FCC Maximum Permissible Exposure (MPE) Limits 128.0 64.0 32.0 16.0 8.0 4.0 2.0 1.0 0.5 0.3 0.1
0.3
1.34
30
30
300
1500
Frequency (MHz) Limits for General Population/Uncontrolled…
1
Source: FCC OET Bulletin 56, Page 14 (August 1999)
100,000
Since I am comparing the transmissions using the FCC’s uncontrolled exposure limits, I am using the worst case assumption that the radios on the poles are transmitting continuously, rather than using a time‐averaged exposure that would take into account the duty cycle of the transmitter (a ratio of how often the transmitter is on versus off in a given time period). I will, however, offer an additional comparison to a cellular phone conversation to help provide a reference that is easy to understand. For that example, I will compare a continuous exposure level from the Tropos radio to a time‐averaged exposure to a cellular phone conversation. This comparison assumes a cell phone with a 50mW transmit power based on FCC data for an HTC Thunderbolt smart phone being held 1 centimeter from the user’s ear as recommended in the manufacturer’s instructions. For each of the three buildings on your property, I have included a summary of the analyses, and will include some more detail on the assumptions used for each of them on the following pages. Western Building Total Output Power (2 radios @ 4W each) 8 Watts Radial Distance from Property to Radio 311.8 ft Power Density (S) mW/cm2 0.000018 mW/cm2 This power density is 55,411 times less than the FCC uncontrolled exposure limits . At this distance , it would take 25.2 months of continuous exposure to receive the same time‐averaged exposure as a 5 minute cell phone conversation with the phone at 1 cm from the ear. Center Building Total Output Power (2 radios @ 4W each) 8 Watts Radial Distance from Property to Radio 232 ft Power Density (S) mW/cm2 0.000033 mW/cm2 This power density is 30,694 times less than the FCC uncontrolled exposure limits. At this distance , it would take 13.9 months of continuous exposure to receive the same time‐averaged exposure as a 5 minute cell phone conversation with the phone at 1 cm from the ear. Eastern Building Total Output Power (2 radios @ 4W each) 8 Watts Radial Distance from Property to Radio 221.1 ft Power Density (S) mW/cm2 0.000036 mW/cm2 This power density is 27,866 times less than the FCC uncontrolled exposure limits . At this distance , it would take 12.6 months of continuous exposure to receive the same time‐averaged exposure as a 5 minute cell phone conversation with the phone at 1 cm from the ear. The RF exposure levels just outside of each of the structures are between 27,000 and 55,000 times less than the levels that the FCC has designated as the MPE for the 2.4GHz and 5.8GHZ frequencies. To put it in perspective, these levels are far below the MPE of 1mW/cm2, which is roughly 50 times less than the levels intended to protect against tissue damage that can result from RF heating from excessive exposures.
Tropos Node Public RF Exposure Levels
8 Watts
FCC Maximum Allowable Continuous (Uncontrolled) Exposure Limit: 1 mW / cm2 Total Output Power (2 radios @ 4W each)
311.8 ft
0.000018 mW/cm2
Radial Distance from Property to Radio
Power Density (S) mW/cm2
Though not likely, signal reflection from ground and buildings is assumed for this case since exposure levels are being determined via theoretical analysis.
This power density is 55,411 times less than the FCC uncontrolled exposure limits . At this distance , it would take 25.2 months of continuous exposure to receive the same timeaveraged exposure as a 5 minute cell phone conversation with the phone at 1 cm from the ear.
Roadway + Bike Path + front lot line offset to building = 311 ft
0.000018 mW/cm2
22 ft to bottom of antenna
“For general population/uncontrolled exposures, the lack of sufficient information or control over how long people are exposed requires the use of much lower public exposure MPE limits that allow for continuous exposure. In other words, as long as the absolute limits are not exceeded, indefinite exposure is allowed." ‐ Source: FCC OET Bulletin 56, Page 14 (August 1999)
Tropos Node Public RF Exposure Levels
8 Watts
FCC Maximum Allowable Continuous (Uncontrolled) Exposure Limit:: 1 mW / cm2 Total Output Power (2 radios @ 4W each)
232 ft
0.000033 mW/cm2
Radial Distance from Property to Radio
Plane Wave Power Density at building surface (S) mW/cm2
Though not likely, signal Reflection from ground and buildings is assumed for this case since exposure levels are being determined via theoretical analysis.
This power density is 30,694 times less than the FCC uncontrolled exposure limits. At this distance , it would take 13.9 months of continuous exposure to receive the same time‐averaged exposure as a 5 minute cell phone conversation with the phone at 1 cm from the ear.
Roadway + Bike Path + front lot line offset to building = 231 ft
0.000033 mW / cm2
22 ft to bottom of antenna
“For general population/uncontrolled exposures, the lack of sufficient information or control over how long people are exposed requires the use of much lower public exposure MPE limits that allow for continuous exposure. In other words, as long as the absolute limits are not exceeded, indefinite exposure is allowed." ‐ Source: FCC OET Bulletin 56, Page 14 (August 1999)
Tropos Node Public RF Exposure Levels
8 Watts
FCC Maximum Allowable Continuous (Uncontrolled) Exposure Limit: 1 mW / cm2 Total Output Power (2 radios @ 4W each)
221.1 ft
0.000036 mW/cm2
Radial Distance from Property to Radio
Power Density (S) mW/cm2
Though not likely, signal reflection from ground and buildings is assumed for this case since exposure levels are being determined via theoretical analysis.
This power density is 27,866 times less than the FCC uncontrolled exposure limits . At this distance , it would take 12.6 months of continuous exposure to receive the same timeaveraged exposure as a 5 minute cell phone conversation with the phone at 1 cm from the ear.
Roadway + Bike Path + front lot line offset to building = 220 ft
0.000036 mW/cm2
22 ft to bottom of antenna
“For general population/uncontrolled exposures, the lack of sufficient information or control over how long people are exposed requires the use of much lower public exposure MPE limits that allow for continuous exposure. In other words, as long as the absolute limits are not exceeded, indefinite exposure is allowed." ‐ Source: FCC OET Bulletin 56, Page 14 (August 1999)
I hope that I have been able to provide you with some useful information. Please do not hesitate to give me a call or send me an email if you have any other questions. Best regards, Kraig Bader Standards Engineering Manager Fort Collins Utilities Light & Power Tel: 970 416 2481
[email protected]