Analysis of the Exposure Levels and Potential Biologic Effects of the PAVE PAWS Radar System (1979) / Chapter Skim
Currently Skimming:
'Characterization of PAVE PAWS Exposure Conditions'
'Characterization of PAVE PAWS Exposure Conditions'
Pages 7-41
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From page 7... ...
The radar at Otis Air Force Base is the subject of this report. This section describes the operating characteristics of the PAVE PAWS radar system, not in the context of engineering and electronic detail, but with reference to how the characteristics are related to human exposure to the radar.
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From page 8... ...
In normal operation, the PAVE PAWS beam is not directed below an elevation of 3° above the horizon. At Otis Air Force Base, the radar is on high ground, and the main beam of the antenna is always at least 100 ft (30 m)
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From page 9... ...
FIGURE 1. PAVE PAWS radar, showing triangular structure and the two antenna faces.
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From page 10... ...
FIGURE 2. Regular array of antenna elements protruding from one face of PAVE PAWS radar.
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From page 11... ...
POWER A long-range radar like PAVE PAWS operates by transmitting brief pulses of energy or short trains of pulses, followed by longer periods during which the transmitter is off and the receiver is sensitive to returning echoes. The radiated power is described in terms of "peak power" and "average power." Peak power is the rate at which energy is emitted during the period of one pulse; that is, peak power is the energy of one pulse divided by the duration (in seconds)
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From page 12... ...
. In round numbers, this is about 3 times the average power transmitted by a typical large TV station and somewhat more than the average power transmitted by a typical high-power-output FM broadcast station (but see the discussion of antenna patterns below)
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From page 13... ...
The average exposure of this observer therefore tends to be comparable with that encountered at an "average" angular position remote from the main beam. Both calculations and measurements on the PAVE PAWS antenna indicate that the average power at ground level near PAVE PAWS should be no greater than one-fifth to one-fourth of the maximum at the peak of a secondary sidelobe.
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From page 14... ...
RADAR CONTROL The detailed pattern of operation of PAVE PAWS radar is controlled by computers that are integral parts of the system. Signals received by the radar are processed and interpreted by a central computer.
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From page 15... ...
Errors that could affect exposure estimates are the following kinds: o A well-formed beam might be steered to an angle below the 3° limit, and reflection from the ground might increase the power density. o A poorly formed beam might have sidelobes more intense than those described.
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From page 16... ...
; Generation of a poorly formed beam requires that a number of antenna elements radiate essentially in a cooperative manner, like a separate small antenna that has been steered in an improper direction. For example, doubling the intensity of one of the secondary sidelobes of the normal antenna requires that about 50 individual radiating elements, properly spaced in the whole array, exhibit improper signal delays in a systematic way.
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From page 17... ...
Even under worst possible atmospheric conditions, the amount of bending is so small that it does not affect the radiation environment described here. MEASURED AND CALCULATED POWER DENSITIES OF THE PAVE PAWS RADAR This discussion is limited to a review of three studies that are directly applicable to the prediction of electric fields and power densities produced by PAVE PAWS at locations outside the boundaries of Otis Air Force Base.
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From page 18... ...
MEASUREMENT METHOD The system used by the Air Force for measuring peak electricfield strength and average power density is shown in Figure 3. Peak power density was calculated from the measured peak electric field strength.
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From page 19... ...
CO oo vO oo *
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From page 21... ...
Table 3 shows the same data arranged ln order of distance and includes the ratios of peak power to average power. Hankln has calculated the electric-field strength and average power density expected to be produced by the radar.
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From page 23... ...
TABLE 3 Ratio of Peak to Average Power Density (Based on Table 2) Test Point Approximate Distance Power Density yW/cm2 Ratio of Peak to Average Power Density Miles km Peak Average 0.061 1 0.6 1.0 19.5 320 15 1 1.6 0.345 0.003 115 16 1.4 2.3 0.008 a -- 11 1.6 2.6 5.23 0.051 103 13 1.7 2.7 0.209 0.001 209 12 2.0 3.2 2.07 0.016 129 2 2.1 3.4 2.7 0.027 100 3 2.1 3.4 0.055 a -- • 4 2.4 3.9 3.6 0.02 180 14 2.8 4.5 0.188 0.002 94 5 4.4 7.1 0.047 0.001 47 6 4.6 7.4 0.006 a ~ 7 5.4 8.7 0.026 a -- 17 7.1 11.4 0.002 a -- 8 7.3 11.7 0.002 a -18 8.8 14.2 0.002 a -- 19 8.9 14.3 0.001 a -- 20 9.0 14.5 a a -9 9.2 14.8 a a -- 21 11.8 19.0 a a -- 10 13.8 22.2 0.002 a ~ aBelow reportable value (less than 0.001 vW/cm2)
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From page 25... ...
i-4 0 P x iFH • 9 • •H 9 oo o> co CN O •o y y c 3 •H "3 4J 5 M w a w S 4J •o y -»» OI 0)
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From page 26... ...
We further assume that there is no exposure to the main beam. Hence, let S-, be the power density associated with primary sidelobe, Sn that associated with the secondary sidelobe, etc.
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From page 27... ...
w JT time FIGURE 4. Schematic representation of pulses from a scanning radar -27
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From page 28... ...
TABLE 6 Distribution of Population around PAVE PAWS Radar Site, Otis Air Force Base, Massachusetts3 No. Census Radius Enumeration No.
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From page 29... ...
at that location; we call this the "specific-source radiofrequency environment.' The quantity most commonly used for specifying exposure to radiowaves is power density. Power density is the rate at which energy crosses a unit area; in the International System of Units, it is given in watts per square meter (W/m )
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From page 30... ...
The highest observed average power density was 0.001 yW/cm .
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From page 31... ...
TABLE 8 Population Exposure in 12 U.S. Cities, 54-900 MHza ~~ Median Exposure, Percent of Population City iiW/cm Exposed at <1 yW/cm Boston 0.018 98.50 Atlanta 0.016 99.20 Miami 0.007 98.20 Philadelphia 0.007 99.87 New York 0.002 99.60 Chicago 0.002 99.60 Washington 0.009 97.20 Las Vegas 0.012 99.10 San Diego 0.010 99.85 Portland 0.020 99.70 Houston 0.011 99.99 Los Angeles 0.005 99.90 All cities 0.005 99.41 aData from Tell and Mantiply.152 -31
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From page 32... ...
TABLE 9 Cumulative Population Exposure at 54-900 MHz0 Power Density, Cumulative Percent of yW/cm Population Exposed 0.002 17 0.005 49 0.01 69 0.02 83 0.05 92 0.1 95 0.2 97.5 0.5 99 1.0 99.5 aData from Tell and Mantiply.152 h For example, 17% are exposed continuously at less than 0.002 69% at less than 0.01 yW/cm , etc.
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From page 33... ...
No. Radars Detected Average Power Density, yW/cm Location Mt.
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From page 34... ...
from those near the ground at equivalent distances. Some FM broadcast antennas have antenna patterns that can produce relatively high power densities at ground level near the antenna tower.
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From page 35... ...
OCGRILS roou HORIZONTAL PL»HE FIGURE 5. Vertical radiation pattern of a UHF TV transmitting antenna.
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From page 36... ...
Roof 7 5 62 97 134 148 Sears Building, Chicago 50th floor Roof 32 201 34 29 Federal Building, Chicago 39th floor 5.7 0.73 Home Tower, San Diego 10th floor 17th floor Roof Roof 18 0.2 119 180 Milam Building, Houston 47th floor 35.8 31.6 aData from Tell and Hankin.150 -36
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From page 37... ...
. For marine radars, the computed average power density for any of the six units that were studied was less than 50 yW/cm at the antenna's turningcircle radius; ^ one of the units has an option for sector scanning, and the maximal power density was about 250 yW/cm when it was operated in this mode.
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From page 38... ...
• 0.0043E2(V2/m2) , where E is the electric field.
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From page 39... ...
Little information is available on the fields produced by handheld walkie-talkies. In one study, electric fields 12 cm from a 2.5-W hand-held unit operating at 27.12 MHz were measured to be as much as 205 V/m, for an "equivalent" power density of 11 mW/cm .
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From page 40... ...
The only fields in excess of this occur ln the immediate vicinity of a few FM antenna towers and on the roofs of tall buildings within a block or so of FM and TV broadcast antennas. With respect to other high-power sources, such as radars and satellite communication systems, the potential for human exposure to obviously high power densities is small, owing to system characteristics that include colllmated beams and, in many cases, a combination of such other factors as remoteness, operation that precludes primarybeam exposure, and motion of the primary beam.
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From page 41... ...
These factors result in estimated time-averaged power densities for most systems that are below 50 yW/cm at distances greater than 0.5 mile from the source. Higher power densities may occur closer to the source, especially if exposure to the primary beam is possible.
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