Cover Image

Not for Sale



View/Hide Left Panel

THE NATIONAL ACADEMIES

Advisers to the Nation on Science, Engineering, and Medicine

National Academy of Sciences

National Academy of Engineering

Institute of Medicine

National Research Council

DIVISION ON EARTH AND LIFE STUDIES

Board on Radiation Effects Research

July 22, 2002

Lt. Col. Richard Ashworth,

Chief

Bioenvironmental Engineering

Office of the Command Surgeon

Headquarters Air Force Space Command 150 Vandenberg Street, Suite 1105 Peterson AFB, CO 80914–4550

Dear Col. Ashworth,

On May 22, 2002, the Department of the Air Force asked the National Research Council (NRC) Committee to Assess Potential Health Effects from Exposures to PAVE PAWS Low-level Phased-array Radiofrequency Energy to provide comments on the plans for Phase IV of the Air Force waveform characterization effort. This request stems from questions raised by the committee during discussions with the Air Force relating to their Phase IV measurement plans.

Specifically the Air Force asked the NRC committee to:

  1. If appropriate, provide any comments, questions, concerns, or suggestions related to the time-domain measurement effort, and

  2. If appropriate, provide any comments or suggestions on an approach to modeling the time-domain waveform to assist in the measurement effort.

The following letter report makes several recommendations related to the time-domain measurement effort. The committee makes no recommendations related to modeling the time-domain waveform.

Prior to this request outlined above, the committee had been tasked to determine whether biologic information obtained from radiofrequency exposures other than phased-array exposures can be used to determine the biologic effects of phased-array exposures. This future determination is to be followed by a committee update of the 1979 National Research Council report on the exposure levels and potential biologic effects of the PAVE PAWS radar system. This committee believes that implementation of the recommendations in this report will provide

500 5thStreet, NW, Washington, DC 20001 USA

(202) 334–2232 (telephone) (202) 334–1639 (fax) national-academies.org



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine National Academy of Sciences National Academy of Engineering Institute of Medicine National Research Council DIVISION ON EARTH AND LIFE STUDIES Board on Radiation Effects Research July 22, 2002 Lt. Col. Richard Ashworth, Chief Bioenvironmental Engineering Office of the Command Surgeon Headquarters Air Force Space Command 150 Vandenberg Street, Suite 1105 Peterson AFB, CO 80914–4550 Dear Col. Ashworth, On May 22, 2002, the Department of the Air Force asked the National Research Council (NRC) Committee to Assess Potential Health Effects from Exposures to PAVE PAWS Low-level Phased-array Radiofrequency Energy to provide comments on the plans for Phase IV of the Air Force waveform characterization effort. This request stems from questions raised by the committee during discussions with the Air Force relating to their Phase IV measurement plans. Specifically the Air Force asked the NRC committee to: If appropriate, provide any comments, questions, concerns, or suggestions related to the time-domain measurement effort, and If appropriate, provide any comments or suggestions on an approach to modeling the time-domain waveform to assist in the measurement effort. The following letter report makes several recommendations related to the time-domain measurement effort. The committee makes no recommendations related to modeling the time-domain waveform. Prior to this request outlined above, the committee had been tasked to determine whether biologic information obtained from radiofrequency exposures other than phased-array exposures can be used to determine the biologic effects of phased-array exposures. This future determination is to be followed by a committee update of the 1979 National Research Council report on the exposure levels and potential biologic effects of the PAVE PAWS radar system. This committee believes that implementation of the recommendations in this report will provide 500 5th Street, NW, Washington, DC 20001 USA (202) 334–2232 (telephone) (202) 334–1639 (fax) national-academies.org

OCR for page 1
a better characterization of the wave-form and exposure levels experienced by citizens residing in the vicinity of the radar beam. This should increase the committee’s ability to determine the applicability of using non-phased-array exposures to evaluate phased-array biologic effects. The additional information obtained if these recommendations are implemented should also facilitate the committee’s ability to update the 1979 Research Council report. One of the questions arising in the PAVE PAWS study is whether the granularity in sidelobe radiation produced by differential time delays over the array produces waveforms that are different from those produced by the antenna main beam (after the power-level differences are taken into account). By granularity, we mean that the fields from the individual elements that make up the phased array add as discrete well-defined elements (i.e., discrete steps) to form the overall beam. This is in contrast to a single source feeding a large reflector covering the same area. To a first approximation, the sum of the fields from each of the radiating elements will add to give the same beam patterns as the corresponding reflector. The difference between these two antenna systems is that one (the phased array) is a discrete-step approximation to the other. At moderate distances, such as at the site-boundary fence, the differences between these two antennas are expected to be small. If the waveforms are essentially the same, existing RF bioeffects data may be applicable. If not, additional studies may be indicated. The Phase IV measurement data will be critical in answering this question, so these data must be measured carefully and expeditiously. 1) Use of Measurement Sensors The PAVE PAWS measurements to date by AFRL/DEHP personnel and contractors have utilized derivative sensors (E-dot and B-dot sensors). These sensors require integration of the received signals to recover the waveform, and have been widely used in electromagnetic pulse (EMP) type measurements. They are especially good for single, short-pulse, wideband signals. However, absolute signal values are not recovered. In fact, their broad pattern and low sensitivity make them susceptible to interference. The Phase IV measurements should be made with a carefully designed and tested TEM horn antenna that directly produces the signals because the PAVE PAWS signals are not short-pulse and are not wideband. Rather, the instantaneous bandwidth is approximately 3% based on a 70 nanosecond rise time. AFRL has considerable experience with TEM horn antennas for the radiation of very high-power, short-pulse waveforms. However, for measurements of the PAVE PAWS system, we recommend that the two conductors of the horn not be straight (flat), but rather, taper out in a roughly exponential fashion. Extensive data have been accumulated for printed-circuit versions of TEM horns, which are sometimes called notch antennas or Vivaldi antennas. A review of

OCR for page 1
printed-circuit TEM horn antennas is available [1]. Although the printed-circuit horn is compact and easy to construct, it has been shown that the best performance results when the horn conductors taper roughly exponentially from the feed to the mouth, and also when the spacing between the conductors tapers roughly exponentially from the feed to the mouth. These antennas are more difficult to construct, but the performance is excellent and the sensitivity is much higher than that of derivative sensors. For transient (wideband) measurements, discontinuities at the throat and at the mouth are crucial. In these antennas the discontinuity at the throat has been ameliorated by tapering smoothly from the parallel plate feed line into the exponential taper. The discontinuity at the mouth can be reduced by employing sawtooth serrations, absorbing sheets, etc. [see 2–4]. It is believed that these well-known design principles will improve on the short-pulse TEM horn “replica antenna” mentioned in the preliminary measurement plan. That antenna was designed for very wideband operation, and is not appropriate for characterization of the signal from the PAVE PAWS system. The purpose of measuring the PAVE PAWS signals with a carefully designed and tested TEM horn antenna is that this antenna would allow a direct measurement of the radiated signal. Since the signal is narrowband, this type of direct measurement should be the primary measurement. If the antenna is carefully designed and tested, internal reflections (ringing) should be significantly reduced. From the results of those measurements, the waveform can be directly observed when displayed on the TDS7404 oscilloscope, and the slope (rise time) immediately determined. Because the gain of the antenna can easily be calibrated over the narrow bandwidth, it is possible to reduce uncertainties in the incident electric field strength that occur with derivative sensors and resulting numerical integrations. The committee therefore recommends that the Air Force develop and use a carefully designed and tested TEM horn antenna. 2) Comparison of Waveforms The committee believes that a comparison of waveforms from a reflector antenna with the PAVE PAWS wave forms will facilitate the determination of whether non-phased array radio frequency information on biologic effects can be used to determine the potential for phased-array biologic effects. A comparison of waveforms from a radar utilizing a reflector antenna and waveforms similar to PAVE PAWS is essential as most biologic and epidemiologic studies undertaken to evaluate risk have been based on traditional non-phased radar signals. Several questions have been raised in the PAVE PAWS study regarding the appropriateness of using those data. First, is whether the granularity in sidelobe radiation produced by differential time delays over the array produces waveforms that are different from those produced by the antenna main beam. Second, is whether there are substantial rise-time differences between reflector antenna and phased-array signals. Third, is whether there are substantial differences in pulse and scan rates of the signals.

OCR for page 1
If a suitable reflector antenna radar system (diameter comparable to the phased array) can be identified, two sets of measurements would suffice. Both measurements would be in the far field (at a distance relevant to the PAVE PAWS facility), one in the main beam, and one in the sidelobe region. Each measurement would consist of a set of snapshots of the waveform (as the waveform changes) at a few locations. These reflector antenna results will be compared with the waveforms measured from PAVE PAWS under conditions reflecting typical usage over a sufficient time period (a few days) and, with calibration measurements (e.g. single-element radiation). Recordings of sufficient duration to characterize and compare chirp repetition and scan rates at the two types of facilities should be obtained. Through these comparative measurements, two critical questions can be addressed: 1) are phased-array waveforms different from those of reflectors and 2) are time-delay effects visible? 3) Location of Population Phase IV Measurements One of the objectives of Phase IV is to characterize the PAVE PAWS radar emissions as they impact the Cape Cod population. On pages 92–93 of the Protocol for Phase II and Phase III the Air Force recommends that “the community be a major player in selecting the sites” of Phase IV measurements. Power-density measurements should be obtained to cover the same populations to be evaluated for incidence rates of morbidity and mortality. A directional antenna (such as a Yagi-Uda antenna) should be utilized, rather than E-dot and B-dot sensors, for those power-density measurements. Exposure data, consisting of radiation-intensity measurements, should be made at, or near, the centroid of each census tract to allow correlation with health-outcomes data. In 1978 there were measurements of radiation intensity made at 21 sites on Cape Cod. Apparently the PAVE PAWS Public Health Steering Group has had an offer of the use of 32 firehouses on Cape Cod for hosting measurement efforts to determine population exposures. Since firehouses tend to be located to represent population densities, it might be well for Phase IV measurements to be conducted in some or all of the firehouses. 4) Summary The PAVE PAWS signals should be measured with a carefully designed and tested TEM horn antenna. This antenna will allow a direct measurement of the radiated signal The committee also recommends that a comparison of waveforms from a radar utilizing a reflector antenna and waveforms similar to PAVE PAWS be performed. It is important to have field measurements from both the phased array and the reflector antennas so that the committee can evaluate the extent to which the biological data resulting from the exposures to reflector antennas is relevant to that from the PAVE PAWS phased array. Scattering and multipath effects can be important and can lead to significant fluctuations in the strength of the measured fields. In many cases, the fields will decrease more rapidly than 1/r2. In other cases, the exposure on a hill top, or

OCR for page 1
with line-of-sight paths to the antenna, may exceed the values measured in a valley or at a location screened by buildings or other objects at shorter distances from the radar. It is important to have the exposure measurements under normal operating conditions. Waveform measurements (time-domain measurements) need to be made at only a limited number of locations over time (on the order of a few days) while radiation exposure needs to be measured at the census-tract level. Sincerely, Frank E.Barnes Chairman Committee to Assess Potential Health Effects from Exposures to PAVE PAWS Low-level Phased-array Radiofrequency Energy