"Radiofrequency-Field Exposure Facility

Two different exposure systems were used during the course of the study: a system designed to simulate a far-field exposure condition during the last 4 days of pregnancy through weaning of the pups at 23 6 2 days old and a near-field, principally head-only exposure system used from day 35 (of age) through 2 years. The exposure systems used in these studies at 1.62 GHz were based on proven designs used for similar studies at 836.55 MHz (6). However, some modifications to the exposure chamber, electronic components, and restraint system were made to achieve the required SAR values at 1.62 GHz as well as to facilitate animal handling and reduce the cost of equipment. The Iridium signal modulation uses differentially encoded quaternary phase shift keying (DEQPSK), a frame length of 90 ms with 11 data slots per frame [For more information on digital telephony signal transmission formats, see ref. (21)]"

"2. Near-field exposure electronics and carousel systems

For the near-field exposure period of the study, a system with 10 animals placed radially around a central antenna was used, similar to that described by Moros et al. (23) (see Fig. 1). The facility contained nine partially shielded double exposure units with each unit housing two exposure carousels (for a total of 18 near-field exposure carousels). The 18 carousels were located in pairs on support structures and provided for exposure of 90 animals per sex per group during each exposure session. Six of the units were assigned to each of the three exposure conditions or zones (1.6, 0.16 and sham W/kg). The exposure condition was rotated every 6 weeks among the three zones. Power for the near-field exposure system antennas at the high and low exposure levels (1.6 and 0.16 W/kg) was provided by six 100-W linear amplifiers driven by the Iridium RF signal generator. Sleeved dipole antennas were connected to the RF sources through power and control circuitry for the exposure system. Attenuation was provided appropriate to the signal generator and amplifier specifications. Amplifier output was monitored by an RF-field detector connected to the coupled output of a directional coupler. A 10-dB directional coupler provided the power for the 0.16 W/kg exposure group via the coupled output. Power distribution to the antennas is accomplished by a cascade of two-way and three-way splitters. Outputs of the three-way splitters were connected to the sleeved dipole antennas on the carousels via directional couplers, each with a crystal detector connected to the coupled output. The laboratory control system (described below) monitored the output signal of each crystal detector and confirmed the presence of the signal. The system also determined whether the signal was within preset limits. Forward and reflected power levels to each antenna were measured using the HP 4418B power meter prior to starting the exposures and then every 6 weeks for the duration of the study. Radar absorbing materials were obtained and placed around each exposure unit. This modification was necessary to prevent stray fields in the shielded room from having an impact on the other systems in the room. With the radar absorbing materials in place, the ‘‘contaminating/ stray’’ fields from one antenna were negligible when measured at the rat positions in other units (see below). The RF-field levels were measured using a Tektronix 2782 Spectrum analyzer (Tektronics, Portland, OR) coupled with a half-wavelength sleeve dipole. The instrument rack was configured to operate at 1.62 GHz, continuous wave. The output power at the high-power sleeve dipole antennas was approximately 120 mW. The output power at the low power antennas was approximately 12 mW. The spectrum analyzer was used to measure the received power at a distance of approximately 10 cm from each antenna. All 12 active antennas were on during the measurements and all radar absorbing material panels were in place. The RF field was assessed in the adjacent room used to house the animals at two heights and ten positions in the room for a total of 20 measurements. Power levels, normalized to the average received power measured at the high power antenna locations, are summarized in Table 1.

Exposure Monitoring and Control

A computer-control system was used to monitor system performance and to record and store observations from the study. Interfacing of the exposure system to the computers was accomplished through National Instruments (Austin, TX) LabView Field Point equipment. This reliable modular interface system provided for multiplexing of input signals, analog to digital input data, as well as digital to analog conversion of control signals. The system automated data collection and recording and eliminated the need for strip chart recording and multiplexer use. In addition to the field sensors installed to sense and allow adjustment of the exposure fields, three other sensors are monitored by the data acquisition unit every 6 min. Identical combination temperature and relative humidity sensors (Omega Engineering, Model HX93V) were installed in the exposure and animal home cage rooms.

1. a.c. magnetic-field measurements

The a.c. magnetic-field flux density was determined at each carousel position in the animal exposure room (Room 194) using the three-axis EMDEX II (Enertech, Campbell, CA) magnetic-field meter. RF fields were energized during the measurements. The EMDEX II device has a detection limit of 1 mT and a 1-mT measurement resolution. The EMDEX II was used in the broadband mode that measures flux density in the frequency range between 40 Hz and 800 Hz. The a.c. magnetic fields at all but four carousel positions were determined to be nominally 1 mT.

2. Static (d.c.) magnetic-field measurements

The d.c. (geomagnetic) field measurements were made using the Bartington three-axis probe and pre-amplifier system at 14 positions in the exposure room (Room 194) and seven positions in the animal housing room (Room 192). The average d.c. RMS magnetic-field flux densities were 413 and 434 mT in the two rooms, respectively."

"The F1 offspring of these groups (700 animals) were assigned to the near-field treatment groups based on body weight within a treatment group and gender such that the average weights of animals in the treatment groups were comparable prior to starting the near-field exposures. For the near-field study, 90 males and 90 females were selected for each near-field treatment group (1) 1.6 W/kg, (2) 0.16 W/kg and (3) near-field sham controls. Another 80 males and 80 females were held as cage controls. Near-field exposures (1.6 W/kg and 0.16 W/kg 2 h/day, 5 days/week) were initiated when the offspring were 35 6 1 days old and were continued until the rats were approximately 2 years old. The exposure regimen was based on exposure times used in a similar study using 836 MHz signal exposure (6) and also a consideration of avoiding prolonged confinement (more than 2 h/day) in the exposure tubes. During exposure sessions the rats were confined head first in cylindrical tubes arranged in a carousel configuration with the RF antenna centered in the carousel."

"To minimize the effect of any subtle variation in environmental factors in the animal room, such as temperature or light intensity, animal cages were rotated weekly within each rack and each rack was rotated within the room. To minimize any potential location or system differences during far-field exposure, the location of cages within the exposure system was rotated each week. During near-field exposures, rats were rotated through the six different carousel positions of the zone. In addition, the exposure time was rotated weekly through the three daily 2-h exposure shifts to randomize any potential effects due to time of day of exposure. The exposure groups were color coded so that study personnel did not know the exposure groupings during the study. The code was broken only after the pathology evaluation was complete."

F1, Fischer 344 rats, female - Near-Field-, Sham-Exposed, Cage Control

HAWC