Thorough testing requires taking measurements both inside and outside the vehicle.
David A. Case, Jaime McLain and Murry Gavin
Cisco Systems, Inc., Richland, Ohio, USA
Experienced compliance test personnel are familiar with the usual spate of RF issues. Tasks range from performing MPE (maximum permissible exposure) studies and SAR (specific absorption rate) testing to fielding innumerable customer questions. Experienced practitioners deal with questions that arise either in-house or when dealing with an external testing facility. In certain circumstances, the EMC professional must focus on on-site EMF testing of some equipment installations.
In fact, part of FCC Rule 1.1307 specifically calls for an environmental assessment. The system integrator or carrier must carry out an on-site electromagnetic field survey to verify that the installation is in compliance with emissions limits. Apart from the issue of compliance with the “letter of the law”, there are the quite understandable concerns of those working in the facility, or close by. They have an obvious, legitimate interest in system compliance. In most cases, the MPE or SAR studies undertaken as part of the initial engineering process will be sufficient.
Still, in some cases, on-site testing will be required either to reassure the operators of the system or, in other cases, to verify the results of the initial studies already completed. In this particular instance, two vehicles were evaluated. The first was a Chevy Suburban equipped with VF, UHF radios, and satellite up/down link, as well as a Mesh network wireless Access Point. The second vehicle was a Sentinel by Wolf Coach custom-built on a 2006 International 4300 chassis and equipped with commercial and amateur HF, VHF, and UHF radios; satellite up/down link; and wireless LAN (WLAN) Mesh AP—as well as a complete conference center with the WLAN AP providing connection within the vehicle.
TEST PARAMETERS
The vehicles in question were confined to use with the United States, so we referenced FCC limits—Part 1.1310, which, in turn, references IEEE ANSI C 95.1(99). Since we could control access to the vehicle and could place warning signs inside the vehicles or by the external antennas, we classified the radio operators’ stations as a controlled environment. Further, we included the access of the roof on the Sentinel as a controlled area. Because the conference room could, in theory, be used by persons not operating the radio, we classified it as an uncontrolled environment. Finally, the HF radio could be set up in areas that people could access easily so any areas where the antenna could be set up were deemed uncontrolled.
Several factors were working in our favor. Except for the satellite and WLAN radios, all the devices had push-to-talk controls. Consequently, most of the transmitters would not be running 24/7, 100 percent duty cycle. Also, running multiple radios would require multiple operators, and the station had not been set up for that arrangement. The testing was to be carried out with the systems operating under normal conditions. A NARDA 3000 meter was chosen to measure the spectrum below 3 GHz, and a NARDA 871b was used to measure the WLAN operating in the 5-GHz range, as well as the HF amateur radio systems.
ON-SITE TESTING ISSUES
On-site testing generally takes a bit of planning as no site is absolutely ideal, and the tester must address various issues. In this case, our testers were carrying out the environmental assessment while another group was testing the capabilities of the conference equipment. Also, unlike the controlled environment of a laboratory real-world situations always include obstacles and difficulties to testing. For example, our team had to deal with the effects of the various laptops that occupants of the conference room were using to upload or download files and the various signals from their cell phones.
TESTING AND RESULTS
Thorough testing mandated taking measurements at several locations, both inside and outside the vehicles. These key spots included the radio operator’s seat; any location outside the vehicle where a person might stand while entering or exiting the vehicle; and the area around the mobile HF antenna. RF levels on the roofs of the vehicles were tested as well (Table 1).
The EMF meters were self-calibrated before the testing began, and the radio equipment was allowed to warm up for approximately 30 minutes while the test setup was finalized. Several members of the team had developed the test procedure and plan, which were based on recommended best practices. For comparison, several ambient scans were carried out both inside and outside the vehicles. The radios were then powered up to give the team a baseline measurement. Overall frequency bands were evaluated, and specific measurements were performed at the operating frequencies of the transmitters.
Test results indicated that measurements at the bottom of the steps leading up to the vehicle were higher than those recorded at the workstation of the radios. This finding was no surprise since stepping outside the vehicle also means losing the attenuation it provides. Notably, there was almost no difference in interior/exterior measurements in the 1.5- to 3-GHz band (Table 2). This situation was related to the 2.4-GHz AP installed inside the vehicle and the laptops being used connected to it being operated outside the vehicle. Still, the levels in this range remained well below the referenced limits.
Since the NARDA meter and antenna were configured to measure only the 30-MHz to 3-GHz range, an 871b meter, which measures below 30 MHz and above 3 GHz was used to evaluate the upper and lower bands. These measurements were calculated as a percentage of the standard limit. These measurements indicated that the systems were compliant and were operating within the applicable limits. Measurements were also taken several meters away from the vehicles to determine if the overall emissions were within the limits of exposure for the general populace. Measurements taken a few meters from the vehicles indicated that they were indeed with these limits.
ADDITIONAL CONSIDERATIONS
To assure accurate testing of compliance, two other issues had to be considered. Specifically, we adhered to specific recommended guidelines for antenna mountings, and we restricted user access. We did not test at distances closer than those recommended by manufacturers as their “keep-away” distances. Since the amateur antenna could be set up in areas where anyone might walk up to it, we decided that when it was in actual operating conditions, cones or some type of tape barrier would be placed around it. No casual passerby could walk up and touch the antenna. Also, warning signs were employed. Users were reminded to determine the status of the radios before accessing the roof, and an interior sign reminded users if someone were on the roof servicing the radio antennas or carrying out routine maintenance.
We also determined that apart from the WLAN and satellite systems, the push-to-talk radios actually helped to reduce the overall exposure since they did not operate at 100 percent duty cycle. Further, the shielding of the truck helped reduce the exposure of operators and those in the conference room.
CONCLUSION
On-site testing can be both challenging and intriguing. One hopes that the testing results in predictable and repeatable results. Based on our testing, we were able to evaluate the WLAN in real-world situations and to obtain valuable insights on the ways our technology alters the RF spectrum in terms of EMFs. In reviewing the data, we noted that our results were similar to those obtained by Dr. Ken Foster in his study of WLAN for Wi-Fi. Most significantly, we determined that operators in the vehicle did not encounter level of exposure in excess of recommended limits. In fact, results were safely below the limits by a measurable margin.
REFERENCE DOCUMENTS
- CFR 47 code of Federal Regulations Part 0-19 2006
- OET 65C Rev 01-01 Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields
- IEEE ANSI C 95.1 (99)
- Radiofrequency Exposure from Wireless LANs Using Wi-Fi Technologies , Dr Kenneth R. Foster, 2007.
ABOUT THE AUTHORS
David A. Case NCE, NCT is the Regulatory Technical Leader and a member of the EMC staff under the Compliance and Certifications, Testing and Infrastructure Group at Cisco Systems. He is responsible for providing high-end guidance on wireless and medical devices for the various groups. He is also responsible for addressing standards and regulatory issues for Cisco, as well as serving as an active member of a number of technical committees. Currently, he is the Vice Chair of the Wi-Fi Alliance Health and Science Group.
Jaime McLain has been employed by Cisco Systems since 2003 as a Project Manager for Tactical Operations. This group supports communications under challenging conditions such as after a disaster. Prior to joining Cisco, Jaime worked for three years in Nortel Networks Optical support group. He had eight years’ experience as a Ground Radio Technician in the U.S. Air Force and was an instructor of radio maintenance for four of those years.
Murry Gavin has been employed by Cisco Systems in RTP, NC. For the last 10 years, he has worked in NSITE (Network Solutions & Integration Test Engineering) organization focusing primarily on lawful intercept and VoIP solutions. Prior to joining Cisco, he worked for three years in the Sprint PCS Technology Integration Center in Lenexa, KS. Prior to joining Sprint, Murry was employed by Bell Northern Research/Nortel in RTP, NC for 22 years. He was graduated in 1971 from West Liberty State College in West Liberty, WV.
