Field Strength Measurements of Speed Measuring Radar Units

Baird, R. C.; Lewis, R. L.; Kremer, D. P.; Kilgore, S. B. · 1981 · ROSA P / United States. National Highway Traffic Safety Administration

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Summary

This 1981 interim report by the National Bureau of Standards (NBS), sponsored by the National Highway Traffic Safety Administration (NHTSA), addresses the need to evaluate potential radiation hazards associated with speed-measuring radar units used by law enforcement. The primary objective was to establish a comprehensive database of microwave radiation emissions from these devices to inform safety standards and performance qualifications. The study focused on measuring field strength and power density in two distinct environments: free-space conditions and typical vehicular operating configurations. The experimental design comprised two tasks involving 22 radar units (15 X-Band and 7 K-Band). Task 1 involved free-space measurements conducted at the NBS near-field scanning facility. Researchers mounted radar units on a model mount and used calibrated receiving probes to scan an imaginary spherical surface around the aperture. Measurements were taken at four specific distances (1, 3, 6, and 10 or 12 feet, depending on the band) to determine field strength as a function of distance and angle. The system employed phase-locking techniques to detect weak signals in side- and back-lobe regions, with an estimated total measurement uncertainty of approximately ±0.95 dB. Task 2 involved vehicular measurements where radar units were mounted in a four-door sedan in eight typical operating positions, including hand-held through windows and dashboard mounts. Calibrated isotropic probes were used to scan the interior volume, specifically monitoring power density at driver and passenger locations. The estimated uncertainty for these complex vehicular measurements was ±1.0 dB. The results indicated that power densities varied by radar model and frequency band, with K-Band units generally exhibiting higher on-axis power densities than X-Band units due to higher antenna gain. In free-space tests, most units showed circular symmetry in their radiation patterns. Inside the vehicle, maximum power densities at the driver and passenger locations were consistently low, often falling below the detection thresholds of the probes (0.001 mW/cm² for X-Band and 0.01 mW/cm² for K-Band). The study found that all tested units were well-designed with effective shielding against leakage and back radiation. The significance of these findings lies in their comparison to existing safety guidelines. The measured power densities at a 12-inch distance from the radar aperture did not exceed the Occupational Safety and Health Administration’s Radiation Protection Guide limit of 10 mW/cm², nor the proposed American National Standards Institute limit of 5 mW/cm², even under continuous operation. However, the aperture power densities for many units represented a significant fraction (25–50%) of these maximum permissible exposure levels. The report concludes that while the devices are safe for operators inside the vehicle, the data provides a critical baseline for evaluating exposure risks for individuals in close proximity to the radar beam, supporting the development of performance standards for law enforcement equipment.

Key finding

On-axis power densities at 12 inches from the radar aperture reached up to 2.82 mW/cm², but interior vehicle field strengths remained well below established safety limits of 5 to 10 mW/cm².

Methodology

lab_experiment

Sample size: 22

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• exposure measurement