A comparison of RF field uniformity in a compact semi-anechoic room and OATS

To provide a uniform RF field for immunity testing, a basic understanding of the antenna beamwidth radiation pattern is required. Ground floor and wall reflections and resonance contribute to the field nonuniformity. During the process of developing an automotive RF immunity test requirement, obstacles to meeting this requirement were encountered due to the absence of an absorber lined ground plane and the frequency limitations of the ferrite tiles placed on the walls and ceiling of the test chamber. The frequency range of this test is 200 MHz to 4 GHz. To visualize the actual effects of the floor and wall on beamwidth patterns, a series of measurements were taken in the semi-anechoic room. Mapping of the antenna beamwidth in a semi-anechoic chamber provides a method to determine the optimum distance to antenna and antenna/target orientation. A secondary benefit of beamwidth mapping is that the actual field strength can be measured and compared to the input power. This will help in determining if the intended amplifier is large enough to produce the required field strength. Additional testing using an open area test site (OATS) sheds light on the contributions of floor and absorber wall reflections. Through an experimental study, optimum antenna orientation is determined to obtain the targeted field uniformity.