Vertical Site Attenuation - A Necessity!

This paper introduces a further improvement in determining the quality of an open area test site (OATS) for making radiated emission measurements. This technique extends present requirements suggested by the FCC in OST55 to characterize the site attenuation of open area test facilities by using transmit and receive antennas which are vertically as well as horizontally aligned with respect to the metallic ground plane. Vertical polarization is generally more sensitive than horizontal in showing certain site imperfections, such as surrounding reflecting buildings, fences, and other obstructions that might adversely affect measurements. Vertical polarization measurements are also more sensitive to recording technique and measurement system (antenna factor, cable loss, and receiver/spectrum analyzer miscalibrations) inadequacies. Areas for further site attenuation analysis are also indicated. National Standards Institute C63 (now called Accredited Standards Com­ mittee C63 Electromagnetic Compatibility) decided that vertical site attenuation was a necessary addition in determining site quality. The Com­ mittee recommended the continued inclusion of horizontal polarization measurements primarily as a check of the instrumentation, test setup, and antenna calibration prior to making the vertical polarization measurement. The Committee could have recommended dropping horizontal polarization altogether. It felt, however, that was premature since there was not enough data to ensure that vertical polarization measurements would catch all site anomalies. The Ad Hoc Committee’s work is contained in a draft which is undergoing the final approval process within C63 prior to publishing as an ANSI Standard.4 Details of this work and supporting experimentation are now summarized.