Site qualification above 1 GHz and SVSWR systemic errors

Site VSR was developed under the belief that a test site can be characterized in the same fashion as a transmission line. The intent of the test is to determine if the spatial impedance of the site deviates from free space resulting in a reflection coefficient at the boundary. The key aspect of transmission line VSWR measurements is continuous movement of the clamp or RF transducer along the transmission line to find the maximum and minimum voltages. This article will review the inherent error in the S_VSWR method written into CISPR 16-1-4. This error is due to the fact that the standard samples only six discrete locations. The spacing of those locations relative to the wavelength of the sampled standing wave results in an inherent compliance bias. In other words, the method in the standard will always result in the site performance being represented as better than it actually is and the true site performance is almost never actually measured. A simple mathematical model was developed to illustrate the problem. Empirical validation tests were conducted with sampling every millimeter. Test results validate the very substantial error term in the method. An alternative approach, time domain reflectivity, provides complete site evaluation faster and more accurately than the VSWR method will be briefly discussed.