An Experimental Investigation into the Radiating Mechanisms of Unshielded Transmission Lines at Microwave Frequencies

An empirical study has been performed to investigate the radiation mechanisms that yield emissions from two-conductor transmission line circuits at microwave frequencies up to 8.5 GHz. The measured signal levels established by radiation from the test configurations within a reverberation chamber were compared with the levels radiated by a wide-band dual-ridge antenna in the same environment. Two basic test configurations were considered, one in which the transmission line segment and full feed structure, including balun and feed plate, were contained within the chamber cavity, and one in which the feed structure and feed plate could be isolated from the cavity. The measurements show that radiation from the line terminations dominates the emission within the cavity that contains it, including when only bulkhead SMA pin adapters were exposed to the cavity, independent of whether the line was a parallel-conductor twin-lead line or a twisted pair. Emissions from the twin-lead line exceeded that from the twisted-pair line only when the lines were passed through apertures in the chamber walls. Moment-method calculation of the radiation from an ideal, balanced twin-lead line showed overall lower emissions than that measured at the lower microwave frequencies considered. The results suggest that the emissions can be modeled by adding a radiation resistance element at the connector location to standard transmission line theory.