An analysis of the effects of feedline and ground screen noise currents on a conical monopole receiving antenna

Abstract

Excessive electromagnetic interference/radio frequency interference (EMI/RFI) degrades the capability of the Naval Security Group (NSG) high frequency direction finding (HFDF) sites to receive signals of interest (SOI). These sites use a circularly disposed antenna array (CDAA) to receive signals in the 2 to 30 MHz frequency range. A conical monopole (CM), an antenna whose bandwidth matches the CDAA's, may provide a solution to the EMI/RFI problem through separation and isolation. Semi-remotely locating the CM away from EMI/ RFI sources achieves the former, while its independence from the noisy RF distribution system accomplishes the latter. This thesis analyzed the susceptibility of the CM to EMI/RFI currents that might be injected onto the feedline from equipment in the building at the center of the CDAA. The Numerical Electromagnetics Code (NEC) was used to model the CM and its buried feedline at the Imperial Beach CDAA site. The numerical model was used to validate proposed limits for the maximum allowable EMI/RFI current on incidental conductors and grounds at receiving sites. A maximum of two microamps of EMI current has been suggested to ensure that no appreciable degradation of SOI reception occurred. The NEC CM model predicted that the two microamp current limit would be adequate for SOI reception on the CM examined. Experimental measurements obtained at the Imperial Beach HFDF site partially validated these results and demonstrated that the CDAA response to the two microamp EMI/RFI limit is within the expected levels.