Radiation Exposure Modelling Strategies for Wire Antenna Arrays

Abstract

This thesis presents a computationally efficient software system to determine the minimum safety distances for human exposure to radio frequency radiation from wire antenna systems. Based on classical analytical methods and original code, a system was developed to calculate the occupational and general public safety zones from electromagnetic radiation emanating from wire antenna systems, according to maximum human exposure limits set by radio frequency communication standards. Isolated dipole and multi-element, Yagi-Uda style wire antenna arrays were considered in free space, as well as antenna systems comprising multiple antennas. The designed algorithm was developed due to the increased industry demand for simpler, more efficient wire antenna exposure assessment techniques, not requiring the complexity of commercially available software.

Different strategies for calculating the currents on the elements and the field-strengths of wire antennas were investigated. Dependent on the operating frequency and the power delivered to the antenna, the antenna radiation hazard zones are shown to frequently protrude into the near radiating zones of the antenna. While the applied analytical techniques are not claimed to be new, and the far-field analysis of wire antennas is also well documented, the analytical near-field calculations as required for computing the radiation hazard zones are not well known, and their application to Yagi-Uda style wire antenna arrays and presented results are believed to be new.

A novel algorithm was designed to determine a suitable, physical antenna structure from specified far-field characteristics if the structural wire antenna specifications are unknown. This research resulted as a response to industry requirements, being a practical dilemma encountered in over 90 % of wire antennas found at antenna sites. It is shown how the antenna's gain and E- and H-plane half-power beamwidths could be used to select a suitable reference antenna from an antenna database for computing the radiation hazard zones of wire antennas with inaccessible structural specifications.

The thesis proposes the use of optimisation techniques in a novel approach for determining the radiation hazard zones. Optimisation techniques are shown to compute the occupational and general public radiation hazard zones of single- and multi-antenna systems with higher accuracy of better than 0.7 V/m and reduced computational demands by a factor of five compared to traditional contouring methods.

Testing and verification of the developed software system was performed by applying the calculations to an accumulated wire antenna test bed, comprising over 130 different wire antenna designs, sourced from the literature. The obtained results were compared to the numerical electromagnetics code (NEC-2), a method of moments code, as well as some measured data.

Obtained results indicate that an algorithm using an analytical near-field-strength technique, based on a three radiation point-source representation of each wire antenna array element, provided the best modelling strategy for determining the radiation hazard zones of wire antenna systems. Radiation hazard zones were conservatively calculated to within a maximum of 3.45 dBi of equivalent NEC-2 models. Implemented on a recent model personal computer, case-study results indicate the radiation hazard zones of a four-antenna system were calculated in 55 seconds using the proposed three point-source wire antenna modelling engine, compared to 285 seconds using NEC-2, a 5x saving in the required computational time. In conjunction with the proposed radiation hazard zone optimisation algorithm, this time was further reduced to 13 seconds.