Exposure zones near parasitic elements in high powered antennas

Abstract

This simple investigation, and results from many other experimental measurements and theoretical investigations demonstrate the following: a) There is a region close to the parasitic element where the electric field strength is greatly reduced and the magnetic field is greatly increased. b) The spatial rate of change of the field strength is very high. Small changes in distance can result in very large changes in electric field strength. This effect is observed in many transmitter situations. c) The presence of an additional conductor in this zone will influence the near fields and so can influence the overall effectiveness of the antenna. These conclusions are important because: a) Grounded towers used in VLF, LF and HF antennas show small regions where the measured and modeled quasi-static fields are greatly reduced. Such regions might be defined as "safe" when radiation exposure standards [6] are applied. b) The near field zone immediately behind Yagi-Uda antennas is not as "safe" as that predicted from far field to near field transform approximations (Ebersbach, Thiel and Leckenby [4]). c) Antenna isolation from nearby conductors using parasitic elements placed close to the driven elements is not particularly effective when compared to a large reflecting screen.