Radiation from an antenna in manhole

This paper reports results of investigation on the radiation characteristics of a vertical rod antenna in manhole at 430 MHz for the purpose of allowing radio access to water pipe sensors. Field distribution in and around the manhole was calculated by FITD 3D simulation. It was found that a portion of generated waves penetrated through the asphalt layer around the metal top to radiate into the space above ground. Results obtained for a region above the ground surface were compared to results of E-field measurement made on a real size manhole built in a car parking lot for experiment. The measured field was roughly twice smaller than expected from the simulation, but shapes of E_z versus distance curves were quite similar, supporting the validity of the simulation. Approximately 1.6% of antenna input power was radiated into the space above ground. Radiation efficiency was admittedly poor. Nevertheless, it was found possible to establish a practical short distance radio link between the inside and outside of manhole using a pair of 10 mW transmitter/receiver units at 430 MHz.