Development of Cast Iron Manhole Cover With Broadband-Radio-Transmission Characteristics Applying Spiral Structure

Abstract

This letter reports on a cast iron manhole cover with a slit structure to improve the efficiency of radio propagation from underground structures. The spiral slit of a broadband antenna was applied to the iron cover to achieve broadband radio propagation characteristics. A full-scale manhole model was fabricated and evaluated, resulting in a maximum gain of 2.5 dB and a bandwidth of 193 MHz [(807 to 1000) MHz]. The results of field tests with the prototype showed an average of −77.9 dBm for the slit iron cover and −100.5 dBm for the conventional product in wireless communication from inside a manhole through the iron cover. The 22.6 dBm improvement in radio transmission while maintaining the functionality of the steel cover is sufficiently practical. This result can contribute to the maintenance and management of aging infrastructure and the spread of disaster prevention systems as a technology to reduce the cost of introducing Internet-of-Things (IoT) systems to underground infrastructure.