A mode based model for radio wave propagation in Storm Drain Pipes

In this paper, we analyze the behavior of Storm Drain Pipes (SDPs) as multimode leaky waveguides for radio wave transmissions between 2.4-2.5 GHz within the Industrial, Scientific and Medical (ISM) band. The previous research on analyzing RF propagation inside underground structures focused on auto, railroad, subway, mine and sewer tunnels of diameters greater than 1.05 m. Our research provides a novel contribution by characterizing RF propagation inside narrow SDPs of diameter as low as 0.30m. Understanding the behavior of SDPs as communication channels is not only important for designing portable military communication equipment, but also for other civil applications. We study the modal content of propagating waves and derive a mode based model (MBM) for predicting the received power. Empirical data gathered at selected sites in Charlotte, North Carolina is used to validate the MBM. Based on analysis of nonlinear curve fitting through the measured data, close agreement is found between the predicted and measured power. We also highlight the requirement of further work to improve the fitting of measured data with the predicted results.