Grating lobes in RF propagation through brick and metal reinforced walls

Abstract

This paper presents correlation of full wave RF predictions with measurements of transmission through standard brick walls. Transmission as a function of frequency and polarization is computed using both time and frequency domain solvers. Results are compared to measurements for three-dimensional brick geometries that include multiple arbitrarily shaped air holes with rounded edges and corners. Although there have been many experimental studies of RF propagation through walls, few references are available that specifically examine the grating lobe issues, Honcharenko et al. (1994) presented an analytical study for 1D periodicity that demonstrated the potential for grating lobe propagation in brick walls; however, very limited experimental validation was provided. Both brick and metal reinforced walls can support grating lobes if the frequency is high enough. For the bricks investigated, grating lobes occur at L-band frequencies for normal incidence, and at lower frequencies for other angles of incidence. At frequencies where grating lobes propagate significant power, transmission measurements using stationary transmit and receive antennas do not account for all of the power transmitted through periodic walls. Very significant amounts of power can be propagated in directions that are not, in general, recorded by a stationary receive antenna over a wide bandwidth. This is because the angular locations of the peaks of the multiple grating lobe beams are a function of frequency. When significant power is directed into the grating lobes the power in the main lobe drops, which is an important effect that is not included in simple slab models. RF prediction tools used to predict total fields inside of buildings most often represent walls as multi-layer dielectric slabs. Because dielectric slabs support only the fundamental mode, they are unable to include grating lobe effects.