High-frequency backscatter from terrain with cement-block walls

The measurements of high-frequency (HF) radar cross sections (RCSs)sigmaof trees on a large flat field were reported a few years ago by Steele. This work used a special adaptation of the standing-wave method to 26 MHz. Shortly afterward, the present author used the same apparatus to measure the (RCSs) of cement walls erected on the same terrain. Wall sizes ranged from 5 ft high by 10 ft wide to 20 by 20 ft. Angles of elevationpsiranged from 2.5 to22.5deg. Using vertical polarization (Efield in the plane of incidence), cross sections increased with wall size. When a wall was thoroughly dampened,sigmaincreased by 3 to 13 dB, depending onpsi, and wall size oversigmawas obtained when the wall was dry or partly damp. Chicken wire covering increased the 10- by 10-ft wall's cross section by 10 dB. Using horizontal polarization, cross sections were immeasurable for the smaller walls. For the 20- by 20-ft wall,sigmawas a smoothly increasing function withpsi. When this wall was wet, its cross section increased by 4 dB. The wall-ground combination was treated as a corner reflector, using a theory based on physical optics. The theory compares fairly well to experimental cross sections for the 20- by 20-ft wall. Extrapolation of the theory suggests that a 150-ft high by 100-ft wide building could have a cross section approaching10^{4}m²at 26 MHz.