High-frequency backscatter from terrain with trees

The standing-wave method was used to measure the radar cross section σ of a flat piece of ground on which stood a single tree. Measurements were made at 26 MHz at angles of elevation γ up to 22.5 degrees, by raising the transmitting antenna with a balloon and moving the receiving probe along the transmitter-target line by means of ropes. For horizontal polarization, σ was a smoothly increasing function of γ. For "vertical" polarization (polarization parallel to the plane of incidence on the ground), σ increased rapidly up to 10 degrees, then became fairly constant with a value of 6 m². The sudden change in the slope of σ(γ) is the knee effect suggested by previous observations. The Brewster angle was 14 degrees, consistent with a theory that the knee should occur a few degrees below the Brewster angle. Below 15 degrees, the cross section for vertical polarization was greater than that for horizontal polarization by as much as 30 dB.