High-frequency measurement of radar cross section using the standing-wave method

The standing-wave method of measuring radar cross sections has been adapted for use at 26 MHz. A target in the middle of a large flat field was illuminated by a transmitter 300 feet away, and a probe was moved along the transmitter-target line to measure the amplitude and position of the standing wave due to the target. Aluminum dipoles of different lengths were first used as targets. Both the target cross section and the phase of the scattered radiation varied with the target length in the same way as predicted from previous microwave measurements. The results demonstrate the validity of extrapolating microwave measurements to the high-frequency part of the spectrum. Calibration of the system was achieved by using the known cross section of a half-wave dipole. The technique was used to measure cross sections of trees, street lights, and vehicles. With vertical polarization, oak trees had cross sections between 30 m²and 70 m², indicating that trees can be an important source of ground backscatter. A street light had a cross section of 240 m², and vehicles had cross sections ranging from 0.7 m²to 28 m², depending on aspect.