An investigation of the polarisation dependence of insect radar cross sections at constant aspect

摘要

A low cost system for measuring insect radar cross section (RCS) as afunction of polarisation orientation when viewed at constant vertical aspect ispresented.A low power continuous wave system (frequency 9.4 GHz) was developedwhich illuminated a target vertically and measured the power reflected back as thetarget was rotated in the horizontal plane. Data was collected by a microcomputer andstored on floppy disk for later analysis. A standard target was measured before eachinsect target and software used this data to calibrate the system for each insect targetmeasurement.A 5 parameter mathematical model, based on the scattering matrix, isdescribed. Software was developed to calculate the parameters from collected datausing a least squares procedure.Measurements from 54 specimens representing 18 species of commonlyavailable insect are reported. Average RCS’s were similar to the RCS of waterspheres of equivalent mass. For small insects, the maximum and minimum RCSoccurred when the electric vector (E-vector) was parallel and perpendicular to thebody axis respectively. As insect size increased, a subsidiary maximum developedwhen the E-vector was perpendicular to the body axis, becoming dominant for thelargest insects measured. The shapes of the RCS pattern were approximately ‘mirror’symmetric, except for the RCS curves from S. gregaria, the largest insects measured,which were asymmetrical and sensitive to the position of the large rear legs.It was found that the ratio of maximum RCS to minimum RCS bore no simplerelationship to the ratio of body length to abdomen width of the insect, even whenadjusted for mass and was not a useful measure of target shape.The implications of this data are discussed and suggestions for further workare presented.