Improvement in the Low Frequency Performance of Geometric Transition Radar Absorbers Using Square Loop Impedance Layers

A technique is described for improving the low frequency performance of geometric transition (GT) radar absorbers based on lossy foam pyramids. The technique makes use of the fact that at high frequencies, only the geometric transition region of the absorber is utilized whereas at low frequencies, the whole absorber thickness interacts with the incident wave. Hence the low frequency performance may be improved, without compromising that at high frequencies, by electrically loading the absorber base layer using one or more frequency selective surfaces (FSS) whose elements are typically in the form of single or nested loops. Other advantages of this technique include minimal increases in weight and manufacturing costs. The paper includes comparative predictions of unmodified and loaded GT absorber reflectivity at both normal and oblique incidence and discusses the effect on absorber performance of tolerance variations in the dimensions and location of the loading FSS elements. Finally, free-space reflectivity measurements on unmodified and loaded commercial absorber blocks are made over the frequency range 1-10 GHz and these confirm the validity of the technique.