Tunable radar absorbers using frequency selective surfaces

Current techniques for controlling the radar cross-section (RCS) of military assets include careful shaping and the use of appropriate materials and construction methods. In general, these techniques are passive in the sense that once utilised in a particular asset, the latter cannot easily be modified to take account of new threat scenarios or operational requirements. Moreover, Rozanov (see IEEE Trans on Antennas and Propagation, vol.48, p.1230-1234, 2000) work has highlighted the bandwidth limitations of radar absorbing materials (RAM) based on passive constituents and these may be summarised in the following relationship: |log_e(ρ₀)|(λ _max-λ_min)<2π²Σ_i μ_id_i where Γ₀=20log₁₀ (ρ₀) is the desired value of reflectivity, λ_max and λ_min define the limits of the band over which this performance is to be maintained and μ_i and d_i are the permeabilities and thicknesses of the layers comprising the absorber. Hence, it might be desirable for future military systems to be capable of some measure of dynamic RCS control so as to achieve increased operational performance, flexibility, survivability and in-service lifetime. Dynamic control of microwave reflectivity might also find application in civilian areas such as automotive and traffic engineering, e.m.c. and other r.f. test facilities and r.f. tagging and asset tracking