A tunable metamaterial-based passive limiter for protection from HPM and UWB sources

The development of a high power in-line limiter utilizing varactor-loaded metamaterial structures is presented. A metamaterial structure is an artificial structure engineered to provide electromagnetic properties not available in nature, more explicitly defined as a material having simultaneously negative permittivity and negative permeability. A singly-negative material (SNG) structure, the split-ring resonator (SRR), is a negative permeability material which acts as a notch filter with resonant frequency f ₀. The resonant frequency of the SRR filter yields itself to tuning since the capacitance between the SRR and transmission lines is easily changeable through the use of varactors. At nominal power levels, f ₀ is significantly offset from the receiving frequency such that the receiving frequency is unattenuated. When an in-band high power microwave (HPM) is incident upon the filter, a DC bias is applied to several varactors and shifts the resonant frequency of the filter to that of the receiving frequency due to the change in capacitance of the varactors. This effectively attenuates the incident HPM. The filter uses a microwave rectifying circuit to extract a DC voltage from the in-band HPM, which serves as the DC bias voltage across the varactors. Ansoft's HFSS was used to accurately model and design the SRR structure to minimize the E-field and maximize resonant effects. Both high and low power continuous wave testing verified minimal insertion loss as well as verification that the use of varactors in conjunction with a split ring would effectively shift the resonant frequency of the notch filter.