Plasmonic nanoantennas with constant input impedance within a wide range of mid-infrared frequencies are designed. For antennas working in lower frequencies like radiofrequencies or microwaves, it could be done by using self-complementary geometries if Babinet's principle is approximately valid. However, the scaling up to the infrared is not trivial since metals are not good conductors in such high frequencies. We have found an alternative way to recover the validity of Babinet's principle and thus the achievement of constant input impedance.
