In this paper, the design, fabrication and experimental characterization of a ultrathin, polarization insensitive, planar microwave absorber are presented. The unit cell of the absorber consists of convoluted square as Frequency Selective Surface (FSS) within a metal grid. The designed absorber results in efficient absorption of the incident microwave energy. The validation of the proposed design is conducted by fabricating a 1.63 mm thin (only 0.054l) absorbing array of 15 ´19 FSS elements which is then tested for normal and oblique incidences through horn antenna on low cost X-band microwave test bench setup using WR90 waveguide. Full wave electromagnetic simulation demonstrates 99.999% microwave absorption at 10.7 GHz with 1.86% absorption bandwidth. The absorber demonstrates very good absorption rate and frequency stability for Transverse Electric (TE) and Transverse Magnetic (TM) polarizations with variation in incidence angle (q) from 0° to 45°. The parametric analysis of the proposed FSS-based absorber is carried out to understand its absorption mechanism for practical applications.

Microwave absorber is a device which absorbs incident radiations and disables all transmission and reflection at operating frequency. Conventional microwave absorbers use Radar Absorbing Materials (RAMs) (Singh et al., 2012), Salisbury screen (Chambers, 1994; and Seman et al., 2011) and some types of dielectric or magnetic composites (An et al., 2010) to absorb incident microwave energy. Here, the main idea of absorption is to tune the real part of permittivity and permeability of the absorbing material by adjusting the dimensions of the electric and magnetic resonant components in the unit cell so that the impedance of absorber match with that of free space. Impedance matching causes minimization of reflection and transmission of incident waves by the absorber for an intended frequency and large losses in the absorber with the incident energy converted into heat. However, the disadvantages of these absorbers include use of specially made resistive sheets (Chambers, 1994; Munk et al., 2007; and Seman et al., 2011), loaded resistances of specific values (Liu et al., 2011), nonavailability of certain types of magnetic or dielectric absorbing composites in desired proportion for deposition on polymeric substrates (An et al., 2004; Flogwras et al., 2010; and Mattiucci et al., 2012). Therefore, it is needed to find alternative solutions which can lead to lower profile, planar, thinner, smaller volume, easy fabrication and high performance microwave absorbers.

Electrical and Electronics Engineering Journal, Ultrathin Narrowband, Convoluted Square Frequency, Surface-Based Microwave Absorber.