Ultra-Wideband 3-D Microwave Absorbers With Composite Slotlines and Microstrip Lines: Synthetic Design and Implementation

This article proposes a class of ultra-wideband 3-D microwave absorbers with composite slotlines and microstrip lines from synthetic design to practical implementation. Firstly, the general equivalent transmission line (TL) model of the proposed 3-D absorber element is formed based on two sets of shunt short-ended stubs and an i-section nonuniform connecting line. Then, the synthetic procedure is theoretically investigated using the equivalent TL model, aiming to construct an ultra-wide absorption band with the Chebyshev equal-ripple response. According to the prescribed absorptive performance, such as maximum reflection coefficient (C) and fractional bandwidth (FBW), the relevant parameters of the TL model can be directly calculated by the established synthesis approach. To validate the proposed concept, two prototypes, namely, absorber-I (i=1) and absorber-II (i=2), are designed, fabricated, and measured. Each prototype element applies a simple 3-D structure composed of slotlines, microstrip lines, and only one absorptive load, which are all etched on a single-layered substrate. Measured results agree well with synthetizations and simulations. For absorber-I, the measured bandwidth of 95.5% in a frequency range from 5.08 to 14.44GHz is successfully realized. Absorber-II obtains a measured FBW of 111.1% in a range from 4.48 to 15.68GHz. Average absorption ratios within the operating band are higher than 95.5% and 93.8% for absorber-I and absorber-II, respectively. Besides, the proposed 3-D absorbers are angularly stable within 600 oblique incidence. Therefore, such ultra-wideband 3-D microwave absorbers with simple structures possess the attractive potential for effectively absorbing electromagnetic waves.