A Thin X-Band Microwave Absorber Using a Center Shorted Spiral Medium

In this letter, the reflection characteristics of a shorted two-arm spiral medium are examined numerically. A magnetic-type phase response zone has been observed at a lower frequency range for both polarizations compared to other magnetic photonic band-gap (PBG) structures, such as the mushroom structure. The proposed structure exhibits good stability on the incident angle. The resonant frequency $(\angle\Gamma=0)$ varies within 1% with respect to the incident angle $(\theta\in(0^{\circ}\ 60^{\circ})$ for both polarizations. An absorption state occurs at the first magnetic-type phase response region. A 0.508-mm-thin X-band microwave absorber is fabricated and verified in a WR90 waveguide. An optimized design with 1.5% absorbing bandwidth is also given. Finally, the absorbing mechanism is discussed.