Novel Suspended-Line Gap Waveguide Packaged With Stacked-Mushroom EBG Structures

A novel low-loss suspended-line gap waveguide (SLGW) supporting transverse electromagnetic (TEM) propagation is presented for millimeter-wave applications for the first time. The suspended line is composed of two parallel strips connected by metal via holes. Periodic stacked-mushroom electromagnetic band gap (SM-EBG) structures are used to package the suspended line by creating a stopband for all parallel-plate modes. According to the different packaging requirements of the circuit, the SM-EBG unit cell can be composed of several stacked mushrooms and metal covers. The air gaps' thickness between face-to-face patches of the adjacent mushrooms is near zero, confining more power in the air-filled guiding region of the SLGW. Furthermore, the SM-EBG structures with near-zero thick air gaps can provide more stable mechanical support for the multilayer structure. Therefore, SLGW has a lower loss and more robust gap stability compared with the conventional air-filled printed versions of GWs, such as printed ridge gap waveguide (PRGW) and inverted microstrip gap waveguide (IMGW). On the other hand, compared with the self-packaged double-sided interconnected strip line (DSISL), the proposed SLGW packaged with SM-EBG structures has the advantage of no electrical contact, avoiding the power leakages in the gap between plates at discontinuities and simplifying the assembly. The design and analysis of the SM-EBG structure, the design of SLGW, and the microstrip-to-SLGW transition design are given in this article. A prototype of the proposed SLGW with back-to-back transition is fabricated packaged with SM-EBG structures to prove the concept.

I. Introduction

With the increasing demand for millimeter-wave wireless systems, the components based on the traditional transmission lines are facing challenges in realizing a low loss, high integration, effective cost, and stable circuit performance at high frequencies. The hollow waveguide has the advantage of low loss and high quality factor, but it is bulky and difficult to be integrated with RF circuits. Microstrip suffers from radiation losses, surface waves, and undesired cavity modes produced by the metal box shielding [1]. For the stripline, any vertical asymmetry between the two ground planes will cause the unwanted higher order modes [2]. Substrate-integrated waveguide (SIW) is a self-packaged structure realizing a metal waveguide in the planar form. Since electromagnetic (EM) waves mainly propagate in the dielectric, SIW suffers from severe dielectric losses at high frequencies [3]. Therefore, air-filled self-packaged multilayer printed circuit board (PCB)-based transmission lines, such as air-filled SIW (AF-SIW) [4], substrate-integrated suspended line (SISL) [5], DSISL [6], and empty substrate-integrated coaxial line (SICL) [7], have been presented to achieve low loss. However, energy leakages are inevitable at high frequencies due to the possible poor electrical contacts between plates, especially at discontinuities.