Contents I. Introduction
Planar slot antennas are very popular in the design of microwave circuits due to its advantages such as low profile, conformability, easy fabrication etc. But it suffers from an evident drawback of its bidirectional radiation characteristics which can be improved by using metallic cavity at the backside of the antenna. The cavity backed slot antenna shows unidirectional radiation pattern and high gain but becomes bulky due to the non-planar metallic cavity structure [1]–[3]. Recently a new technology known as Substrate Integrated Waveguide (SIW) has emerged which helps to implement waveguide like structures in planar substrate [4]. Design of low profile, planar substrate integrated waveguide cavity backed slot antenna was proposed by Luo et.al. which replaces the bulky metal cavity backing with SIW cavity to make the design fully planar [5]. The proposed antenna is fed by microstrip line along with microstrip to grounded coplanar waveguide (GCPW) transition along the center of the cavity and the design shows a bandwidth of 1.7% with a gain of 5.4 dBi. Several design techniques to improve the bandwidth of this type of antenna have been reported in recent years [6]–[8].
