  A novel shunt longitudinal slot planar array antenna based on a ridged
substrate integrated waveguide (RSIW) with a good cross polarization and
isolation is designed and fabricated. The slots in the proposed structure are
placed on the centerline of the broad wall of the substrate integrated
waveguide (SIW), in order to reduce cross polarization. Also, the asymmetric
field distributions on both sides of each slot, which are required for making
radiation possible, are realized through using ridges close to the narrow walls
of the SIW. The admittance of a single slot in the proposed structure is
obtained regarding the different values of the main parameters of the
structure. An eight-element longitudinal slot array antenna on the broad wall
of the RSIW is designed based on Elliots design procedure. In this design,
internal and external mutual coupling effects are taken into account through
calculating the active admittance for each slot of the array. A 2D graph is
presented which contains the real and imaginary parts of the equivalent
admittance of a slot based on the changes in the widths of the ridges and
length of the slot. Using this graph, the widths of the ridges and length of
the slot can be revised in a way that the mutual coupling effect is taken into
account. Since the array antenna is composed of collinear elements, it
demonstrates a better cross polarization and isolation in comparison with
conventional slot array antennas. Simulation results show good agreement with
measurement results of the fabricated proposed antenna.
