Design of Periodic Shorting-Vias for Suppressing the Fundamental Mode in Microstrip Leaky-Wave Antennas

The design of periodic shorting-vias for suppressing the fundamental mode in microstrip leaky-wave antennas is studied in this paper. When a microstrip leaky-wave antenna is loaded with periodic shorting-vias along the centerline, the fundamental mode has a stop band in which the mode is strongly attenuated. The stop band is investigated to best match the (leaky-wave) antenna-mode region of the first higher mode. An approximate formula is derived to estimate the start of the stop band. When the fundamental mode is suppressed in the required band, a microstrip leaky-wave antenna can be efficiently fed with a simple asymmetrical feeding structure and no additional out-phase network is needed. A microstrip leaky-wave antenna fed by a coaxial probe and another one fed by a microstrip line are demonstrated. With the proper design of shorting-vias for suppressing the fundamental mode, excellent performances of the microstrip leaky-wave antennas are achieved.