A consideration on parallel plate mode suppression of triplate-type planar antenna

A planar antenna for which the feed system is constructed with triplate transmission lines has been studied extensively, since unwanted radiation from the feed system can be suppressed. However, this type of feed system excites the parallel plate mode (TEM mode) propagated between the upper and the lower ground plane due to the feeding line discontinuity. This TEM mode causes the unexpected coupling between antenna elements and makes inferior the design capability. As the TEM mode suppressing method, the three dimensional method which connects the upper and the lower ground plane using the short-circuited pins was reported by Ohmine et al. (1992). In this paper, we propose a new method for suppressing the TEM mode. First, the basic configuration of the triplate-type planar antenna (TPA) with the TEM mode suppressing system is presented. Second, the configuration and radiation properties of the linearly polarized TPA element (LP-TPA) is described, and the effect of the TEM mode suppression is confirmed. Third, this method is then applied to the circularly polarized TPA element (CP-TPA) and the TEM mode suppression effect is also confirmed.<>