I. Introduction
Negative-refractive-index (NRI) metamaterials based on the composite right/left-handed transmission lines (CRLH-TLs) have been proven to be suitable materials for the design of antennas with compact size [1]–[7]. Recently, various compact antennas operating at zero or negative resonant modes of CRLH-TLs have been reported. In [1]–[4], a series of zeroth-order resonant antennas have been designed. Because of infinite wavelength operation, these antennas were more compact than conventional half-wavelength antennas. However, the antennas operating at the zeroth-order resonant mode often suffer from low radiation efficiency and low gain. According to [5], these drawbacks result from two factors: the small radiator size and the inherent loss caused by the artificially engineered structures. To tackle these problems, a family of substrate integrated waveguide (SIW) slot antennas operating at the negative-order mode were designed, including the open-ended type and the short-ended type [6], [7]. A substantial degree of miniaturization in both the transversal and the longitudinal dimensions could be realized compared with the conventional waveguide slot antennas because the operating frequencies of these antennas were far below the initial cutoff frequency of the SIW. Also, these antennas provided higher radiation efficiencies and gains than those of the CRLH zeroth-order resonant antennas in [1]–[4].