This paper presents a solution to overcome the inherently limited bandwidth of substrate-integrated waveguide (SIW) slot antennas. It is analytically shown that by decreasing the permittivity of a dielectric loaded slot antenna, the resulting bandwidth increases significantly, where the widest bandwidth can be achieved when the permittivity of the dielectric is less than unity. To demonstrate this concept, a rectangular SIW slot is loaded by an array of thin wires to realize the desired low-index metamaterials (MTMs), which consequently results in a single-layer, compact, and cost-effective structure. We have measured an impedance bandwidth (|S11|<−10dB) of 36.2%, covering the millimeter-wave (mmWave) frequency range of 19.7–28.4 GHz. The radiation efficiency is above 90%, providing at least 7.5 decibels relative to isotropic (dBi) gain through the entire frequency band, making it a potential candidate for industrial, scientific and medical (ISM) and/or automotive radar (24.125--24.25 GHz) and 5G (24.25--28.35 GHz). Measurements show that the proposed antenna not only has a broad impedance bandwidth but also an improved radiation bandwidth.