A quarter-wavelength-thick unprinted dielectric slab is placed above a slot antenna to achieve high gain with a large bandwidth. By optimally truncating the high-permittivity superstrate to a small area of 2.25λ20, 8-10 dB enhancement in the directivity of the slot is achieved over a 3 dB directivity bandwidth of 16-18%. A prototype antenna was fabricated and tested at 11.5 GHz. Its measured peak gain is 16.95 dBi and the 3 dB gain bandwidth is 16.25%. The proposed antenna is simple and, more importantly, has a lower profile with a total height of 0.593λ0,.