A multilayer leaky-wave antenna (LWA) is presented for microwave and millimetre-wave frequencies. By placing two superstrate dielectric layers on top of a base grounded dielectric slab, a practical resonant cavity structure can be realised for high gain at 17.39 GHz. The compact 6 x 6 cm LWA is fed by a printed surface-wave source integrated in the bottom ground plane. Measured gain values are greater than 11 dBi at broadside with half power beam less than 7 degrees. The presented low cost and low profile LWA may be useful for radar applications and surveillance systems where lightweight and compact antenna designs are of interest.