In a single microstripline, the dominant mode is "quasi" TEM, which is a non-radiating surface mode. The higher-order modes, however, become leaky when the propagation constant is less than that of the free-space wave number, k/sub 0/. One of the simplest ways to realize a leaky-wave antenna is to use a regular microstripline properly excited by a coaxial probe. The major problems in this case are input impedance matching and suppression of surface-mode excitations. In the proposed antenna, a double-layer microstrip structure is used to facilitate the impedance variation at the feed. The field distribution at the feed location is altered to match the input impedance by varying the locations and widths of the metallic patches on the two layers. Once the input impedance is matched to a particular leaky mode propagation, the surface modes will be likely to be suppressed because of impedance mismatch to all modes other than the intended leaky mode. Compared with other leaky-wave antennas, the proposed antenna is planar and can be easily implemented in an MMIC environment.