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A narrow-beam antenna using an artificial dielectric medium with effective permittivity less than unity has been investigated theoretically and experimentally. Complex propagation constant for a wave propagating along the structure is evaluated by solving the dispersion relation. The analysis indicates the presence of a leaky-wave mode. The radiation pattern is calculated by using Kirchhoff-Huygens integration method. Experimental results for an antenna of this type designed for operation at band are reported and compared with theoretical calculations. Beam position varies from to when the frequency is changed from 8.6 to 11.5 GHz. Over this frequency range the half-power beamwidth varies from to . An expression for the half-power beamwidth of a leakywave antenna of finite length has been derived and discussed in the Appendix.