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We present the analysis and design of a microstrip antenna array with a conical beam. The antenna array employs a periodic structure derived from a microstrip on a perforated ground plane. The microstrip antenna array consists of eight leaky lines evenly distributed around a circle. Each line radiates mainly a pair of forward and backward beams, forming a conical beam. The new leaky line design employs the concept of space-harmonic modulation on the EH0 mode launched at the common joint of the antenna array. The scattering and dispersion characteristics of the leaky line are thoroughly investigated. It is theoretically and experimentally validated that the perturbation of the photolithographically etched antisymmetric holes excites the leaky EH1 mode. Scattering analyses of the surface currents on the microstrip reveal in detail the physical operation of the leaky line. The dispersion characteristics are, thus, established and shown in a Brillouin diagram, indicating the role of each space harmonic, denoted by its complex propagation constant, γm,n±. Constant γm,n± represents a traveling-wave component of the nth higher-order spatial component in association with the EHm mode: the superscript, + (-), signifies a forward (backward) traveling wave. For the particular leaky line design, γ0,-1+ and γ1,0+ space harmonics, both showing odd-symmetric field polarization, and both outside the triangular Brillouin region, form the backward and forward leaky waves, respectively. When the leaky lines are evenly and collinearly tied, an 11.5 GHz radial antenna array prototype is formed, and emits a conical beam, showing a 3 dB beamwidth of 24° at the flare angle of 31°.