Abstract: This paper proposes a planar conical-beam leaky-wave antenna (CBLWA) that achieves a broad elevation scanning range and excellent out-of-roundness (OoR) level. Concentric annular leaky-wave structures are etched onto the surface of a parallel-plate waveguide, which can convert the guided electromagnetic waves into conical-beam radiation waves. To effectively widen the elevation scanning range, the dispersion characteristics of the unit cell is theoretically analyzed. By establishing the equivalent circuit of the leaky-wave structure, the circuit parameters of the electromagnetic model are effectively extracted. The impact of various probe configurations on the OoR level is evaluated. This analysis ultimately led to the design of an eight-port equal power divider feeding network utilizing a grounded coplanar waveguide. Measured results demonstrate the antenna can operate within the 30.0 GHz to 37.0 GHz frequency range, achieving a fractional bandwidth of 21% and a 45° scanning range. The CBLWA maintains stable omnidirectional radiation with azimuth plane OoR below 1.6 dB and cross-polarization levels consistently below -22 dB. The design could find promising applications in millimeter-wave (mmWave) systems, such as satellite communications and fuze detection.