Gradual continuous transverse stub antenna array for true broadside radiation

Continuous Transverse Stub (CTS) antennas are widely used in high-gain aperture systems, but they inherently suffer from a fundamental trade-off: achieving good impedance matching typically requires choosing an inter-element spacing slightly smaller than the guided wavelength, which introduces a longitudinal phase gradient and prevents true broadside radiation at the design frequency. Here, we propose a Gradual Continuous Transverse Stub (GCTS) antenna array in which the guided wavelength inside the slow-wave region and the inter-element spacing on the radiating layer are jointly tapered along the initial section of the structure. This co-tapering strategy enables each radiating cell to operate under a locally phase-balanced condition, so that the elements are excited with matched amplitude while maintaining phase alignment at the design frequency. As a result, the antenna achieves true broadside radiation without beam tilt under nominal excitation, while still supporting wide-angle mechanical beam steering similar to Variable-Inclination CTS (VICTS) apertures. A prototype GCTS antenna was fabricated and experimentally characterized. The measured results confirm improved impedance matching, broadside radiation at 11.7 GHz, and efficient beam scanning over a wide angular range, in good agreement with the theoretical concept. These results demonstrate that the proposed GCTS architecture provides an effective approach for mechanically steerable CTS and VICTS apertures.