Compact Shared-Aperture Wideband Endfire Antennas Embedded in a Heteromorphic Cavity

In this letter, a novel scheme for dual wideband endfire antennas operating within a shared aperture is presented. Confined by the installation aperture, we have developed a low-frequency (S-C band) endfire antenna (L-EFA) and a high-frequency (X band) endfire antenna (H-EFA), both coexisting in a compact metallic heteromorphic cavity. The L-EFA is developed from a half-mode tapered slot radiator, integrating slotted lines and gradient pectinate top loading to achieve optimal impedance matching while preserving a low-profile (0.13$\lambda_\text{L}$) design. Positioned in front of the L-EFA, the H-EFA employs a small and lightweight dielectric lens to ensure wide and stable azimuth radiation patterns. The design of the compact metallic cavity has been optimized to enhance wideband impedance matching and maximize radiation efficiency for both antennas. To validate this scheme, we fabricated and measured prototypes embedded within a metallic cylinder. The dimensions of the cavity are limited to 0.38$\lambda_\text{L}$ × 0.54$\lambda_\text{L}$ (where $\lambda_\text{L}$ represents the wavelength at the lowest operational frequency). Measurement results demonstrate that shared-aperture antennas effectively cover dual bandwidths of 2.1 GHz to 6 GHz and 6 GHz to 18 GHz, while exhibiting stable endfire radiation performance with high radiation efficiency (≥85%) across both operational bands.