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A spoof surface plasmon polariton (SSPP) antenna with dual-band endfire gain and small frequency ratio (FR) is presented. It uses an asymmetric structure to support two independent surface waves with different nonlinear phase constants. Due to the different phases of the superposed $E$ -field at the terminal of the proposed structure, dual-band endfire gains and endfire radiation null are achieved. The odd- and even-mode feedings, providing additional phase differences, are also investigated. Three prototypes, with FRs of 1.06, 1.11, and 1.5, are designed, simulated, and measured, respectively. All results validate the mechanism and demonstrate that the FR of the proposed antenna can be achieved easily in the range of 1.06–1.5 by choosing different unit cells and feeding modes. Moreover, the measured endfire out-of-band gain suppression, over 17 dB, presents a good directional interference inhibition. This small-FR endfire antenna shows a broad prospect for band coverage and directional noise inhibition with very close operating frequencies. [ABSTRACT FROM AUTHOR]

An endfire frequency scanning tapered slot antenna (FSTSA) is proposed and investigated. The proposed FSTSA is a two-conductor structure with a gradient slots array etched only on one of two inner edges in the FSTAS. The asymmetrical inner edges enable the antenna to excite two modes with different phase constants simultaneously along the two inner edges. Due to dispersion characteristics of spoof surface plasmon polaritons (SSPPs), the phase difference between the EM waves along the two conductors changes versus frequencies, which enables the main lobe of the FSTSA to scan through the endfire direction. An analysis model is conceived to provide insights into the design, which accurately predicts main lobe directions and initial values of the proposed antenna. The proposed antenna is fabricated, and the measured results show that the main lobe of the antenna scans from −15° to 14° (endfire direction at 0°) from 6.60 to 7.70 GHz, with the scanning rate of 26.4°/GHz. This antenna could be suitable for radar, plane integrated communication system, and certain other beam-scanning applications. [ABSTRACT FROM AUTHOR]