###### Abstract  
In this paper, a titled-beam antenna based on spoof surface plasmon polaritons (SSPs) transmission lines (TLs) is proposed. The parallel SSPPs-TL is a slow-wave TL, which is able to limit waves in the TL strictly. By periodically introducing a set of tapered stubs along the SSPPs-TL, the backward endfire beams are formed by the surface waves in the slow-wave radiation region. Then, through the placement of a big metal plate below the endfire antenna, the backward endfire beams are tilted, and the tilted angle of the beams are steered by the distance of the metal plate and antenna. Over the band of 5.7 GHz-7.0 GHz, the tilted antenna performs constant shapes of radiation patterns. The gain keeps stable at around 12 dBi and the 1-dB gain bandwidth is 20%. The measured results of the fabricated prototypes confirm the design theory and simulated results. t 2021 2021 6 May 2  
Meanwhile, it also has the drawback of a bulky structure because of a large number of the periodic unit cells. Besides, a tilted beam with high gain also can be formed by arraying the radiation elements and electrically manipulating the phase of the elements [10], whereas the phase manipulation needs the electronic phase shifters or phase control circuits that increase the cost and complexity of the antenna system. Leaky-wave antenna (LWA) with fixed tilted beams have attracted increasing interest for point-to-point or point-to-multipoint communication systems. One of the biggest problems of the fixed-beam LWA is the beam squint that leads to gain losses at the desired radiation direction. To overcome this problem, some complementary frequency-dependent metasurfaces are proposed to cancel the dispersion of the LWA [11; 12; 13]. In [13], two complementary dispersive prisms are proposed, one with mirror symmetry and the other one with glide symmetry, which are coupled to the LWA radiation aperture. The beam squint is reduced by the two prisms. A 11% bandwidth centered at 58 GHz is obtained with a beam squint less than \(\pm 0.9^{\circ}\) (mirror) and \(\pm 1.7^{\circ}\) (glide). Spoof surface plasmon polaritons (SSPPs) have been widely studied since it was proved that the periodical subwavelength structure is able to support SPP waves in microwave and gigahertz frequencies [14]. In recent years, several designs based on SSPP waveguide with endfire radiation performances have been reported [15; 16; 17; 18; 19]. One way to obtain endfire radiation beams is by loading a main radiator and the directors at the end of an SSPPs-TL [15], the working theory of which is similar to Yagi antenna. Furthermore, another efficient method for endfire radiation is to excite the odd leaky mode of the dual-side corrugated-groove SSPP waveguide [16; 17; 18], to which the phase constant is well tailored by the corrugated grooves to achieve endfire radiation with the approximate satisfaction of the Hansen-Woodyard conditions. In [16], a feeding structure of a microstrip-to-slotline converter and a differential-mode exciter is used to excite the odd-mode signal on the SSPP structure. Additionally, a tapering end is introduced to the SSPP radiator, realizing a broader upper bandwidth. In [17], the TE\({}_{1}\) leaky-wave mode is strategically excited by an electric slot in the center of the metal strip with dual-side corrugated grooves, which is a very simple structure exciting the odd mode. In [18], an equivalent radiating aperture is formed by gradually decreasing the grooves of an odd-mode plasmonic structure. The mechanism of SPP wave radiating out through the aperture is clarified by referring to a horn antenna. In this paper, a fixed tilted-beam leaky-wave antenna is proposed. It is a travelling wave antenna based on the parallel SSPPs-TL. By loading a set of metal stubs, the confinement property of SSPP waves in the SSPPs-TL is broken and converted to the radiation electromagnetic waves. The proposed antenna has a phase constant for backward endfire in a wide region through the slow-wave region and the fast-wave region.