  This paper proposes a low-profile and highly efficient endfire radiating
travelling-wave antenna based on spoof surface plasmon polaritons (SSPPs)
transmission line. The aperture is approximately
$0.32\lambda_0\times0.01\lambda_0$ where $\lambda_0$ is the space wavelength at
the operational frequency 8 GHz. This antenna provides an endfire radiation
beam within 7.5-8.5 GHz. The maximum gain and total efficiency reaches 9.2 dBi
and $96\%$, respectively. In addition to the endfire operation, it also
provides a beam scanning functionality within 9-12 GHz. Measurement results are
finally given to validate the proposed SSPPs antenna.
