  In this paper, we study a plasmonic horn nanoantenna on a metal-backed
substrate. The horn nanoantenna structure consists of a two-wire transmission
line (TWTL) flared at the end. We analyze the effect of the substrate thickness
on the nanoantenna's radiation pattern, and demonstrate beam steering in a
broad range of elevation angles. Furthermore, we analyze the effect of the
ground plane on the impedance matching between the antenna and the TWTL, and
observe that the ground plane increases the back reflection into the waveguide.
To reduce the reflection, we develop a transmission line model to design an
impedance matching section which leads to 99.75% power transmission to the
nanoantenna.
