  Three dimensional (3D) radiation pattern of an antenna mounted at a drone can
significantly influence the air-to-ground (A2G) link quality. Even when a drone
transmitter is very close to a ground receiver, if the antenna orientations are
not aligned properly, a significant degradation can be observed in the received
signal power at the receiver. To characterize such effects for a
doughnut-shaped antenna radiation pattern, using an ultra-wideband (UWB)
transmitter at the drone and a UWB receiver at the ground, we carry out A2G
channel measurements to capture the link quality at the ground receiver for
various link distances, drone heights, and antenna orientations. We develop a
simple analytical model to approximate the influence of 3D antenna patterns on
the received signal strength (RSS), which show reasonable agreement with
measurements despite the simplicity of the model and the complicated 3D
radiation from the UWB antennas. We also explore how the signal strength can be
improved when multiple antennas with different orientations are utilized at
transmitter/receiver.
