  This paper presents an experimental study of the air-to-ground (AG)
propagation channel through ultrawideband(UWB) measurements in an open area
using unmanned-aerial-vehicles (UAVs). Measurements were performed using UWB
radios operating at a frequency range of 3.1 GHz - 4.8 GHz and UWB planar
elliptical dipole antennas having an omni-directional pattern in the azimuth
plane and typical donut shaped pattern in the elevation plane. Three scenarios
were considered for the channel measurements: (i)two receivers (RXs) at
different heights above the ground and placed close to each other in
line-of-sight (LOS) with the transmitter (TX) on the UAV and the UAV is
hovering; (ii) RXs are in obstructed line-of-sight (OLOS) with the UAV TX due
to foliage, and the UAV is hovering; and, (iii) UAV is moving in a circular
path. Different horizontal and vertical distances between the RXs and TX were
used in the measurements. In addition, two different antenna orientations were
used on the UAV antennas (vertical and horizontal) to analyze the effects of
antenna radiation patterns on the UWB AG propagation. From the empirical
results, it was observed that the received power depends mainly on the antenna
radiation pattern in the elevation plane when the antennas are oriented in the
same direction, as expected for these omni-azimuth antennas. Moreover, the
overall antenna gain at the TX and RX can be approximated using trigonometric
functions of the elevation angle.
