Communications, navigation, and other related systems need to have a well-defined antenna radiation pattern. In onboard vessel systems, the radiation pattern can be much different than the one obtained for an isolated antenna (because of the vessel's structure and other nearby radiating systems interference). Finding out the onboard antenna's radiation pattern is a well-known problem for any shipbuilder/owner. The conventional method consists of measuring radiation patterns from a fixed antenna on the coast while the ship is navigating in circles. Recent electronic systems in the market now allow for an alternative method: keeping the ship static while an unmanned aerial vehicle (UAV) circles it, measuring the antenna's transmitted power. This research paper examines the airspace volume and optimal flight path of an off-the-shelf UAV system for measuring the onboard antenna's radiation pattern in the presence of physical constraints such as the vessel's dimensions, safety zone, distance to base, Fresnel's and multipath distances, and considering the loss due to polarization decoupling between the antenna under test and UAV's antenna.