Antenna location optimization for an airborne in-situ measurement probe using the FDTD

Large VHF radar systems use ground reflections from the surrounding terrain to form the radar antenna pattern. It is therefore necessary to use in-situ measurements to determine the performance of the radar antenna. An airborne measurement probe consisting of a helicopter-mounted dipole antenna was employed to perform these in-situ measurements. The radar antenna radiation patterns are reconstructed from data collected on in-bound or out-bound flights through its main beam. Results from initial measurements aroused suspicions that the radiation patterns of the helicopter antenna probe were not uniform enough to accurately measure the radar antenna patterns without implementing a probe correction. Either the radiation pattern of the helicopter probe must be made uniform over some specified azimuth and elevation angles, or its deviation must be known in real time so as to enable a probe correction during flight. In either case, it became highly desirable to understand the radiation characteristics of the helicopter-mounted antenna. This paper documents the characterization of the original helicopter probe configuration, and the results of an attempt to find, using numerical modeling with the FDTD, a dipole mounting location on the helicopter which would yield radiation patterns of sufficient uniformity over the critical azimuth and elevation angles.