If you are an operator or a manufacturer of high-power broadcast antennas you need to know their true, on-site performance in terms of radiation patterns and radiated powers, otherwise coverage  may be less than you or your customers are expecting.

A transmitting antenna for television or radio broadcasting is a complex structure and much can go wrong between leaving the manufacturer and its final installation on the mast. Directional beams may be mis-aligned or antenna panels may be incorrectly assembled.

The figure on the right shows how faulty antenna panels can affect the horizontal radiation pattern resulting in reduced coverage in certain directions. Ground-based field strength measurements cannot identify these problems with sufficient certainty or accuracy due to the adverse effects of terrain and ground clutter on the received signal.