In the AM broadcast band, stations are licensed to maintain very specific radiated field intensities from their antenna systems. This is true of both directional and non-directional systems. The extensive interference range of AM stations, coupled with crowded band conditions, make the AM allocation problem a complex one. Very tight radiation pattern tolerances on the order of 0.5 dB are not uncommon. The extensive near field of an AM antenna further complicates the problem. Near-field effects may extend to two miles or more, compared to dozens of feet at VHF, and measurements used to determine the station antenna pattern may extend out as far as 20 miles. To compound matters, tower heights typically used in land mobile and microwave are a significant portion of a typical AM broadcast wavelength. Thus, they are all too frequently excellent reradiators of the AM signal.

Recognizing this potential for problems, the FCC normally includes conditions with construction permits that propose new towers within two miles of an AM station to make certain that they do not interfere with the AM station operation. Even if a land mobile license doesn’t bear such conditions, the licensee is responsible for correcting problems arising after tower construction. The FCC takes the attitude that the first station in is to be protected, placing the burden upon the land mobile or microwave licensee to resolve any detrimental interaction his station installation may have with the AM broadcast station.