Complicating matters further is the behavior of the horizontal component of the radiation, the portion received best by horizontal antennas. When the antenna is mounted on the side of narrow pole or small cross-section tower, this portion often resembles a circular pattern, but as the tower width increases, very strange patterns may result, sometimes with minimum radiation in the direction of antenna mounting. Enterprising station engineers learned that although the FCC classified their antenna as omnidirectional, in truth it was not, and rather than rolling the dice with their station's coverage, some have been known to tweak the antenna mounting while monitoring signal levels in critical market locations, seeking the most favorable results. The line is clearly drawn when it comes to actually modifying the structure by adding additional, or parasitic elements near the antenna bays. There is a case on record where a station actually installed screens behind each antenna bay that served as reflectors to direct the rf energy toward the market, a clearly illegal modification.
Some operators install a side-mounted antenna on a small pole at the tower top.
This technique adds cost and may impose structural limits on the number of bays that can be used, decreasing antenna gain, but produces a more predictable pattern. These are usually very broadband, and are often the choice of stations that share one antenna between two or more transmitters. They aren't as popular as the side-mounted flavor due to their high cost. The important feature with this system is that the horizontal bracing lattice work repeats at precisely the same interval as the bay mounting, ensuring that the range testing results will truly be recreated in the field. Repeatability is the key.