In a decision of the FCC Media Bureau’s Audio Division that may be of interest to the more technically minded broadcasters, the Commission found that an FM station’s supposedly nondirectional FM antenna should be treated as directional. This decision was in response to a complaint from another broadcaster on the same channel, arguing that the broadcaster in question was exceeding its licensed effective radiated power in the direction of the complaining station (which was also the direction of Dallas, toward the more densely populated areas that it was trying to serve). The station that received the objection argued that the apparent effect on its antenna pattern was simply the result of being side-mounted on the broadcast tower that it was using, and this kind of effect was common in the industry and impossible to avoid. Yet, in reviewing the pleadings filed by the parties, the FCC found that the supposedly nondirectional station looked far too much like a directional one, and ordered the licensee to reduce power to keep its ERP (effective radiated power) in the direction where it was greatest to a value within that set out in its license. What impact will this decision have on other FM stations with sidemounted antennas?
First, it appears that the this case is one where, at least according to the FCC decision, the station had specifically designed an optimized pattern that resulted in its significantly exceeding its permitted power in the direction of the complaining station. The FCC found that, in the direction in which its maximum power was being radiated, the station had an effective ERP of 274 kw, far in excess of its licensed 100 kw ERP. The Commission noted that the direction of the highest radiation was actually not in the direction of the station’s city of license. The FCC also found that the ratio of the power in the direction of maximum radiation to the power in the direction of the minimum radiation was 19.18 dB, far exceeding the maximum 15 dB ratio permitted for directional antennas. Finding these great discrepancies in what was supposedly a nondirectional antenna led the FCC to its decision that the antenna was designed to do what it did –radiate more than permitted in the direction of the complaining station. But does this decision have potentially greater impact?The decision does suggest that stations need to keep their effective radiated power totally within licensed values in all directions, even when taking into account the effects of towers on which their antennas are sidemounted. But the FCC also goes on to cite other cases, where complaints were dismissed, where the effects of the sidemounting were smaller, or where the antenna systems were not so carefully designed so as to maximize power in one direction. It also cites a 1984 Policy Statement suggesting that any sidemounting designed to intentionally distort the pattern of an FM station should be licensed as a directional pattern. Note the highlighted word “intentionally,” which I highlighted, not the FCC. And, of course, much has changed in the last 30 years, as the station involved here tried to argue in its defense.
It appears that the Commission recognizes that small variations in patterns may exist due to the reflected signal that results whenever an antenna is sidemounted, and that small unintentional variations may not be a concern. But the decision does not clearly and broadly draw lines between what is permitted and what is prohibited – and what is intentional and what is not. I am sure that engineers will be concerned that accounting for variations not cause great complications in licensing nondirectional FM stations. The decision here is a decision in a single case, where there are seemingly bad facts for the station employing the optimized antenna. What its true meaning will be for other sidemounted FM stations will no doubt be discerned from future decisions. In designing antenna construction for new facilities, station owners should have their technical team carefully read the discussions in this case, to be sure that their proposed operations do not run afoul of the limits set forth herein. And broadcasters should be watching to see if there is a further appeal of this decision, or an attempt to get more clarity on this issue from engineering groups.