Dramatic “optimizing” of FM antenna gets the hairy eyeball from the Audio Division
In the FM radio world, there are supposed to be only two kinds of antennas: directional and non-directional. While it has long recognized that that simplistic, idealized notion is not entirely valid, the Audio Division hasn’t acted on that recognition – until now.
In a decision that likely disappointed at least one Texas FM licensee, the Division has ordered that licensee – whose station is licensed to operate, nondirectionally, with ERP of 100 kW – to explain why its license shouldn’t be changed to specify directional operation. Such a change would result in a reduction by more than half (from 25 kW to 9.1 kW) of the station’s transmitter output power.
Non-directional antennas (a/k/a/ “non-D’s” or “omni’s”), of course, are supposed to transmit an equally strong signal in all directions. On the other hand, directionals – or “DA’s” – are designed to produce a signal that is stronger in some directions than others. They come in handy when a station needs to avoid interfering with a co- or adjacent-channel station in one direction.
But things are not as simple as they might appear – mainly because, thanks to technical considerations, omni antennas do not necessarily provide an idealized circular signal contour. Perhaps most obviously, if a non-directional antenna is mounted on the side of a tower, rather than the top, the interaction of the signal with the tower structure itself can distort the signal in a number of ways. Recognizing this, antenna manufacturers have sought to adjust some omni’s to “optimize” their performance, i.e., to counteract such distorting effects.
But once you start down the “optimization” road, things can leave the rails pretty quickly.Continue Reading...