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.

After all, if you can adjust an omni’s performance in some regards, you can adjust it in others. And sure enough, for more than 30 years various efforts have been made to convert ostensibly omnidirectional antennas into de facto directional antennas through an array of devices. Those include use of frequency-matched “lambda” towers specifically designed to support an antenna operating on a particular frequency. The frequency matching effect of the tower can significantly alter an omni’s signal in various ways. Another device is the attachment of “parasitic elements” onto the antennas in various places. These, too, affect the signal.

While such devices are usually touted as efforts to compensate for common distorting factors, observers have long understood that they may also be used to achieve favorable directionalization that can extend a station’s signal well beyond its predicted omnidirectional contour.

The Commission was onto this back in 1984. It issued an obscure, one-page public notice (not published in any official publication, and not easy to track down in 2015 – until this post, at least), in which it warned:

In making allotments and in issuing construction permits and licenses the Commission assumes that FM non-directional broadcast antennas have perfectly circular horizontal radiation patterns. Actual antenna patterns shall conform to the ideal as closely as is practicable. The use of any technique or means (including side mounting) which intentionally distorts the radiation pattern of what is nominally a non-directional antenna makes that antenna directional and it must be licensed as such.

Having rattled that saber, though, the Commission carefully placed it back in its scabbard and locked it away, never to be wielded again. Until now.

The Commission received a complaint from a Class A FM in Texas whose signal was, according to the complaint, getting creamed by a 100 kW Class C0 station. The C0 is an omni, and its predicted contour indicated that it was protecting the Class A as it should. But in real life, that wasn’t the case.

The Division asked the Class C0 licensee about its transmission system and was told that the station’s antenna had been “optimized”, but not directionalized. Digging further, the Division determined that the C0’s signal in the direction of the Class A was the equivalent of a signal transmitted with between 260-275 kW of ERP, more than twice the C0’s authorized 100 kW ERP. Moreover, the maximum-to-minimum ratio for the supposed non-D antenna turned out to exceed the maximum value allowed for directionals in the horizontal plane. And there was evidence indicating that the directionalization was intentional: the antenna’s supporting structure was a lambda tower matched to the station’s frequency, and the licensee and its engineer had performed “pattern optimization” studies prior to construction. So the C0 licensee “knew in advance” how the antenna would perform.

The Division’s understated conclusion: “[I]t is difficult to credit [the C0 licensee’s] position that its facility should be considered non-directional.”

Since the Class A is entitled to the level of protection from the C0 predicted based on the C0’s supposed omnidirectional antenna, the Division has ordered the C0 to explain why its license shouldn’t be modified to specify the de facto directional antenna (or, more precisely, the directional pattern) which it’s using – but with power reduced to afford the Class A protection. As indicated above, such a major league reduction would be bad news of the C0.

One take-home lesson from this order is that the FCC is prepared to help stations who believe that they are not getting the protection to which they are entitled as a result of another station’s “optimized” antenna.

Exactly how many such situations exist, however, is far from clear. If a station had been experiencing such interference, it would presumably already have thrown the flag at the Commission. As a practical matter, it’s possible, if not likely, that lots of aggressively “optimized” antennas are currently in operation – but, since they aren’t causing interference, there’s probably not much chance that the FCC will start raising any questions about them.

But the order provides another take-home message. It sends the unmistakable signal to anyone who might be thinking about installing an aggressively “optimized” antenna that care should be taken to avoid any interference to anyone as a result of the optimization.