The Commission has given the thumbs-up to the use of moment method computer modeling for directional AM proofs in many, but not all, circumstances. This should ease one of the most time-consuming and expensive burdens traditionally borne by AM licensees.

Directional AM antenna systems are designed to generate carefully sculpted patterns of radiation – with the signal intentionally suppressed in one or more directions in order to prevent interference. Normally, these systems are designed on paper, using that voodoo that consulting engineers do so well. But the FCC, not to mention other licensees, expects the signal produced when the design is actually constructed will conform to the theoretical design. Accordingly, the Commission has historically required directional AM licensees to submit “proofs” of their antenna patterns, proofs obtained by the taking of a boatload of measurements at a boatload of points along several bearings at varying distances from the transmitter. It is an understatement to say that doing proofs is cumbersome – or, as the FCC describes it, “daunting, slow, and expensive.”

As computer programming has developed, however, various alternatives to the hand-crank proof method have evolved in the form of programs known generically as “moment method” programs. These verify antenna performance based on measurements of the current and phase measured at specific locations on the antenna elements. While there is some debate in engineering circles as to whether moment method programs are a valid substitute for field measurements, the Commission decided to accept proofs generated by the moment method, subject to a number of caveats.

First, stations relying on moment method proofs will still have to take and submit a limited number of reference field strength measurements at three to eight locations. These values, together with descriptions of the measurement locations, will have to be included with the proof in the license application (Form 302). Second, the proof will have to contain a complete description of the station’s antenna sampling system. As the FCC sees it, the accuracy of the moment method hinges to a substantial degree on the accuracy of the sampling system, so the Commission wants to be able to confirm that the sampling system is up to snuff. Third, only systems that are series-fed may be verified by the computer method.

AM signal prediction is, and always has been, a pretty iffy proposition. As the Commission itself acknowledges, any station’s actual service area is affected by a wide variety of often-unpredictable factors, such as short- and long-term variations in antenna performance, seasonal changes in ground conductivities, and even sunspot activity (to name a few). As a result, even the seemingly unarguable results obtained from field measurements provide, at best, only a freeze-frame, instantaneous glimpse of how the antenna happened to be behaving at that particular point in time. While the moment method may, once it gets rolled out and tested in practice, turn out to be less than perfect, we should remember that that is probably not much worse than the margin of error we’re already dealing with.

AM licensees who have proofs due to be filed in the near future should check with their consulting engineers to see if the moment method may be an alternative. Technically, the new rules permitting the moment method will not be effective until OMB has approved them. However, it is at least possible that the FCC staff may be open to waivers pending OMB approval. Again, check with your consulting engineer.