Alexander: Bring Consistency to AM Measurements

NRSC is likely to be one of the lesser problems an AM could have
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Cris Alexander, CPBE, AMD, DRB, is director of engineering for Crawford Broadcasting as well as technical editor of Radio World Engineering Extra. He comments here in his Crawford role.

I have read with great interest the opinions of some of my colleagues regarding the usefulness of annual occupied bandwidth measurements for AM stations. Certainly some engineers earn a significant portion of their income from making such measurements, so I understand that a little bias in the direction of keeping such measurements might exist on the part of some with that underlying motivation.

[Read: Should Required NRSC Measurements for AM Stations Continue?]

From a licensee’s perspective, however, I see very little value in making annual occupied bandwidth measurements. These measurements are a burden to licensees, and in my experience, they seldom reveal any issues. Indeed, the noise floor is so high in many areas that in some cases it is impossible to resolve emissions beyond 75 kHz removed from carrier that might otherwise be above the 80 dB or 43+10log(P) limit.

There are many transmission parameter limits imposed on broadcast stations, both AM and FM, for which there are no statutory measurement intervals. What comes immediately to mind is monitoring points for conventionally-proofed AM directional stations. Field intensity limits are set for monitoring points, and licensees are required to maintain fields below those limits, but other than “as often as necessary to ensure…,” stations with approved sampling systems have no requirement to measure monitoring points at any particular interval. Arguably, there is a much greater likelihood of interference to a neighbor resulting from a misadjusted or malfunctioning directional antenna than a transmitter producing out-of-band emissions. As such, wouldn’t we be well served by taking a similar approach to AM occupied bandwidth measurements?

It would seem to me that we would all be better served if the rules were consistent and the AM emission requirements in §73.44 mirrored the requirements for FM stations in §73.317. If a problem exists and a spectrum neighbor complains, measurements would then be made to determine whether the AM station is indeed the source of the problem, and then the underlying emission limitations of §73.44 would compel the AM licensee to bring his emissions into compliance. I believe such an arrangement would work very well, and it would save broadcast licensees the often substantial costs, time and trouble associated with compulsory annual measurements.

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