The National Radio Systems Committee recently published the results of a study on AM band noise.
The report, principally authored by John Kean, is titled NRSC-R102, “Measurement of AM Band RF Noise Levels and Station Signal Attenuation.” It was released in January.
The study consisted of several measurement series conducted along roadways both in urban and rural areas, measuring the signal strength of three different Baltimore/Washington 50 kW AM stations as well as the RF noise on three different unoccupied AM band frequencies.
Measurements were made across five environments: rural, rural-suburban, suburban, urban and dense urban. The results were mostly as one would expect, but there were a few surprises.
One was that field strengths from the test stations were sometimes far below predicted values and that reception in what would normally be considered “quiet” areas was often noisy.
Another surprise, at least for me, was that suburban environments often produced some of the best, most noise-free reception. There were certainly noisy spots as you would expect, but strong field strengths from the stations generally produced good, noise-free listening.
Not surprising, power lines, traffic — possibly electric vehicles — and electrical equipment were identified as the main manmade noise producers.
That certainly tracks with my own experience. Overhead power lines, light rail catenaries and other power-carrying conductors along roadways have long been sources of hash in AM reception.
Also not surprising, heavily built-up areas tend to attenuate AM signals. Local noise sources within these areas, in combination with the reduced signal strengths of AM stations, often result in reduced signal-to-noise ratios, even making AM reception impossible in some situations. There is nothing new there.
Tweedles and chirps
The report concludes with several recommendations, including strategic transmitter placement, the use of single-frequency networks, improved car AM antenna efficiency and better receiver design.
I would agree with all these recommendations in principle, but some are not practical in many cases.
In recent years, our industry has seen a trend of AM transmitter sites being sold off and the stations moved to locations more distant from the city center. AM tower sites soak up a lot of real estate, and often urban growth has made those tower sites open-space islands in built-up areas. The dirt is worth much more than the radio station.
The result is, in many cases, lower signal strengths in the city, not greater. Stations that used a number of towers in a directional pattern that put a strong main lobe over the city often downgrade to a single tower with a much lower non-directional power level. The resulting reduction in signal levels is predictable.
One source of RF noise in the mobile listening environment not dealt with in the report — in fairness, it is well beyond the scope of the report — is on-board computers and electronics.
Long before electric vehicles were a thing, I began observing this in my travels. I would rent a car with what looked to be a good entertainment system aboard, but it would quickly become clear that reception of even strong AM signals was a challenge.
Two factors were apparently at work. One was the “shark fin” or other low-profile antenna used and the other was noise generated by the car’s electronics.
My wife’s car is a good example of this. Even our local 50 kW AM station that produces a lot of signal strength has all sorts of tweedles, bleeps and chirps audible in the background. Shut the car off and all those rhythmic noises go away.
In recent years, we have seen what is essentially the same thing with EVs. Motor control electronics, traction motors and onboard computers produce so much hash that some manufacturers have “solved” the problem by removing AM reception from the entertainment system altogether.
Some vehicles, and not necessarily EVs, are rolling noise generators that affect not only reception for their occupants but also affect nearby vehicles. Get stuck in traffic near one of those and you may find yourself switching to FM or some other source.
Clearly most all these things are beyond the control of the broadcaster.
So what is the solution to noise on the AM band? We’re talking about signal-to-noise ratio here, and if we can’t do anything about the noise, maybe we can do something about the signal.
Increasing power would improve the signal-to-noise ratio of any station at any location within the coverage area. AM power is limited in most cases by co- and adjacent-channel protections. I think it’s time that the FCC recognizes — even from the data in the NRSC report that shows even 1.5 mV/m field strengths to be problematic in rural environments — that the 0.5 mV/m protected contour is no longer valid. There’s just too much noise out there to make a half-millivolt signal comfortably listenable in most locations.
Raising the normally protected contour to a more reasonable value — say, 2 mV/m — would permit many stations a significant power increase, and with that would come improved signal-to-noise ratios and better listening experiences.
Would station owners make that investment if they could? Many would, but I suspect many more would not. Still, for those who are willing to make the investment to better serve their listeners, having that option would be a good thing.
(The NRSC’s Measurement of AM Band RF Noise Levels and Station Signal Attenuation report.)
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