Interference in the form of RF noise has been associated with AM broadcasting since it came into being more than 100 years ago. One early engineer remarked that static noise “is like the poor; it will always be with us.”
The industry managed to overcome the effects of natural and manmade noise for decades. Unfortunately, manmade RF noise has been a source of increasing concern, causing some to write off AM as a lost cause. Others take a more hopeful view on AM’s viability; one of these is consulting engineer John Kean.
His research suggests that good AM radio reception is still possible in large metropolitan areas, and he has been investigating common sources of noise that may drive listeners away. The project has culminated in a series of webinars produced under the auspices of the Society of Broadcast Engineers.
Radio World: What prompted these webinars? Are they a followup to your National Radio Systems Committee study involving AM reception in urban areas?
John Kean: They are connected, because the NRSC’s working group views RF noise in the medium-wave part of the spectrum to be one of the biggest technical problems for AM radio by far. This continues to drive listeners away. The content may be something they want to hear, but they don’t want to listen to the noise. Younger listeners may not even consider trying AM simply because of its reputation for low quality.
I was asked to chair the NRSC’s document working group in 2015, and there were some documents dealing with AM that needed updating. This got me interested in working with the AM improvement group, where I heard a lot of discussions about noise. This prompted me to do some studies. I discovered that AM noise wasn’t that bad on the highways. I also discovered that urban clutter in built-up areas can greatly affect signal strength.
RW: How bad is interference in home listening environments now?
Kean: I found in my own home situation that it takes a signal of at least 25 millivolts per meter to overcome the noise. This is a really large signal compared to the 2 mV/m figure used for regulatory allocations and station marketing purposes.
Most of this is attributable to switching power supplies. They’re compact and cheap, and for years the FCC has allowed these to be sold and distributed in the U.S. It’s clear that many of these devices are not FCC Part 15-compliant. While broadcasters may have been aware of this, there’s been very little done to enforce Part 15. Switching frequencies are usually below 150 kHz, but with harmonics sometimes extending across the band and up into the HF spectrum.
RW: What key points do you cover in the webinars?
Kean: In the first session, I address how noise comes about. Although we might assume that this is something everyone is familiar with, there are some engineers who may be new to the industry, particularly who came from other areas such as computers or audio, where RF noise isn’t really a big concern.
I introduce participants to potential noise sources and the concept of unintentional radiators. I differentiate between noise that occurs on roadways and how it can be different from that occurring in indoor environments.
I’ll discuss noise from switching supplies in detail, including how it is conducted, and how to locate and measure it. And I’ll be addressing such things as “self-interference,” which may also be new to those who have never worked in a situation where studios and transmitters are co-located. I’ll discuss techniques that are helpful in keeping transmitter RF out of analog and digital lines and equipment.
RW: How deep a dive into noise abatement will this be?
Kean: I’m planning to discuss shielding techniques for blocking unwanted signals from studios and how to construct your own interference filters.
RW: What are some of the other things on the webinar agenda?
Kean: I’ll also look at interference to AM stations from adjacent-channel broadcasters. The 10-kHz spacing we use is a century-old norm, but it results in “splatter” to your next-door neighbors. Over the years, changes in program content and audio processing have exacerbated this problem. I think that even with a minor change in occupied bandwidth, broadcasters can substantially reduce mutual interference and even improve their own audio.
Another thing I plan to talk about is the degraded nighttime coverage of stations due to the increased number of stations on the air and reduced separation between them.
RW: Is there anything new that may be a concern and that we should be aware of?
Kean: There is another relatively new noise source that could affect both indoor and outdoor or mobile AM reception. This is the technology that’s being developed for wirelessly charging electric vehicles. These are very powerful RF interference sources — 10 kW power supplies in one’s garage. This is something that broadcasters really need to get the FCC to focus on. We have to make sure that emission levels are limited.
I still believe that AM is viable. It has the ability to reach millions of listeners, but it’s up to the AM broadcasters to protect themselves from harm. They have to convince the FCC to protect them from this new source of interference.
RW: How can readers access the webinars, and who else has been involved in helping?
Kean: I did the first in August and the second is scheduled for late October. The third will take place in November. Plans are to archive them. Information is available on the SBE website.
David Layer at the NAB has been supportive and helped quite a bit in preparing material for some of my earlier presentations. Gary Cavell of Cavell, Mertz & Associates is joining me in the webinars and has been helpful in presenting some of the material and providing greater insight.
RW: Are you involved in any other initiatives for improving AM?
Kean: I’m working on a project for enhancing AM station coverage in urban areas through the use of on-channel repeaters or single-frequency networks. Stay tuned
