A current Radio World ebook explores what to know about FM translators. This is an excerpt.
Bob Clinton is a senior engineer with Cavell Mertz & Associates, where he has worked in RF transmission, computer networking, telephone systems, WiFi, audio and video systems and drone measurement work.
Radio World: How has the role of translators evolved?
Bob Clinton: Historically the idea of a fill-in translator has been to improve coverage in specific areas where a station’s signal tends to be weak or terrain-limited.
In the 1980s and ’90s, in the eastern part of the country, translators were primarily noncommercial educators looking to extend coverage outside of the normal 60 dBu contour, while in the mountainous west, they were used to solve terrain problems, because even if you put up a big 50 or 100 kW station, the terrain there would kill you.
Then AM revitalization came along.
RW: It expanded the number of translators significantly. And we saw the impact of HD Radio, where a translator could rebroadcast an HD multi-channel, carrying that as a fresh signal on a different frequency.
Clinton: Yes. Sometimes these days it even feels like some broadcasters want a translator even if they don’t really have a business need for it. Interest has been almost on the order of a fad that many get into for fear of missing out.
[Related: “FM Translators Remain a Popular Tool”]
RW: We’ve got an awful lot of them on the FM band now. What kind of projects are you active in?
Clinton: Most of my recent work has been on the consulting side, where we help people apply for or modify the translators. New translators require a filing window, which the FCC will announce only in a fairly long while. There is no schedule, and it can take several years for them to propose another round. Anyone interested in a translator right now would have to find an existing one to repurpose.
A lot of translators for AM stations are not allowed to be sold unless you take the AM with you; the translator has to stay with the parent station. But there are plenty of translators that can be sold or change ownership.
Much of our work lately has been with fill-in translators, where a station wants a stronger signal in a community.
If you’re going to purchase a translator and move it, the change must qualify under the minor modification rules. Minor mods can be done essentially at any time, unless a filing freeze has been instituted.
It could be a channel change to a first-, second- or third-adjacent channel, or to the IF (intermediate frequency), which is 53 or 54 channels away. Or it could be a physical move of a short distance. The move normally requires that you have contour overlap — whatever you propose must have a certain amount of 60 dBu contour overlap with the existing license. It’s not difficult to demonstrate this; but it depends on how big your license is. If it’s for five watts, you’re not going to be able to move very far with the new one, whatever you propose.
But a neat thing about translators is that if you can demonstrate that you’re receiving a significant amount of interference from an existing facility, you can change to pretty much any channel in your same band, whether you’re in the educator band or the commercial band.
RW: Do many projects involve the sale of a translator to an owner who wants to rebroadcast a different station?
Clinton: Some, but most already have a translator and are unhappy with the coverage.
But keep in mind that a translator is a translator; it’s usually not going to deliver anywhere near as much coverage as a Class A station.
Some clients are just getting clobbered by incoming interference, and we can make a channel change to improve that; but there are parts of the country, like North Carolina, California and parts of Texas, that are so densely populated with FM stations and translators that finding an alternate channel is almost impossible.
RW: What is the process once they come to you?
Clinton: We do a preclusion study, which looks at all the channels that they can potentially move to. The next step is a contour study to see how close other nearby contours are to your location. Based on that, we can run incoming interference studies on any promising channel.
Back when everything was manual, it took a day to run a study to predict interference. Now we can run one in five minutes.
RW: Are there misconceptions out there about translators?
Clinton: We mentioned expectations about coverage. Take a 1 kW AM station, for example. Everyone knows AM has challenges with interference and noise. The AM might think, “Oh we’re going to get an FM translator. We’re going to be on the FM dial. Now we can get all kinds of good, quality coverage.” Unfortunately even a 250-watt translator is not going to have the coverage of even a 1 kW AM, so they’re disappointed.
The best solution is to locate the translator in the community that they really want to reach and optimize the population.
Be aware that while the rules permit a maximum of 250 watts for a translator, there’s no height limit. You can have 250 watts at a crazy height above average terrain and actually exceed the coverage of a typical Class A, which is limited to 6 kW at 100 meters, and above that height you have to reduce power to stay within your contour distance.
So you’ll see exceptions where a translator is on the top of a mountain with 250 watts and they cover an entire valley. It’s just amazing.
But not everybody can do that.
RW: Are there other particular considerations to be aware of?
Clinton: Something to note about non-fill-in translators is the ERP limit. Where fill-in translators are limited to 250 watts as long as they fit inside the parent station’s 60 dBu contour, a non-fill-in translator is limited generally to something lower. The rules use a formula we refer to as the MERP, which is the ERP based on the maximum height above average terrain of 12 radials from the transmitter site. There’s a lookup table that provides the ERP limit, based on locations east or west of the Mississippi. As a rule, translators west of the Mississippi can operate with more power than those east of the Mississippi.
There can be other special conditions at times. For example, translators are required to protect co-channel and first-, second- and third-adjacent channels. Sometimes you can actually locate a translator inside of the protected contour of a second- or third-adjacent, which means that you need to reduce power or have a special antenna to demonstrate protection to that second- or third-adjacent channel. So we might specify a type of antenna to minimize the downward radiation at ground level near the transmitter site — perhaps a three- or four-bay antenna with three-quarters-wave spacing — so the population will not be impacted by the interfering contour. And the construction permit typically will then list it as a special condition to avoid interference.
Also there are rules that permit someone who is not the owner of a parent station to rebroadcast that station in another part of the country; this is often used to cover small communities, typically with religious broadcasting or educational stations such as NPR affiliates.
Another thing to be aware of is that in 2018 the rules changed to allow existing stations to file complaints against a translator inside of their 45 dBu contour, which is much larger than a 60 or 54 dBu contour, depending on your class. Class Bs are protected to their 54 while a B1 is protected to their 57 dBu contour. Opening that up to all the way to the 45 dBu contour means that the co-channel interfering contour would be as low as 25 dBu, which is enormous.
The existing facility has to demonstrate that they’ve had listeners who want to still hear the station and are willing to assist by submitting a complaint in the complaint process. Then you need a certain number of listeners in that interference area who say they want the translator to go away; then the translator would have to change channels or move, to clear that interference.
RW: Are there places where translators play a particularly important role? I’m envisioning very big areas like Alaska.
Clinton: Alaska is unusual. There are a number of translators there with special protections, because population is clustered in some locations, but elsewhere there’s nobody for miles, so it would almost be a waste of effort and money to build a 50 kW station. The FCC has allowed a number of Class D stations there that essentially have the same protections as a full-service station.
RW: What else should we be thinking about?
Clinton: I’m surprised more people haven’t looked into putting HD Radio on translators. I know of one translator that has filed a digital notification with the FCC. Perhaps that could be a future use of translators, to get HD Radio into areas where the main HD signal is having issues. The cost of adding HD to a translator obviously is a drawback at this point.
RW: How do boosters relate to the translator discussion?
Clinton: Boosters are on-channel with the main facility, by definition. They’re not limited to 250 watts, but if I’m correct, their 60 dBu contour cannot exceed the 60 dBu contour of the parent station. I believe the 60 dBu contour limitation is true even of Class B stations.
So you can have a significant amount of power on a booster, but if it’s covering an area inside of the protected contour of the main station, you generally will need terrain blocking or you’ll cause interference to yourself on channel.
RW: New rules allow boosters to originate programming for “geotargeting” purposes. This is mostly a topic for another day, but have you had any experience with the GeoBroadcast Solutions ZoneCasting system, which uses synchronized on-channel boosters?
Clinton: No. I’ve just run computer interference studies. From that, my sense is that without terrain blocking, you still will have significant areas of self-interference, even with synchronization. This may limit how many of those systems they can build — unless you have a fairly significant community that you can cover with a booster and there’s no population near it for a number of miles, which means you minimize the size of any population harmed by the interference.
RW: What else should we be aware of?
Clinton: We mentioned that sometimes a translator needs to change channel because they have too much incoming interference. Interestingly, translators are permitted to ignore incoming interference when they file an application. It’s kind of weird, but it lets them wedge themselves into places that a full-service station would not be allowed.
A minimum Class A is still 100 watts, which is pretty small, yet a 100-watt translator would be allowed to fit into places where 100-watt Class A would not because it’s allowed to ignore incoming interference.
So some broadcasters have thought, “Great, I can ignore incoming interference, I’ll pick this channel and use it here.” But then they discover that the incoming interference actually is a significant issue and they ultimately have to change channels. Especially when it’s co-channel. There’s nothing you can do to fix it, you can’t add a notch filter to the receiver.
That’s one of the “gotchas” about translators.
