Public Radio Frequencies, Repeaters, and Translators: What's the reasoning?

My local Public Radio affiliate is Yellowstone Public Radio:

But it doesn’t end there, as the link above shows. Through a few repeaters and a metric buttload of translators, YPR manages to cover Montana and northern Wyoming with the same content on slightly different frequencies: Most of the stations operate at less than 100 MHz, a substantial portion operating at less than 90 MHz.

All of the foregoing raises interesting questions in my mind. To wit:
[li]Why can’t they all be on the same frequency? KEMC, the mother station of the YPR brood, operates at 91.7. What would be wrong with all of the repeaters and translators operating at that frequency?[/li][li]Why are the frequencies clustered pretty tightly around the 88-100 MHz range? There are a few outliers, but if you keep your FM dial between 88.5 and 100 in the state of Montana there’s a good chance you’re listening to YPR.[/li][li]How does KEMC get its content to all of the various stations it serves? I can think of a few methods, but I’d like some straight dope.[/li][li]What’s the difference between a repeater and a translator? I get the feeling a repeater is manned and a translator is not, but does this mean repeaters add their own content?[/li][/ul]This should be an interesting thread to read while I’m listening to YPR. :slight_smile:

Just one bump, before this slides into complete obscurity. I’m sure someone knows something about this.

I’ll bump this again. It’s an interesting question, and we just need the right person to come along an answer…

So many questions. Let’s see what we can do.

The FM frequenices 88-92 are reserved solely for non-commercial stations, like NPR affiliates. Way out in sparsely populated areas, where there aren’t that many commercial broadcasters, non-commercial stations may be licensed across the commercial portion of the spectrum, rather than all being bunched up at one end of the dial. That’s why you’ll see many, but not all of those stations at 91.9 or below.

The reason they don’t put all the stations on the same frequency is the same reason they don’t put any nearby station on the same frequency – interference. In fact, the FCC has engineering guidelines of how far apart stations on the same frequency should be located, based on antenna height, transmission power and a lot of other stuff. Even a weak signal can cause interference, especially at the far ends of a station’s coverage area.

There are four basic ways to get the programming from one station to another:
Actually picking up the signal over the air and rebroadcasting it.
High-quality telephone lines
Microwave transmissions
Satellite. In fact, here the story specifically notes that many Montana stations are fed by satellite

I’m finding differing definitions of the difference between “repeater” and “translator.” As near as I can tell, a repeater station can only rebroadcast a signal, while a translator also rebroadcasts, but is capable of transmitting something from another source.

I know a bit about radio history, but I only have approximate answers.

The FCC authorized the creation of low-powered non-commercial FM stations in 1948. They were limited to the lower end of the FM spectrum. I’m not sure if this was for technical reasons, but the effect was to create a ghetto for educational, college, and religious broadcasters who couldn’t afford more powerful equipment. NPR and other non-commerical stations who can afford the good stuff can place it anywhere on the dial.

Transmitters can create interference and dead spots if they’re on the same frequency. Putting them at different frequencies avoids problems where the signals overlap.

I think signals are just sent out line-of-sight, and then captured and retransmitted. I’m not sure about the distinction between repeaters and translators but note that all the repeaters have regular radio call letters while all the translators have a different style of call letters I associate with ham radio and experimental stations. That would imply some fundamental difference, probably of the sort the OP posits.

Wow, after several bumps a simulpost on preview. Looks like I’m mostly correct.

Just a guess, but I would guess that at repeater just retransmits a signal on the same frequency to extend range. A translator, on the other hand, would retransmit a signal on a different frequency to extend range but also to reduce interference.

kunilou, Exapno Mapcase: Very good! I knew my fellow Dopers would fight this ignorance.

Satellite isn’t a method of distribution I would have seriously considered in this five-and-dime state, but looking at it satellite makes more sense than microwave relays or even straightforward rebroadcasting assuming the birds are already in the sky and accessing them isn’t an issue. It’s indeed very interesting.

I didn’t think of this, obviously. I suppose destructive interference (if that’s what you’re talking about) would indeed occur in that scenario.

The index of my copy of the ARRL handbook* was oddly silent on the subject of repeaters and translators, so I did some googling.

The two seem to be fairly close.

Translators receive a program on one frequency and transmit the same program on another. Repeaters were mentioned as doing the same thing, but were also mentioned in ham-radio contexts, where a repeater would be a fixed station that could pick up a weak signal from a mobile two-way radio and retransmit it with greater power… I’m not sure about the frequency in this case.

[sub]*The ARRL Handbook is the bible of amateur radio in the US and Canada. Unfortunately my copy is a yard-sale find from 1961.[/sub]

Some television stations have UHF repeaters as well, but I couldn’t lay my hands on a cite.

The NPR station where I work broadcasts all over the Florida Panhandle with translators on different frequencies, other than the three main ones. They are placed strategically in areas on the fringe of one signal, so that you can pick it up on one frequency or the other. The reason for the different frequencies is that you have to take what is available in your area. One of our translator frequencies is 106.1. You have to transmit at a certain power level and in a certain pattern that will not interfere with any other station on that frequency or its adjacent on either side.

Montana Public Radio, which serves western Montana, has 12 transmitters. Only 2 are on the same frequency, the stations for Kalispell and Great Falls, which are 242 miles apart.