# FM station that's slipped its moorings? **URL:** \ **Category:** Factual Questions\ **Created:** [October 15, 2000, 11:21pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049 "2000-10-15T23:21:43Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [October 15, 2000, 11:21pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/1 "2000-10-15T23:21:43Z")
While motoring around this afternoon I decided to listen to something different from my usual classical station. Just punchin’ my way through the spectrum with the seek button, I came upon a station I had not heard of before. It’s an oldies station whose ID jingle goes, “Golden oldies one-oh-seve-point-five!” Well, that made me curious, because they were at 97.1 on my dial, so I listened for a bit, and sure enough the DJ identified them as 107.5. I thought for a minute I might be getting some fluke reception from another city, but no, it turns out they’re a relatively new Houston station. So I checked the other stations, and they’re all where they’re supposed to be. And nothing was received at 107.5. I will note that reception of this station included a wee bit of static (unusual for FM). The situation was unchanged when I rode in the car again a couple of hours later. What’s happening here? Could some unusual atmospheric events be causing this (weather here today is beautiful; ~75°, nary a cloud in the sky)? Could these guys really be screwing up and broadcasting on the wrong frequency? Should I call’em and tell’em? ---
**Author:** ![ubermensch](https://avatars.discourse-cdn.com/v4/letter/u/bcef8e/32.png) [@ubermensch](https://boards.straightdope.com/u/ubermensch)\ **Post date:** [October 15, 2000, 11:54pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/2 "2000-10-15T23:54:03Z")
well, there’s a few things that could be happening. the only thing that needs to be said for the id to be legal is the station’s call letters and station of license. but they could probably still get into trouble with the fcc for broadcasting misleading statements. maybe they’re about to move frequencies. maybe it’s an automated station with the wrong legal ids stuck in the cart player, or computer maybe it’s a satellite station and the legal ids on your side aren’t firing at all. the static kinda leads me to believe a computer is involved. can’t say why, it just does. out of curiosity, i’d call em. make that a plan for tomorrow, would ya? ---
**Author:** ![LIONsob](https://avatars.discourse-cdn.com/v4/letter/l/76d3ee/32.png) [@LIONsob](https://boards.straightdope.com/u/LIONsob)\ **Post date:** [October 15, 2000, 11:54pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/3 "2000-10-15T23:54:43Z")
Hey Beatle 94.5 and 107.5 swaped channels about a month or so ago with a bunch of hype about how each station was going off the air. We also have a new \Classic 80’s rock\ station 106.9, it used to be a christian station before the change. Later Peace LIONsob ---
**Author:** ![Cabbage](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@Cabbage](https://boards.straightdope.com/u/Cabbage)\ **Post date:** [October 16, 2000, 6:08pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/4 "2000-10-16T18:08:20Z")
I’m surprised nobody’s mentioned radio “images”, which is what it sounds like to me. It happens to me all the time, where I’m from there’s a station I like to listen to at 96.3. Whenever I drive near the local station (101.7) it interferes with 96.3, and I can hear 101.7 through 96.3. I don’t know enough about radio images to really give an explanation, but I found this, and I’m sure someone here will be able to give you a better idea what cause it: > [@](#): > > Aside from noise generated within the locale of the antenna, most other types of interference are easily dealt with by the characteristics inherent in a competent FM tuner design. The problem is that interference can make itself known at different points within the tuner’s circuitry. > > The circuit’s ability to deal with interference from other station signals lies mainly in its ‘selectivity’. Some tuners may not have this quality in abundance as it could detract from the sensitivity of lesser capable ‘front end’ designs. Without the necessary selectivity, the rf stages in this design can ‘saturate’ with signal when an antenna with more gain than a dipole is used. The symptons of such a problem appear as a higher-than-usual noise level and strong, local stations presenting their unwelcome ‘images’ at more than one point on the dial. [http://www.fanfare.com/rfms-bk.html](http://www.fanfare.com/rfms-bk.html) ---
**Author:** ![DougC](https://avatars.discourse-cdn.com/v4/letter/d/7feea3/32.png) [@DougC](https://boards.straightdope.com/u/DougC)\ **Post date:** [October 16, 2000, 7:10pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/5 "2000-10-16T19:10:24Z")
- - - A couple stations in St Louis recently changed formats, as have a couple DJ’s. - The reason you heard the station at the wrong place on the dial was because if you have a commercial license, you have to transmit something. It is required. Obviously there’s not a lot you can do about technical breakdowns, but you can’t just sign off for two weeks and send all the employees on vacation. If a station is undergoing a large staff restructuring, (by some agreement) they temporarily “borrow” another existing stations’ broadcast. - In St Louis, there’s a classic rock station at 94.7. Up until a few days ago, there _used_ to be another at 97.1, but 97.1 has decided to switch formats. Their staff won’t be ready for a couple weeks, so they are transmitting 94.7’s signal for that long. 94.7’s identification says that it is being broadcast on both frequencies until the other goes to the talk format. ~ - Radio “images” are something else entirely: when I was in San Antonio, I could only get four FM stations across the whole dial: two Mexican, one Christian/gospel and one country. The Mexican stations use ancient transmitting equipment that sends “ghosts”: 93.7: you’d hear it here at 40% volume, 60% static 94.3: you’d hear it here at 55% volume, 45% static 94.9: you’d hear it here at 70% volume, 30% static 95.4: you’d hear it here at 85% volume, 15% static 96.1: This was where the Mexican station was actually “transmitting” at- 96.9: you’d hear it here at 85% volume, 15% static 97.5: you’d hear it here at 70% volume, 30% static 98.3: you’d hear it here at 55% volume, 45% static 99.1: you’d hear it here at 40% volume, 60% static ! Three Mexican stations just about cover the whole dial. - MC ---
**Author:** ![BobT](https://avatars.discourse-cdn.com/v4/letter/b/8e8cbc/32.png) [@BobT](https://boards.straightdope.com/u/BobT)\ **Post date:** [October 16, 2000, 7:20pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/6 "2000-10-16T19:20:31Z")
My AM reception in LA has been bouncing around, but I think it may be a problem with the tuner. One preset station will jump to another preset station for a few seconds and then go back. ---
**Author:** ![TheNerd](https://avatars.discourse-cdn.com/v4/letter/t/858c86/32.png) [@TheNerd](https://boards.straightdope.com/u/TheNerd)\ **Post date:** [October 16, 2000, 9:47pm UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/7 "2000-10-16T21:47:22Z")
Like Lion said, 94.5 and 107.5 swapped a while back. Before the swap 107.5 was the Buzz, and broadcasted on both 97.1 and 107.5. I believe the 97.1 tower was the “cleveland-Houston” (Don’t ask me, I’m just remembering the commercials) broadcast, and the 107.5 broadcast was from Conroe. So it’s not some weird slippage of frequency, they really were broadcasting on 97.1. 106.9 hasn’t been good for anything since it stopped being ZRock. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [October 17, 2000, 12:06am UTC](https://boards.straightdope.com/t/fm-station-thats-slipped-its-moorings/37049/8 "2000-10-17T00:06:47Z")
It’s not an image frequency problem unless you’re listening on a heathkit. Modern tuners have become very good at filtering out the image frequencies. I don’t mean to belittle heathkit (are they still in business?), it just took me back to my days teaching RF theory & tuner design. Half way through the course all the students had to build tuners on breadboards. Due to the cheesy nature of the kit & lack of shielding, image filtering was a big problem. For the record, any frequency (besides the desired signal) that beats together with the receiver’s internal oscillator to produce a 10.7MHz (for FM) sum, difference, or multiple of the sum or difference signal is an image. In your example, you were tuned to 97.1MHz. The nearest image frequencies would be: 97.1MHz + 10.7MHz = 107.8MHz 97.1MHz - 10.7MHz = 98.4MHz[sup]\*[/sup] Therefore, any strong signals at these frequencies will result in an image being generated in your tuner. 107.8 is close to 107.5 (the station you imagined you were hearing) but I don’t think it’s close enough. FM boadcasts have a bandwidth of 200KHz (.2MHz) with a dead space “no man’s land” guardband of 25KHz on each side (straining to remember that number- I haven’t taught this crap in years). The image of 107.7MHz is the next available broadcast slot, and _that_ is the station you should have heard, if any. Your tuner would have to be in really bad shape not to be able to reject an image that far away. Having seen many many bizarre “that’s theoretically impossible!” problems with tuners as well as many other consumer devices, all I’m willing to say is that it was _probably_ not an image you heard. I’m leaning more towards **Lion’s** answer. [sub][sup]\*[/sup]And so on using harmonics of 10.7MHz[/sub]