How can the same FM station appear at two different spots on the dial?

How can the same FM station appear at two different spots on the dial?

To the person who complained recently that I was off topic, I'm sorry. This is not about any repair it would be feasible to make. It's only about electronics.

Where I live there are two FM radio stations, 88.1 which is only a few miles away, and 88.5 which is 40 or 50 miles away.

Right now, only my expensive KLM radio plus any car radio gets the second one well, but I've had some cheap radios that do almost as well.

I lose track of which radios those are, so I'll start tuning at 88.1 and tune up very gradually. After a period of silence, when I get above

88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again.

How is that happening? I know about harmonics, but that doesn't apply, does it?

P.S. This means 88.5 doesn't come in at all. I've tried stretching out the power cord, which on the cheap radios is usually the antenna. Sometimes that helps but on most of the radios, 88.5 won't come in at all.

P.P.S. 88.1 is WYPR Baltimore. 88.5 is WAMU in DC. Sometimes they play the same thing, like during the top of the hour news, Diane Rehm, etc. although WAMU is on a 5 or 10 second delay most of the time. Because the topic and the voices can be the same it means I can't tell for a while if I've gotten 88.5 or just another 'instance' of 88.1.

Reply to
micky
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My initial thought was that perhaps the station is operating a "translator" (repeater) on 88.7-ish, but a search of the FCC database doesn't turn up anything. There are two translators licensed to Maryland on 88.7 MHz: W204BA in Oakland and W204CL in Lexington Park. Both belong to Grace Missionary Church (d/b/a Grace Christian School). Some Googling shows those affiliated with a small religious radio network, but it's possible they could be re-transmitting 88.1 for some reason. Both transmitters are fairly low power, as is typical of translators (250 and

55 watts, respectively), and given the distance (2-3 hours away) I doubt there would be much overlap in coverage area, if any.

So... that possibility fairly well eliminated, I think the best bet is to zip off an e-mail to the station and ask what's going on. Looks like those are public radio stations, and my experience has been that the engineers at those types of facilities are typically pretty helpful when it comes to resolving reception concerns and addressing technical questions. If you do that, I'd be curious what you dig up.

Since you are able to reproduce the behavior on multiple radios, I doubt it's a problem with the receivers.

Reply to
Steve Crow

Wild Ass Guess here; you are hearing a Translator station rebroadcasting the main station's programming. These mini-stations fill in nulls or shadows. check fccinfo.com

Reply to
dave

Here's a list of FM licenses. Translators have a 3 digit number in the callsign. If it's a flaky receiver you may be interfering with aircraft.

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Reply to
dave

There are at least three ways in which you can end up with a strong FM station at two locations on the dial.

(1) As somebody else suggested, it might be a "translator" - a second transmitter carrying the same program material on a different channel. Translators are sometimes used to "fill in" a station's service footprint - e.g. to provide service to an area on the far side of a mountain from the primary transmitter.

(2) Image. FM receivers are almost always superheterodyne receiver... they have a local oscillator which is tuned either above, or below, the station's frequency by a fixed amount (most commonly 10.7 MHz). "Mixing" of the station frequency and the local oscillator create an "intermediate frequency" signal at (e.g.) 10.7 MHz which is then filtered, amplified, and decoded.

This architecture can cause a station to "reappear" on the dial, if you're tuned away from it by twice the intermediate frequency (e.g. by 21.4 MHz) - a second "image" of the station appears on the dial. Good FM receivers have enough selectivity built into their "front end" to keep this problem to a minimum - the tuner "filters out" the station at the image frequency efficiently enough, before mixing with the local oscillator, to keep it from "reappearing" or interfering with a desired station (image rejection is often 90-100 decibels, if I recall correctly).

(3) Intermodulation. If you have two strong stations nearby, their signals can mix (either in the receiver front end, or elsewhere) and create "spurious" signals located on either side of their true locations on the dial. These spur signals will often be noisy and distorted.

What you're describing doesn't sound like an image problem (#2) because the second "copy" of 88.1 is so close to it on the dial. It might be intermodulation, or the 88.1 station may have a translator off in the distance.

Due to recent consolidation of radio-station ownership (both commercial service and "noncommercial" FM), the signal at 88.7/88.8 might be a formerly-independent station in another market, which has been "bought up" by the ownership of 88.1 and is now simply rebroadcasting its signal.

Reply to
David Platt

This sounds very likely; if it is due to front-end nonlinearity, it's possible to test/treat it by inserting an attenuator between the FM antenna and the receiver (assuming the receiver has a plug-in antenna). Lower the signal level, and the spurious response should go away.

Alternately, one can attenuate (filter) either the interfering FM station or the (presumably AM) difference-frequency station: this can be done with a lossy antenna+load placed near your radio, so can apply without access to antenna terminals.

Reply to
whit3rd

** Not really possible since the FM band is only 20MHz wide.

For a low side local osc:

88.1-10.7 = 77.4 = lowest local osc f 77.4+20.0 = 97.4 = highest local osc f 97.4+10.7 = 108.1 = higher f than any station.

** Plus contain the audio modulation of both signals.

Be a real pain if the two FM carriers differed by 10.7MHz ...

FYI:

The 2nd harmonic of strong carriers can intermod with the 2nd harmonic of the local osc to produce a new signal on the dial.

In this case, the FM deviation is doubled so may be distorted by the detector.

... Phil

Reply to
Phil Allison

I used to work on Radio Row in Houston. One day the FCC came to visit KILT FM 100.1 because they were causing squeals on the aeronautical band. It wasn't any of the station's pro gear making the interference; it was an old console FM receiver in the station lobby. 100.1 + 21.4 =

121.5. Radio row was on the direct approach to Hobby Airport or this old mis-aligned radio would have never been busted.
Reply to
dave

I thought that was some of the basis of the ban on electronic devices on airplanes.

Certainly there is folklore that when AM/FM transistor portables became cheap and available, suddenly people were using them on airplanes, and that did or could have caused interference, precisely because the local oscillator radiated and in the aircraft band.

It's murky whether that was the specific cause of the rule or not, and probably made murkier since it's been forty years since I read about this.

Michael

Reply to
Michael Black

If you have the AFC on, the station will pop up at different dial locations depending on which direction you are tuning. At least my old portable does.

Paul

Reply to
Paul Drahn

I was thinking along that line, except thinking of pointing out that for whatever reasons, not great selectivity or a noisy synthesizer, a station can be heard on more than one frequency. But, I can't recall that happening when there's an adjacent station, then the first station being received further up.

If that second station wasn't there, AFC is a good suggestion, and something we might not think of much anymore, with so many fm receivers digitally tuned. But I'd think it would "lock" to the statino further up, that presumably is stronger at that point than the first station.

Michael

Reply to
Michael Black

I think I've noticed this too.

But the AFC wasn't on, because that would have made it almost impossible to get a weak station like 88.5.

Well, I'm calling it weak because most radios won't get it, but Wikip says that it's 50,000 watts ERF (sp?) but 88.1 is only 15,500 watts. (also ERF? It didnt' say.) So maybe I'm calling it weak because it farther away, in DC, not Baltimore where I live, but actually, there are places north of here, farther from DC, the Westminster, Md. area, where

88.5 comes in well and 88.1 barely comes in. A friend moved to Finksberg and she had to change to 88.5.

But maybe the FCC makes them arrange their antennas so that in the city of Baltimore and its populous suburbs, 88.5 doesn't overpower 88.1. But the frequencies are different, and there's no Baltimore 88.5, so why would 88.5's antennas have to avoid the populous part of Baltimore, or any part?

Reply to
micky

In message , Phil Allison writes

I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal.

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Ian
Reply to
Ian Jackson

I recently had a station on 107.5 also have a signal at around 87.xx, don't know exactly I was using an analog radio. I almost called the station, but waited until the next day and the lower frequency signal was gone. I know it was just a single day event because I listen daily to a transmission at 87.5Mhz.

Mikek

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Reply to
amdx

I had a local situation at 94.5MHz interfering with 94.3MHz. The

94.3MHz station is an out of town station and signal strength is weaker. The interference was on all my radios. I called the Radio station engineer and he suggested the engineer from the out of town station probably put me up to making the call, this was not true. From the conversation, I think he had got a lot of calls about the interference, but he assured my the station was in compliance with FCC Reg's. It was Hip Hop vs O'Reilly back then. It went on that way for years until the station changed from Hip Hop to some other format, then the interference went away. Today I can't even find a semi local 94.3 MHz station.

Mikek

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Reply to
amdx

Precisely If it were lower, you'd greatly increase the possibility of images.

Reply to
William Sommerwerck

In message , William Sommerwerck writes

Even with the local oscillator above the station frequency, if you're near an airport, you can get the air traffic control traffic (120MHz +/- quite a lot) breaking through - especially if the planes are passing more-or-less overhead. My kitchen radio gets hit when it's tuned to

97.3MHz, by out-bound flights which have just taken off from London Heathrow, on around 118.7MHz. But, of course, it all depends on the 'front end' selectivity of the radio. However, this doesn't explain the OP's problem - which indeed does sound as if it's simply that the same program being carried by more than one transmitter.
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Ian
Reply to
Ian Jackson

Isn't it more precisely, that by putting the LO higher, the image falls where fewer strong signals are?

You don't want images to be below the FM broadcast band, then you end up with TV stations 2 through 6. But above the FM broadcast band, you get a decent stretch of aero band, amateur radio, public service, weather. Channel 7 doesn't start until somewhere above all that.

Michael

Reply to
Michael Black

That would depend on band allocations and transmitter power. I'm thinking of images from within the FM band.

Given that the FM band is 20MHz, and twice 10.7 MHz is greater than 20MHz, if the LO is above the incoming signal, images would come from stations above

107.9MHz (outside the band). If the LO were below the incoming signal, you could have in-band images starting at 98.9MHz.
Reply to
William Sommerwerck

No. Taking the upper band edge as 108, 108 - 2x10.7 = 86.6 (well below the lower band edge). This is within old US TV channel 6 - and as you tune lower, you will hit channel 5.

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Ian
Reply to
Ian Jackson

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