Home |
Search |
Today's Posts |
|
Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
Reply |
|
|
LinkBack | Thread Tools | Display Modes |
#1
Posted to sci.electronics.repair
|
|||
|
|||
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. |
#2
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On Mon, 27 Oct 2014, micky wrote:
How can the same FM station appear at two different spots on the dial? --snip-- 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? 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. |
#3
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On 10/27/2014 09:32 AM, micky wrote:
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. 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 |
#4
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On 10/27/2014 09:32 AM, micky wrote:
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. 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. http://fccinfo.com/CMDProFacLookup.p...lace=Baltimore |
#5
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
In article ,
micky wrote: 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? 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. |
#6
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote:
In article , micky wrote: How can the same FM station appear at two different spots on the dial? (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. 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. |
#7
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
David Platt wrote:
(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. ** 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. (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. ** 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 |
#8
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On 10/27/2014 01:48 PM, whit3rd wrote:
On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote: In article , micky wrote: How can the same FM station appear at two different spots on the dial? (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. 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. 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. |
#9
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On Tue, 28 Oct 2014, dave wrote:
On 10/27/2014 01:48 PM, whit3rd wrote: On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote: In article , micky wrote: How can the same FM station appear at two different spots on the dial? (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. 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. 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. 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 |
#10
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On 10/27/2014 9:32 AM, micky wrote:
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. 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 |
#11
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On Tue, 28 Oct 2014, Paul Drahn wrote:
On 10/27/2014 9:32 AM, micky wrote: 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. 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. 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 |
#12
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
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. I can understand the authorities getting a bit ansty about that one. 121.5 is the international aeronautical VHF distress frequency ... :-) Arfa |
#13
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black wrote:
On Tue, 28 Oct 2014, Paul Drahn wrote: On 10/27/2014 9:32 AM, micky wrote: 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. 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. 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? 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 |
#14
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
snip 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.) ERP - Effective Radiated Power ? The 'real' transmitter output multiplied by the 'gain' of the transmitting antenna. So maybe I'm calling it weak because it farther away, in DC, not Baltimore where I live, There are many many factors that affect the propagation of a VHF signal over a lower frequency one, some of which will degrade that signal, and others of which can, under the right conditions, enhance it. VHF signal reception is a lottery, once you are outside the designed service area of the station. 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? Without seeing a published map of the station's service area, it's impossible to say. However, something as simple as a tall building in the direction of the transmitting site, can be enough to cast a 'radio shadow' across a large swathe of territory on the 'downstream' side Arfa Michael |
#15
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
In message , Phil
Allison writes ** 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. I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. -- Ian |
#16
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On 10/27/2014 3:48 PM, whit3rd wrote:
On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote: In article , micky wrote: How can the same FM station appear at two different spots on the dial? (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. 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. 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 --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
#17
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On 10/29/2014 1:11 AM, micky wrote:
On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black wrote: On Tue, 28 Oct 2014, Paul Drahn wrote: On 10/27/2014 9:32 AM, micky wrote: 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. 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. 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? 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 --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
#18
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
"Ian Jackson" wrote in message ...
I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. Precisely If it were lower, you'd greatly increase the possibility of images. |
#19
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
In message , William Sommerwerck
writes "Ian Jackson" wrote in message ... I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. Precisely If it were lower, you'd greatly increase the possibility of images. 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. -- Ian |
#20
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On Wed, 29 Oct 2014, William Sommerwerck wrote:
"Ian Jackson" wrote in message ... I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. Precisely If it were lower, you'd greatly increase the possibility of images. 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 |
#21
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
"Michael Black" wrote in message
xample.org... On Wed, 29 Oct 2014, William Sommerwerck wrote: Precisely. If it were lower, you'd greatly increase the possibility of images. Isn't it more precisely, that by putting the LO higher, the image falls where fewer strong signals are? 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. |
#22
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
In message , William Sommerwerck
writes "Michael Black" wrote in message news:alpine.LNX.2.02.1410291558180.4171@darkstar. example.org... On Wed, 29 Oct 2014, William Sommerwerck wrote: Precisely. If it were lower, you'd greatly increase the possibility of images. Isn't it more precisely, that by putting the LO higher, the image falls where fewer strong signals are? 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. 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. -- Ian |
#23
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
Ian Jackson wrote:
** 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. I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. ** In the examples I have checked ( both tube and SS ), it was always 10.7MHz lower. Makes the LO more stable is one reason. .... Phil |
#24
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
William Sommerwerck wrote:
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. ** Nonsense. Long as a particular band has less width than double the IF frequency, no in-band images will occur. ..... Phil |
#25
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
In message , Phil
Allison writes Ian Jackson wrote: ** 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. I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. ** In the examples I have checked ( both tube and SS ), it was always 10.7MHz lower. Then don't you get a lot of image trouble from the two TV channels below the FM band? Makes the LO more stable is one reason. I would think the benefit would be pretty marginal. While I'm sure there are exceptions, regardless of the frequency they're receiving, there are probably very few 'normal' radios or TV sets etc where the LO runs below the tuned frequency. -- Ian |
#26
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
Ian Jackson wrote: ** In the examples I have checked ( both tube and SS ), it was always 10.7MHz lower. Then don't you get a lot of image trouble from the two TV channels below the FM band? ** Who is this "you" - white man ? The tube FM receiver was made in USA ( mono, 75uS de-emphasis) and used a 12AT7 local oscillator - barely able to run at 100MHz. ..... Phil |
#27
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
micky wrote:
How can the same FM station appear at two different spots on the dial? 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. ** What make and model radio is doing this ?? Does it have a TDA7000 IC inside, by any chance ?? Those have an internal IF frequency of only 70KHz and image rejection is by purest magic. FYI: If this problem exists on only one radio, it must be the fault of that radio. FYI 2 Your post is 99% incomprehensible drivel. .... Phil 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. |
#28
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
On Wed, 29 Oct 2014 09:30:32 -0000, "Arfa Daily"
wrote: snip 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.) ERP - Effective Radiated Power ? Oh, yeah. P... F... closely related (just add a curved line to the F.) The 'real' transmitter output multiplied by the 'gain' of the transmitting antenna. Thanks. So maybe I'm calling it weak because it farther away, in DC, not Baltimore where I live, There are many many factors that affect the propagation of a VHF signal over a lower frequency one, some of which will degrade that signal, and others of which can, under the right conditions, enhance it. VHF signal reception is a lottery, once you are outside the designed service area of the station. 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? Without seeing a published map of the station's service area, it's impossible to say. However, something as simple as a tall building in the direction of the transmitting site, can be enough to cast a 'radio shadow' across a large swathe of territory on the 'downstream' side Arfa Michael |
#29
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
On Wed, 29 Oct 2014 14:01:57 -0500, amdx wrote:
On 10/29/2014 1:11 AM, micky wrote: On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black wrote: On Tue, 28 Oct 2014, Paul Drahn wrote: On 10/27/2014 9:32 AM, micky wrote: 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. 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. 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? 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. Not surprising. The Hip Hop people are a bunch of trouble-makers. From the miscellaneous drawer, that same expensive KLM radio that gets 88.5 well all the way from DC got 88.1 well too, not surprising since it's a Baltmore station and that's where I am. But about 6 months ago, there started to quite a bit of static (FM static? Maybe I should listen to it again. Anyhow, it was hard to listen to.) on the local station on the expensive radio. But all** the other much cheaper radios continue to get the local station just fine. So sometimes it pays to be cheap. ** I keep buying radios from the 60's and 70's at hamfests, looking for one that will get 88.1, 88.5 and 101.1 Today I can't even find a semi local 94.3 MHz station. LOL Mikek |
#30
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
On Wed, 29 Oct 2014 22:09:28 -0400, micky
wrote: ** I keep buying radios from the 60's and 70's at hamfests, looking for one that will get 88.1, 88.5 and 101.1 Not 101.1. 90.1, C-Span radio, which I guess I've lost interest in. It's boring as all get out during the committee hearings, and the 7AM program used to be great, but it's been discovered by the wackos. Weekends, especailly evenings and nights, can be great. BookTV very good. It or they had a long series about every president and another series about every first lady, and their playing of the LBJ tapes was enlightening (I'd wondered for decades if he really was pro-civil rights or if his votes as senator were the real LBJ. It was the first.) |
#31
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
On Wed, 29 Oct 2014 12:28:16 -0700, "William Sommerwerck"
wrote: "Ian Jackson" wrote in message ... I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. Precisely If it were lower, you'd greatly increase the possibility of images. Nonsense, it would just change their location. Right into the VHF low TV broadcast band (at least in the US). ?-) |
#32
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
In message , josephkk
writes On Wed, 29 Oct 2014 12:28:16 -0700, "William Sommerwerck" wrote: "Ian Jackson" wrote in message ... I'm pretty sure that the local oscillator nearly always runs 10.7MHz HIGHER than the radio signal. Precisely If it were lower, you'd greatly increase the possibility of images. Nonsense, it would just change their location. Right into the VHF low TV broadcast band (at least in the US). ?-) You don't change the mechanism for the interference, but because the two TV channel allocations are where they are, if either was used in your area you would certainly increase the possibility of interference from them. However, as I've said, I think it's unusual for the LO to be on the low side (probably for exactly this reason). -- Ian |
#33
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
Ian Jackson wrote:
You don't change the mechanism for the interference, but because the two TV channel allocations are where they are, if either was used in your area you would certainly increase the possibility of interference from them. However, as I've said, I think it's unusual for the LO to be on the low side (probably for exactly this reason). ** A man who prefers his ignorant opinions to facts is a complete fool: The 6AQ8 along with the 12AT7 were the most common tubes used for LOs in FM tuners from the early 1950s onwards. http://www.r-type.org/exhib/aav0008.htm They were invariably used as low side oscillators. Were not several TV channels tucked right under the FM band back then ? All FM tuners have "image rejection" and benefit from "capture effect". The former ranges from -40dB to -80dB while the latter ranges from 1 to 3dB. So any image signal would be at least 100 to 10,000 times weaker than a good signal on the FM band - and it took only a 40% amplitude difference to make the stronger signal *completely* swamp the FM discriminator. ..... Phil |
#34
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthedial?
amdx wrote: 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. Adjacent channel interference like that is caused by the IF bandwidth and the skirt. The IF transformers aren't brick wall, the amplitude drops away slowly outside the desired bandwidth. That allows a local station to be strong enough to cause problems. AFC can make it worse, by pulling the L.O. towards the stronger station. -- Anyone wanting to run for any political office in the US should have to have a DD214, and a honorable discharge. |
#35
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
In message , Phil
Allison writes Ian Jackson wrote: You don't change the mechanism for the interference, but because the two TV channel allocations are where they are, if either was used in your area you would certainly increase the possibility of interference from them. However, as I've said, I think it's unusual for the LO to be on the low side (probably for exactly this reason). ** A man who prefers his ignorant opinions to facts is a complete fool: The 6AQ8 along with the 12AT7 were the most common tubes used for LOs in FM tuners from the early 1950s onwards. http://www.r-type.org/exhib/aav0008.htm They were invariably used as low side oscillators. The fact that some what might now be considered 'highly desirable collectibles' had low-side LOs doesn't mean it became a standard. Were not several TV channels tucked right under the FM band back then ? Not the present FM band. However, in the USA FM started life between 42 to 50MHz* but this was essentially experimental. After the war, it was allocated the present band (87.8€“107.9 MHz). *Now the analogue TV IF range - which otherwise would have been Channel 1 - and hence TV starts at Channel 2. All FM tuners have "image rejection" and benefit from "capture effect". The former ranges from -40dB to -80dB while the latter ranges from 1 to 3dB. So any image signal would be at least 100 to 10,000 times weaker than a good signal on the FM band - and it took only a 40% amplitude difference to make the stronger signal *completely* swamp the FM discriminator. But that doesn't stop London Heathrow ATC (AM, of course) breaking through on 97.3MHz (at least on my kitchen radio)! -- Ian |
#36
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
"micky" wrote in message ...
From the miscellaneous drawer, that same expensive KLM radio that gets 88.5 well all the way from DC got 88.1 well too, not surprising since it's a Baltmore station and that's where I am. I assume you mean KLH. KLM is an airline. Sammy Davis Jr once did a print ad for KLH in which he said "I used to think KLH was an airline". |
#37
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
"Phil Allison" wrote in message
... William Sommerwerck wrote: 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. ** Nonsense. Long as a particular band has less width than double the IF frequency, no in-band images will occur. Did you actually read what I wrote? The second sentence says that. In the third sentence, I said "If the LO were //below// the incoming signal..." Do the math: 88.1 minus 10.7 plus 21.4 equals... what? 98.8? I wasn't the one who brought up the point about having the LO below the incoming frequency. |
#38
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots on the dial?
"Phil Allison" wrote in message
... The 6AQ8 along with the 12AT7 were the most common tubes used for LOs in FM tuners from the early 1950s onwards. http://www.r-type.org/exhib/aav0008.htm They were invariably used as low-side oscillators. Fascinating. I never knew this. |
#39
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
On 10/29/2014 9:09 PM, micky wrote:
On Wed, 29 Oct 2014 14:01:57 -0500, amdx wrote: On 10/29/2014 1:11 AM, micky wrote: On Tue, 28 Oct 2014 16:39:16 -0400, Michael Black wrote: On Tue, 28 Oct 2014, Paul Drahn wrote: On 10/27/2014 9:32 AM, micky wrote: 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. 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. 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? 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. Not surprising. The Hip Hop people are a bunch of trouble-makers. The station is part of a group, and their 94.5 was interfering with a station at 94.3 playing O'Reilly while their programing had Limbaugh on. From the miscellaneous drawer, that same expensive KLM radio that gets 88.5 well all the way from DC got 88.1 well too, not surprising since it's a Baltmore station and that's where I am. But about 6 months ago, there started to quite a bit of static (FM static? Maybe I should listen to it again. Anyhow, it was hard to listen to.) on the local station on the expensive radio. But all** the other much cheaper radios continue to get the local station just fine. So sometimes it pays to be cheap. ** I keep buying radios from the 60's and 70's at hamfests, looking for one that will get 88.1, 88.5 and 101.1 Today I can't even find a semi local 94.3 MHz station. LOL Tried my truck radio today, I got 94.3MHz, JOY FM a religious station. Covers the AL./FL. line near Dothan. Poor signal though. Mikek |
#40
Posted to sci.electronics.repair
|
|||
|
|||
How can the same FM station appear at two different spots onthe dial?
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. The station engineer might have been telling the strict truth... it would have taken a spectrum analyzer or modulation meter to be sure. Commercial FM is generally allowed a +/- 75 kHz carrier deviation. Due to the way FM works, and due to the fact that the station is transmitting a stereo subcarrier (centered on 38 kHz, with its own sidebands going out as much as 15 kHz on either side), the FM station's actual RF "footprint" can easily have significant energy 120 kHz on either side of its nominal carrier frequency. That's more than half-way out to the "alternate" channel center, 200 kHz away. If the station tends to run "loud" (highly compressed audio, cranked all the way up) then the "wide footprint" is likely to be present much or most of the time. Things can be even worse these days, since many stations are also transmitting in-band/on-channel digital subcarriers which go out even further. A lot of FM radios/receivers have fairly "broad" intermediate- frequency filters... e.g. one or two crystal filters with 220 kHz or even 250 kHz bandwidth. Such broad receptivity lets almost all of the "desired" station's signal in... and that's good for low-distortion stereo reception since you get the whole stereo subcarrier. Unfortunately, if there's a strong signal on the "alternate" channel (200 kHz away), that signal's outer sidebands will end up getting through the filter, and will probably affect the stereo subcarrier and increase distortion or "break through" into audibility. If you're trying to tune in a weak, distant signal that's on an "adjacent" channel to a strong local (100 kHz away) the problem is even worse. There are ways to work around this: - Use an FM tuner which has a narrower IF bandwidth. Better tuners often have a wide/narrow switch setting, with the narrow setting using different (or more) crystal filters with reduced bandwidth - 200, 180, 150, or even 110 kHz. The narrower filters can eliminate a lot of adjacent- and alternate-channel bleedover. The price is higher distortion (especially in stereo) since the outer FM sidebands of the desired station are also eliminated by the narrower filters. - Use a directional FM antenna, and aim it in the direction which gives the best results. This may be "aimed towards the desired station" (increasing its relative strength), or "aimed at an angle away from the undesired station" (to put the interfering station in a "null" in the antenna's reception pattern). |
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Halogen Spots | UK diy | |||
Glue spots | Woodworking | |||
Removing dial bezel on silver Trav A Dial? | Metalworking | |||
Recommendations for soldering station and Desoldering station or rework station. | Electronics Repair | |||
Replaement dial cover for B & S 6" Dial Daliper | Metalworking |