I live on the outskirts of Louisville, KY where the strongest local AM signals are 790, 840, 900, 970 and 1080. I happen to live near the AM 970 transmitter site and have noticed that I am picking up all the local AM stations from their original location as well as their equivalent distance from the reverse side of AM 970. For example, I can receive AM 900 (70 Khz down from AM 970) on AM 1040 (70 Khz up from AM 970). I am picking up AM 840 (130 khz down from 970) on AM 1100 (130 Khz up from 970). I can hear AM 790 (180 khz down from 970) on AM 1150 (180 khz up from 970). AM 1080 (110 khz up from 970) is also heard on AM 860 (110 khz down from 970). Will someone explain what is taken place here? Thanks in advance for sharing the wisdom.
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Reception Question
- Thread starter scanman1809
- Start date
Hi Scanman - this sounds like an Intermodulation problem - your receiver is so close in distance to one or more of the AM radio stations that you have mentioned that the electronic circuitry of your receiver is unable to reject the undesired signals. see http://en.wikipedia.org/wiki/Intermodulation
There are several options here. If you are able to change your listening location (to DX, I'm not saying move your residence), then that should clear up much of the problem - this of course will take some experimentation. Does this problem of stations appearing at multiple places on the dial occur on all of your receivers, or some of them? Then there's reducing the sensitivity of the receiver - your receiver might have a sensitivity control, either a Local/DX switch or a variable attenuator. That might help clear up some of the image frequencies. If an external antenna is connected to the receiver, perhaps the antenna is too long, and too much voltage is being induced by the antenna to your receiver. Try a shorter antenna. Lastly, and this may or may not be an option - purchase a receiver with better image/spurious/IF rejection specifications. I'm sure other DXers and Engineers will have other suggestions as well.
There are several options here. If you are able to change your listening location (to DX, I'm not saying move your residence), then that should clear up much of the problem - this of course will take some experimentation. Does this problem of stations appearing at multiple places on the dial occur on all of your receivers, or some of them? Then there's reducing the sensitivity of the receiver - your receiver might have a sensitivity control, either a Local/DX switch or a variable attenuator. That might help clear up some of the image frequencies. If an external antenna is connected to the receiver, perhaps the antenna is too long, and too much voltage is being induced by the antenna to your receiver. Try a shorter antenna. Lastly, and this may or may not be an option - purchase a receiver with better image/spurious/IF rejection specifications. I'm sure other DXers and Engineers will have other suggestions as well.
stormy01 said:
Stormy,
Thanks for the info. To answer your question, this problem occurs on all of my receivers, including my car radio. I still may try your suggestion of a shorter antenna for some of my indoor receivers.
I made note of some similar situations I have experienced myself on the "1110-1310 AM Daytime" and "1320-1520 AM Daytime" threads. A transmitting station very close to my house on 1270 kHz blasts a 5 kilowatt non-directional signal during daylight hours and causes the following to happen: 1420 kHz (the 2nd strongest day signal in my neighborhood) is also heard on 1120 kHz (1420 and 1120 are both 150 kHz away from 1270), 1230 kHz is heard on 1310 kHz (both 1230 and 1310 are 40 kHz from 1270), and 1170 kHz (the 5th strongest day signal in my neighborhood) is heard on 1370 kHz (1170 and 1370 are both 100 kHz from 1270). Radios with even less selectivity might also hear the local 960 kHz station on 1580 kHz (960 and 1580 are both 310 kHz away from 1270 but 960, being further away from 1270 than the rest, doesn't seem to cause as much of this Intermodulation interference as the rest of the stations do, nor does 960 get interfered by the 1270 signal nearly as much as the others; and not at all on many radios). 1230 gets and gives the most intermodulation interference being only 40 kHz away from 1270, and, if you're literally right next to 1270's towers, the 1270 signal will often cause Blanketing interference and completely wipe away everything else on the AM dial and beyond, unless your radio has excellent selectivity. I even used to hear AM 1270 in my telephone a little bit whenever the station would broadcast its non-directional daytime signal. My location is East Moline, Illinois, in what is known as the "Quad Cities" area.
Thank you for the definition, stormy01. I knew there had to be a term for this situation and now I finally know what it is.
Thank you for the definition, stormy01. I knew there had to be a term for this situation and now I finally know what it is.
As background, you might consider Googling the term “super-heterodyne”. This concept has been the basic design of receivers for many decades and involves the mixing of incoming signals with either 455 or 262 kilocycles to establish an “intermediate frequency” or “IF” for simplicity of tuning and recovery of the audio signal from the RF carrier. In this case, a very strong local signal can OVERdrive the initial stages of a radio into generating other mix combinations that are added into the IF stages by the mixer.
If you have no means of attenuating the incoming signal, you indeed may have to move. ;D
If you have no means of attenuating the incoming signal, you indeed may have to move. ;D
Now this is not a remedy .... I just have to point out the situation the Meadowlands of North New Jersey.
First off, the Meadowlands only came to be called that in more recent decades, when marketing and development needed a friendler name than what the place USED to be called by the locals within range -- The Secaucus Pig Farms. It was a vast, bleak, swampy morass of livestock, refineries, high-tension wires, bats, quicksand, snakes, junkyards, rats, smokestacks and ..... it had great ground systems for AM stations who wanted to shoot their signal SE into Manhattan and as much of the Five Boroughs as possible.
If I recall correctly, WMCA, WOR, WINS, WEPN, WNEW, WLIB, WADO, WBNX, WHOM and WWRL used to have their towers there and might still. And each of them had the multiple towers peculiar to directional AM stations.
The mixing of signals was beyond ferocious there. WLIB and WNEW shared time on 1250 (at the same time), completely obliterating WMTR.
The 1090 mix of WINS and WHN would stop any radio in 'seek' mode (in fact, at times, we got that 1010 + 1050 equals 1090 simulcast as far away as Laurelton in SE Queens.
There was even a frequency on which THREE stations from lower channels mixed.
And at a spot along the NJ Turnpike extension, the devil-knows-what kind of isotopes from this alien landscape converged and completely covered 50,000 watt local WFAN 660 -- rendering it silent for a half mile.
Scanman : You mentioned radio**. What kind are they, and what kind of DX antenna(e) do you use? SOmeone here might have a way to get rid of SOME of those 'spurs'.
First off, the Meadowlands only came to be called that in more recent decades, when marketing and development needed a friendler name than what the place USED to be called by the locals within range -- The Secaucus Pig Farms. It was a vast, bleak, swampy morass of livestock, refineries, high-tension wires, bats, quicksand, snakes, junkyards, rats, smokestacks and ..... it had great ground systems for AM stations who wanted to shoot their signal SE into Manhattan and as much of the Five Boroughs as possible.
If I recall correctly, WMCA, WOR, WINS, WEPN, WNEW, WLIB, WADO, WBNX, WHOM and WWRL used to have their towers there and might still. And each of them had the multiple towers peculiar to directional AM stations.
The mixing of signals was beyond ferocious there. WLIB and WNEW shared time on 1250 (at the same time), completely obliterating WMTR.
The 1090 mix of WINS and WHN would stop any radio in 'seek' mode (in fact, at times, we got that 1010 + 1050 equals 1090 simulcast as far away as Laurelton in SE Queens.
There was even a frequency on which THREE stations from lower channels mixed.
And at a spot along the NJ Turnpike extension, the devil-knows-what kind of isotopes from this alien landscape converged and completely covered 50,000 watt local WFAN 660 -- rendering it silent for a half mile.
Scanman : You mentioned radio**. What kind are they, and what kind of DX antenna(e) do you use? SOmeone here might have a way to get rid of SOME of those 'spurs'.
I spent my junior year of high school in Honolulu. The place where I lived was basically within walking distance of two or three sticks....which in turn were diplexed and multiplexed. The result was "spurs" all over the dial at home and in the car. This rendered nighttime dx mostly impossible (although I did have one blank spot on 710, where I'd regularly snag the Phillipines). The phenomenon extended to shortwave where the lower portion of the spectrum (up to about 6 or 8 mhz) was also affected.
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