Interference in st. louis

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teufler

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Got a request to check out some interence a local hospital is getting on their paging network.
Theirs is a two part system. They transmit a signal on 75.5 mhz and this is relayed on 152.24 mhz. The problem is they are getting another signal on 75.5 mhz that creates garbled pages.

I listen to 75.5 mhz and I can get a signal from 97.1 fm talk radio. I have logged and recorded interference from about 75.35 to 75.650. Using Freescan, log indicates RSSI 568 till it drops off to nothing. This is a 2-3 bar signal on the scanner.. 97.1 I get a high 8+ reading, 5 bars on a 396xt scanner. Now when the mode was nbfm, the interference was just "burps, chirps, scatchy audio. I then tried fm, and wbfm and finally bfm. By the time I get to broadcast fm, the audio is perfectly readible . Since the interference was 21.6 mhz lower than the 97.1 main transmission, I checked 97.1 + 21.6 and got nothing. I checked some other stations in the area , 21.6 lower than the main transmitter and got nothing. I df'ed with a beam, made for 145.00 but it still indicated a direction that indicated the signal direction. When compared with Google earth maps, and the FCC lat long of 97.1, , I had indications of the approximate direction..

Could others in the St. Louis area check , from their locations, are they getting audio on 75.5 fmb mode and post what kind of readings they get.
 

jim202

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By any chance is this being used as a link or is it an image in your radio?

If you take the FM channel of 97.1 minus 75.5, you get 21.6 which might just be an IF of the radio being used as a receiver. Find out what the radio is and check to see what the IF is that the receiver is using.

A solution if this is the IF of the receiver is to change the injection to the other side and re-tune the receiver.

Need to do some homework on just what radio and model number is being used.

You might even have an intermod mix from another signal source that is causing the problem.
 

KevinC

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By any chance is this being used as a link or is it an image in your radio?

If you take the FM channel of 97.1 minus 75.5, you get 21.6 which might just be an IF of the radio being used as a receiver. Find out what the radio is and check to see what the IF is that the receiver is using.

A solution if this is the IF of the receiver is to change the injection to the other side and re-tune the receiver.

Need to do some homework on just what radio and model number is being used.

You might even have an intermod mix from another signal source that is causing the problem.
I believe he stated he could hear it on a different radio also, so it shouldn't be an image.

I'm leaning toward a link, either legal or illegal.
 

teufler

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The hospital has reported interference on their link or output at 75.5 mhz. They wonder what is the interference. I have checked, with the only radios I have, a 396xt and a 780xlt. The 780xlt hjas a small attached antenna, inside type and I get voices. The 396xt has a large outside antenna which I can perfectly. What kind of radio at the hospital, I don't know, as I am seeking information by other here in st louis as to if they are receiving interference also. Just checking for the range of interference. So far it appears like a range of about 50 miles to the south. Now I don't think FM broadcast stations have links. I have checked a few other FM stations, dropping down 21.6mhz and hear nothing.
 

jim202

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Got a request to check out some interence a local hospital is getting on their paging network.
Theirs is a two part system. They transmit a signal on 75.5 mhz and this is relayed on 152.24 mhz. The problem is they are getting another signal on 75.5 mhz that creates garbled pages.

I listen to 75.5 mhz and I can get a signal from 97.1 fm talk radio. I have logged and recorded interference from about 75.35 to 75.650. Using Freescan, log indicates RSSI 568 till it drops off to nothing. This is a 2-3 bar signal on the scanner.. 97.1 I get a high 8+ reading, 5 bars on a 396xt scanner. Now when the mode was nbfm, the interference was just "burps, chirps, scatchy audio. I then tried fm, and wbfm and finally bfm. By the time I get to broadcast fm, the audio is perfectly readible . Since the interference was 21.6 mhz lower than the 97.1 main transmission, I checked 97.1 + 21.6 and got nothing. I checked some other stations in the area , 21.6 lower than the main transmitter and got nothing. I df'ed with a beam, made for 145.00 but it still indicated a direction that indicated the signal direction. When compared with Google earth maps, and the FCC lat long of 97.1, , I had indications of the approximate direction..

Could others in the St. Louis area check , from their locations, are they getting audio on 75.5 fmb mode and post what kind of readings they get.
If you do an FCC search on the 75.5 frequency in the St. Louis area, there are 2 locations that show up. One is at 1034 South Brentwood University Club Tower in Richmond Heights, MO. The other is at 1710 Martin Luther King Drive in St. Louis.

Those are both under the license of KRS635. There was an expired license listed, but we know there is no way the hospital could be using a frequency with an expired license.
 

teufler

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well not my concern about a license , I'm just seeking any information on if anyone else can copy what sounds like 97.1 on 75.5. They are supposed to be just on 97.1. I will put the info in my report that , check on license dates.
 

scanman1958

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OK. Did a check (very basic) on the 75.5 MHz frequency. I used a 396xt, stock rubber duckie antenna and was mobile (but parked) in the area of Manchester Rd and Brentwood Blvd.

I tried what you did (or at least what I think you did) and scrolled through the different modulation settings and got different results.

On all settings I heard what I am assuming was the pager digital noise.

AUTO - The signal defaulted to FM on the scanner and I did hear what I thought was a very faint FM radio station bleed over.

NFM - The signal "volume" was much louder and clearer and at times seemed to be overloaded with static of some sort.

WFM - Heard voice bleed over (almost like a backround noise) from radio station. See below.

FMB - Not a good signal but did hear a morse code signal twice.

AM- The worst signal of all. Just barely readable.


The radio station I think I heard was a religious one. I was able to identify someone repeating phrases like ........ "________ is holy. ________ is holy. ________ is holy." Could not make out the first word or two in the phrase(s). It may be 90.5FM (?) That is a local religious station.

Hope this helps.
 

teufler

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Help on interference

Thanks Jeffry, seems the interference is more localized. I am getting definately 97.1 as I can go there, get 5 bars, go to 75.5 and drop to 2-3 bars but the exact same conversaition.
 
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