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Interference .


TimChgo9
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Kind of OT, but I get interference on my scanner, at various times during the day.  It sounds like a transmission that barely breaks the squeich, but I can't squelch it out but turning up the squelch. 

 

I get it mainly on our local fire frequency (154.250). However, it also affects 154.370, 154.265, 154.415, and 154.800 (our local police department), as well as 154.635  .  Additionally it affects 161.100 (BNSF Chicago Subdivision) and 160.650 (BNSF Chillicothe Subdivision) ,.  It affects these frequencies simultaneously, and it comes and goes.  Sometimes it appears to be dependent on weather, such as on clear nights, it happens more frequently. I am wondering if there is an improperly grounded transmitter out there causing this.  Is this possible?  The sound is annoying, and forces me to lock out many frequencies on the scanner so I don't have to listen to it. Some days it lasts for a few minutes, and on others it goes on for hours.

 

As a side note, I am about 5 blocks from the police and fire stations, and the high school across street has an FM radio station that broadcasts at 88.1Mhz. Also, I do not get this interference while outside and using a handheld scanner. 

 

Any ideas? It was gone for most of the winter, it seemed, but now that warmer weather has returned, so has the interference.  Kind of hoping to find a solution perhaps.  Relocating the scanners in the house hasn't helped.

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This is a common occurence in many areas. With the exceptionally high concentration of transmitters these days across the entire spectrum and with more and more 'digital systems' being used, the incidences of "intermod" gets higher and higher each month.

 

When transmitters transmit at the same time in relatively close proximity to each other, the signal beat together and produce sum and difference harmonics and sub harmonics. These random signals can pop up on or near other frequencies and tricks a "scanner" into thinking that there is a legitimate signal there to be received.  Many scanners do not have the ability to block intermod and if one component of the original beat frequencies is digital, the noise that is produced in the scanner audio can be quite annoying. 

 

There may be other sources of RF contamination, but intermod is one of the high probability sources.

 

Intermod in detail: http://www.midians.com/html/intermod-explained.php

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