Need help identifying possible interference

[kmacka]

I'm currently trying to setup my milair scanners with new frequency banks and I'm having some trouble. I have the banks set up the way I want them but I believe I'm getting interference from my local public safety radio system. While I do hear my local (Lackawanna County) dispatch (EMS), there are also other types of interference. I made a recording of some of the things I'm hearing, I hope someone can identify what it might be.

http://nepamilair.hypermart.net/123007.wav

You may have to click link and then hit save as.

The above link will take you to a .wav recording that I made of some of the stuff I hear. I have different frequencies that make this type noise, ie. 148.05 range and also 307.8 range frequencies. I'm wondering if I need some type of notch filter or something along those lines. Just so you know, my main thing right now is milair, that's what my scanners are setup for.

 

[slicerwizard]

Intermod / overloading. You can hear two stations at the same time; one sounds like a WX station. Are you close to it?

 

[N1BHH]

That is intermod caused when one station transmits at the same time as another, causing your receiver to pick up a different station. There are two frequencies being used and your receiving a third one. You may have a business talking while your local police are talking and the two mix, causing you to hear the weather station. If you know the weather frequency and you are trying to hear your local police, you can add the two together and then divide by 2 to determine where the offending station is operating. That's only one way, there are many more. Read this on intermod: http://users.tpg.com.au/ldbutler/Intermodulation.htm
You should find that offender with the correct calculation.

 

[kb2vxa]

Uh oh, that doesn't even begin to cover third order IMD products so he's right back where he started. Let's keep this simple, intermod is unwanted mixing products caused by front end overload in the receiver. There are only three practical solutions for the scanner buff, locate the offending signal and notch it out with a filter, reduce the signal input to below the point at which the intermod begins or buy a better scanner.

The first is only practical when one frequency is involved such as the taxi company down the block or nearby pager tower, but since more may be involved this can be difficult at best. If you hear one voice or data stream common to all the interference you're in luck, if not you're outta luck.

One final thought, if you're using a preamplifier commonly called an antenna amp, get rid of it. More often than not they cause more problems than they solve. That BTW takes you back to reducing signal input to below the point where intermod begins.

 

[kmacka]

Uh oh, that doesn't even begin to cover third order IMD products so he's right back where he started. Let's keep this simple, intermod is unwanted mixing products caused by front end overload in the receiver. There are only three practical solutions for the scanner buff, locate the offending signal and notch it out with a filter, reduce the signal input to below the point at which the intermod begins or buy a better scanner.

The first is only practical when one frequency is involved such as the taxi company down the block or nearby pager tower, but since more may be involved this can be difficult at best. If you hear one voice or data stream common to all the interference you're in luck, if not you're outta luck.

One final thought, if you're using a preamplifier commonly called an antenna amp, get rid of it. More often than not they cause more problems than they solve. That BTW takes you back to reducing signal input to below the point where intermod begins.

The scanner in question is connected to an electroline eda 2400 which is distributing the signal of my scantenna to 4 scanners. I believe this model of the drop amp, provides 7db of gain. I'm no expert but maybe this drop amp is giving me too much signal?? I'm pretty sure there are other electroline models that don't provide any gain, maybe I need one of those.