Intermod

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KevinC

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How do I get rid of intermod so I can have an enjoyable listening experience on all my Uniden scanners?
If it's truly intermod you figure out the source(s) and eliminate it or reduce it enough so it no longer causes you problems...provided it's actually happening inside your radio and not externally.
 

Wally46

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I think it may be the nearby hospital paging system. For now everytime I hear it I just push the avoid button, but I'm afraid I may be blocking out frequencies that might be interesting.
 

nd5y

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Compared to expensive two way radios, scanners have consumer grade wide coverage receivers that are poorly designed. Turning on your attenuator can help. Installing expensive filters can reduce intermod caused by a particular strong station. You can't eliminate the cause, you can only hide the symptoms.
 
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SB-Wi

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Compared to expensive two way radios, scanners have consumer grade wide coverage receivers that are poorly designed. Turning on your attenuator can help. Installing expensive filters can reduce intermod caused by a particular strong station. You can't eliminate the cause, you can only hide the symptoms.
One way to hide it is to use CTCSS or DCS tones.
 

phask

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Compared to expensive two way radios, scanners have consumer grade wide coverage receivers that are poorly designed. Turning on your attenuator can help. Installing expensive filters can reduce intermod caused by a particular strong station. You can't eliminate the cause, you can only hide the symptoms.
But - if it's truly intermod - it's the same frequency as the one you want. Unless their is a tone (ctss etc.) you can't eliminate it.

- our local ham had intermod on the input from a paging system - we finally re-located after spending way too much time and $$$=
 

whsbuss

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But - if it's truly intermod - it's the same frequency as the one you want. Unless their is a tone (ctss etc.) you can't eliminate it.

- our local ham had intermod on the input from a paging system - we finally re-located after spending way too much time and $$$=
CTCSS tone just prevents the squelch from opening until the correct sub audible tone is received. But true inter mod can still cause havoc once the squelch is opened.
 

Wally46

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OK... I turned on the attenuators on my scanners and unlocked all frequencies I locked out with the avoid button and it's so much better so far. Quiet until there is a good signal. no annoying interference noises every two minutes. Awesome!
 

nr2d

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If it is a 3rd order/2 frequency intermod you can easily find the 3nd frequency that is causing the problem. The formulas are:
(2*F1) - F2 = F3
(2*F2) - F1 = F3
(2*F1) + F2 = F3
(2*F2) + F1 = F3
where F1 is the frequency that the intermod is being heard on and F2 is the paging transmitter's frequency and F3 is the 2nd frequency that may be causing the intermod.

It also could be a 3 frequency/3rd order intermod and the formulas for that are:
F1+F2+F3 = F4
F1+F2-F3 = F4
where F1, F2 and F3 are the 3 frequencies, 1 of which is the victim frequency, mixing to cause the intermod, F4.

It could also be an image problem. That can be calculated with this formula:
(2*scanner IF frequency) + or - F1 where F1 is the victim frequency. This why in the FAA, where I work, on the older receivers the IF was always on the high side of the operating frequency. If not local FM broadcast stations would cause image problems. Here is an example F1 = 133.000 MHz and IF frequency of the old receivers is 20.6 MHz, thus (2*20.6) - 133.00 = 91.8 MHz. Even though this isn't FM broadcast channel in the US it would still cause problems in the receiver.

Remember that the scanner's "front end" has very little if any filtering and any strong close RF source can cause a problem. This would be a receiver intermod and from what you said that adding attenuation made the problem better then I would think it is a receiver intermod.

It also could be a transmitter intermod or a "rusty bolt" intermod. These would be much harder to fix. The formulas to calculate the problem frequency (ies) are the same but the method that is generating the intermod is different.

In a transmitter intermod it is generated in one of the 2 or 3 transmitters and a "rusty bolt" bolt intermod is generated by the corrosion of 2 metals where they touch. This creates a diode effectively and in the presence of 2 or 3 high RF sources causes this "diode" to oscillate and this creates the source of the intermod.
 
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