Intermod

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yorkphotog

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Clearly I've got something set up improperly...

Quick example -- I'm using a BCT15 scanner with a Larsen SPEC800 antenna, mag mounted. I've also tried a tuned (2 3/4") whip. Both cause so much intermod(?) that at certain intersections where I know there are a lot of cell towers, I get absolutely no signal. Meanwhile, I know a guy who has basically the same setup as me (the only difference is that his antennas aren't mag mounted) and doesn't have intermod problems that are at all comparable to mine. The built in attenuator barely helps. I've also tried (as some have suggested) to use a VHF whip for 800 Mhz stuff, and it doesn't make a difference.

Thoughts?
 

zz0468

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Are you getting intermod, or are you getting absolutely no signal? Intermod is when two or more signals mix. You may hear multiple conversations at the same time. Or maybe it's voice and paging. The loss of signals could be fundamental overload, where the receiver simply stops working because of a really strong signal, or it could be desensed - similar cause, but the effective receive sensitivity is reduced to the point you can't hear the desired signal.

If it's happening near a cell tower, you may be experiencing the classic symptoms of the Nextel interference.
 

yorkphotog

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I'm not sure if what I'm getting is intermod or not.. the systems work, but in certain areas it loses basically all signal. I never hear two conversations at a time, but tons of static and crackling.
 

jon_k

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I always thought intermod was two frequencies that combine to form a new frequency. I could imagine that effecting your receiver negatively but I don't think it'd cause overload.

I've heard what I think is intermod on the amateur radio 70cm band. Near the top of the band I heard guys who would bleed in and out talking on nextel radios. I keyed up on frequency and tried to get their attention and nothing happened. I don't think they were bootlegging on the amateur radio spectrum. It just sounded like a normal radio conversation, except it faded in and out a lot.

If intermod would create overload it would have to be in the kilowatt range I figure -- as intermod isn't going to be that powerful anyways most of the time. Even then you would hear the resulting signal from the intermod.

(I /have/ experienced overload on a personal scale. I'd be listening to one frequency on my handheld radio. I would key up on the other 50 watt radio -- first radio goes dead quiet. All on different frequencies. I don't know for sure but I think that one frequency bombards the receiver so much that no other frequency can get to the reciever.)
 
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zz0468

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I always thought intermod was two frequencies that combine to form a new frequency. I could imagine that effecting your receiver negatively but I don't think it'd cause overload.
Your definition of intermod is correct. Overload can cause intermod, not the other way around, generally. But a really strong mix COULD overload a receiver, I suppose.

I've heard what I think is intermod on the amateur radio 70cm band. Near the top of the band I heard guys who would bleed in and out talking on nextel radios. I keyed up on frequency and tried to get their attention and nothing happened. I don't think they were bootlegging on the amateur radio spectrum. It just sounded like a normal radio conversation, except it faded in and out a lot.
I doubt you heard Nextel radios on the 70 cm band. It could have been in band, or it could have been an image.

If intermod would create overload it would have to be in the kilowatt range I figure -- as intermod isn't going to be that powerful anyways most of the time. Even then you would hear the resulting signal from the intermod.
Not so. There is no high power requirement in order to create an intermod mix. Just the right conditions - two or more signals, and a non-linear device to act as a mixer. That non-lineqr device can be another transmitter, a receiver, pre-amp, rusty bolt on a tower, and the list goes on and on.

(I /have/ experienced overload on a personal scale. I'd be listening to one frequency on my handheld radio. I would key up on the other 50 watt radio -- first radio goes dead quiet. All on different frequencies. I don't know for sure but I think that one frequency bombards the receiver so much that no other frequency can get to the reciever.)
That's called desense. The receiver gets so overloaded it ceases to function properly.
 

zz0468

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I'm not sure if what I'm getting is intermod or not.. the systems work, but in certain areas it loses basically all signal. I never hear two conversations at a time, but tons of static and crackling.
That sounds more like an overload situation, not intermod.
 

CCHLLM

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It most definitely wouldn't have been Nextel. No analog radio made is capable of decoding Nextel traffic, no matter what the circumstance.
 

SAR923

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The OP was asking abut his Uniden BCT-15. We all know that you think Radio Shack scanners suck. It does nothing to help this thread by posting such drivel here.
 
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This may seem like a silly question, but have you checked your cables and fittings to make sure everything is in good working order? I've had plenty of experiences where I beat my head trying to figure out what was wrong with something, only to find out it was as simple as replacing a fitting or a cable.
 

yorkphotog

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Yeah, not to be a jerk here.. but this thread has totally turned into Radio Shack hate mail. I'm trying to figure out what the problem is with my rig, not hear about other manufacturers.

I have 4 of the scanners, basically running side by side. The antennas are all mounted in the same area. Could this be a part of the problem or no?
 

yorkphotog

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Imcleish, yes sir, it normally happens in the same areas. I think it has something to do with cell phone overload...because it happens in areas that have a lot of cell phone towers, and I often hear the "Mike/Nextel clicking" even when my Mike phone is nowhere near the radios.
 

imcleish

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Unfortunately that means there is no fix for it, as you know the iden/mike phones will interfere with everything. My guess is the tower is knocking/interfering with the mike phone, it in turn is trying to re-acquire/up its transmit power to keep signal, and you are getting hammered by it.


Try turning off the mike phone for a few minutes in the problem area, and see if the problem is less.

Paging towers cause the most trouble for your scanner....
 
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