HAM overlaping Public Service band

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This is the second night that I have noticed this. Never noticed it before though. I am in Blackford CO. Indiana, and I am hearing this on the police dispatch channel 151.04 When the police o dispatch keys up, I hear them and HAM people talking at the same time. As soon as police or dispatch clear the air, I don't hear either again. To hear what I am saying check the archive on my feed Hartford City and Montpilier Fire and EMS Dispatch, Fireground, and County EMA Live Scanner Audio Feed Archives its on 11-19-2009 at 12:29am. I am using a pro-2018 hoked up via an audio cord to my computer to stream. I cant help but think it has something to do with the computer since I have never heard this before, and my feed is new. Any ideas?
 

zz0468

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You're describing a symptom of intermod. In simple terms, it's a condition created when several signals mix together and produce interference on other frequencies. It's most likely being created in your scanner. Does it have an attenuator setting? Turning that on may reduce or eliminate it.

p.s. As I'm not a premium subscriber, I'm unable to listen to the archive.
 
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Hartford City, Indiana
You're describing a symptom of intermod. In simple terms, it's a condition created when several signals mix together and produce interference on other frequencies. It's most likely being created in your scanner. Does it have an attenuator setting? Turning that on may reduce or eliminate it.

p.s. As I'm not a premium subscriber, I'm unable to listen to the archive.
If it does I have no idea where to look. Its not a fancy scanner at all, non trunking. Both of them are clear, I mean... I can hear the police talking crystal clear, and I can hear the HAM's crystal clear, they are just both on at the same time..
 

zz0468

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Maybe if you posted the make and model of your scanner, someone who's familiar with it could tell you if it has an attenuator. In the meantime, if you're getting the interference on an outside antenna, try an inside antenna to see if the situation is better.

Scanners can be easily overloaded, and when that happens, they're quite prone to this sort of thing. There are dozens of threads here about this very thing. Yes, hearing two sets of voices at the same time is almost certainly an intermod mix. The location where the mix takes place can be almost anywhere, but it's most likely inside your own scanner.
 

kb2vxa

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What you're hearing is clearly intermod of the worst kind. The question is where the intermod is coming from, not necessarily the front end of your receiver since this appears to be a recent development assuming you're using the same receiver as before.

OK, a couple of questions; do you also hear it in the speaker or only in the feed? Does it go away when the scanner is disconnected from the computer? What happens when you shut the computer off? I'll jump the queue on this one, I suspect a "birdie" generated in the computer is mixing with the on air signals and generating unwanted mixing products, intermod. Since the third signal has no modulation you can't hear it but it's there just the same. If that's the case the usual ferrite bead approach just might fix it but first you have to find the source and confirm it.

Notice I originally said not necessarily the receiver, there are some very surprising sources of unwanted mixing products having nothing to do with the receiver in any way. We may get into that later, but first let's round up the usual suspects and put them in the lineup for identification.

Alabama, the PL tone or DPL code on the wanted signal will open the squelch only when present as it should but the unwanted signal will still ride in whenever the squelch opens. Think of the purpose of it and how it operates and you'll understand your error.
 
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mikepdx

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It happens to the best of receivers, too.

One of our local 2m amateur repeaters has such an issue
- on (fortunately) rare occasions, we hear both the local Fire Dept
tone out on 154.19 repeated along with NOAA Weather Radio
on 162.55 (both located at the same mountaintop site).

It's intermod, that's for sure...
 
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I'm not sure if it only does it when it is hooked up to the computer or not, I have not tried that because I was trying to keep my live up as long as possible. Today (before reading these reply's) I switched my scanner and feed over to my desktop rather than my laptop. I'm going to listen as much as I can Friday and Friday night to see if maybe that has fixed it. If not I will let you guys know. I really appreciate all of your advice.
 

kb2vxa

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While we're waiting on Waterfreak:
"It happens to the best of receivers, too."
In the case of a repeater the highly selective "cans" pretty much eliminate the receiver, since a great many use commercial transceivers and the rest use specially made repeater transceivers mixing in the front end is pretty much out of the question.

In my previous post I was also thinking of dissimilar metals and rusted towers, point contact rectification. I remember two classic cases, the first being a State Police tower site that was a real hair puller. It took months to isolate some "bad connections" and corroded tower joints, hardware was replaced and the joints jumpered over, problem solved. Some years later a similar problem was found and rectified (;->) at a water tower where a local 2M repeater is located.

As an aside, hangar 1 at Lakehurst NAS is familiar to most being the site of the Hindenburg disaster. What many perhaps don't know besides it's still in use as an airship hangar is the fact the loft is stuffed with radio equipment from soup to nuts, everything from Navy to federal to a local fire company, even the Jersey Shore ARC has a packet node/BBS and a 70cM repeater up there. The roof is an antenna farm par deluxe, I wonder how they manage to keep it all straight. The view from up there is as fantastic as the radio range, 200' above flat terrain sure is an advantage!

Now here is a giggle for you, every once in a while the Amateur packet station goes off the air and it's always the same, a power failure. Here's the giggle, a technician working up there will trip over the power cord for the rack and pull it out of the wall socket and leave it lying there on the floor. It's pretty aggravating getting our techs who have security clearance together and go there just to plug it back in. There is a stupidly simple solution, replace the plug and socket with a twist lock type but there is a catch 22, no problem with the plug but the socket is Navy property! THOU SHALT NOT TAMPER WITH NAVY PROPERTY for I am a jealous god and thou shalt have no other gods before me. There is an Amateur station in the radio room of the USS New Jersey moored at Camden but I digress....... the Navy going to war with the Navy?
 

litavecz

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Highland Indiana
I'm having the same problem on my pro 2055 scanner.On 2 meter ham I'll get local ambulance dispatch with NOAA weather in the background.This is VERY annoying.I'll also be listening to a conversation over 2 meter band and get intermittent static over the broadcast.This happens pretty much over any frequency in the band.I don't know if this related,but any info would be appreciated.Thanks.
 

dougr1252

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What are the exact freqs that cause the problem? It may be because the difference between the NWS freq and EMS is almost or exactly 1/2 of a freq used by your scanner internally as part of the down-conversion process (10.7 / 21.4 MHz). Try to reduce the strength of one of the signals. Relocating the radio sometimes helps.

I'm having the same problem on my pro 2055 scanner.On 2 meter ham I'll get local ambulance dispatch with NOAA weather in the background.This is VERY annoying.I'll also be listening to a conversation over 2 meter band and get intermittent static over the broadcast.This happens pretty much over any frequency in the band.I don't know if this related,but any info would be appreciated.Thanks.
 
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Hartford City, Indiana
So far so good. I have not had this problem since moving the scanner, and also switching computers that I was streaming to from my laptop to my desktop. Will keep listening, but hopefully this has solved the problem.
 
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