Interference problem

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izzyj4

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I need a little help here. I recently installed two antennas in my attic (an modified RS mag mount and a Diamond D-130J). I ran two separate RG-8X cables down from my attic to my listening post in my basement. Both cables are run side-by-side coming down from the attic to my shack. I have a 780XLT hooked up to the RS antenna and a 996T hooked up to the Diamond lead. I'm getting interference on the 33MHz band to the 996T and it varies which frequency the RF comes in on (I'll get it on 33.50 and 33.56 but not 33.52 or 33.54). If I turn the squelch all the way up I still get the interference.

Same thing with the 780XLT but it RF comes in around 46.16 up to about 46.22 but if I cut the squelch the interference is squelched. I then took each scanner and hooked them up to the other antenna. Same results with the opposite scanner. Also if I turn the squelch down I do get the interference on both 46 & 33 MHz ranges s previously described.

The scanners are near my computer and I have traced the majority of the interference on the 33MHz band to the computer. However once I disconnect the coaxial lead to either scanner and run it without an antenna, I do not get any interference even if I have the squelch just barley up. I put a wick antenna that comes with the scanners on it and I get the interference back. t seems that with running both scanners with their wall warts and having the coaxial hooked up to the ANT jacks, I get the interference. SO for ha-ha's, I decided to run an extension cord from another room's outlet (different electrical circuit in the house) had the 780 and 996 hooked up to the original antennas as I state above and got the interference again.

Now here's the kicker, I have a portable scanner (396T) and I ran it on batteries and was able to trace the 33MHz interference to the computer and also my monitor (CRT). I would have to put the antenna sort of close to either unit to recreate the interference. I think the 46MHz interference may be coming from by cable modem or router (hard wire only) but I couldn't replicate this interference with the portable radio.
So then I hooked the portable scanner up to each of the leads and ran them for a while and did not get any interference over it, basically picked up everything okay during a service search of the VHF and UHF bands.

I'm at a loss. Any suggestions???

Izzy
 

GTR8000

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Computers, CRT's and routers wreak havoc on low band, especially right in the 33 and 46 ranges. Using a Linksys router, by any chance? Those are usually the worst offenders, they send out horrible noise over the Ethernet lines. It nearly completely wipes out anything on 46.18, just my luck. Also, if you have the scanners connected to the computer in any way (audio, USB, etc) that could definitely be another source of noise. I notice the reception on low band takes a nose dive when I connect the scanner's audio out to the computer's sound card.

As to why the handheld isn't picking the same interference up as the two base/mobiles, it could just be that the 396's front end isn't as sensitive to it as the other two are.

Really, what you need to do is keep the scanners as far away from the computer as possible. Obviously this is counterproductive, as most of us listen while working on the computer, or like to have the scanners connected to the computer for various reasons. So you try to minimize the effects. An LCD monitor will pretty much eliminate the interference from the CRT, so that's one step you can take. A wireless router will cut down on a lot of the Ethernet noise, and for the connections that are wired, use high quality, short patch cables to minimize the damage. If it is a Linksys router you're using, you might want to replace it with something that is in a metal box so it has better shielding.

Good luck, believe me I know firsthand how frustrating the low band vs interference can be.
 

izzyj4

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Bingo,I got a Linksys router!!!! I though it might be that. I might be changing some of my set up and location of my desk any way or might just try and change the location of the router. I usually never skimp on cables but I'll have to check on what I got for Cat5's anyway.

The problem is too my antennas are two floors up from me and I have about 40 ft. of co-ax in length. I figured I wouldn't pick up that much interference from the computer. I'm also wondering if I'm also picking it up through the electrical wiring circuit as well. Unfortunately, the portable scanner I have right now has to be refurbed. Had a little "incident" with it where I can't use a wall-wart with it.

I'll try a few things and give a report back. Any other suggestions?
 

GTR8000

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Yeah you can definitely have noise sneaking in through the power grid, no question about it. Take a look at some line conditioners/power filters. Most of the companies that make UPS units carry them...APC, Tripp Lite, Belkin, Monster, etc. Clean power will certainly help the scanners operate more efficiently.

Keep that Linksys box wayyyyy away from the scanners! Keep any CAT5 runs way away also, and shorten up the patch cables between the modem/router/computer to as short as feasible. Those things coming out of a Linksys router act like a huge radiating antenna, it's frightening how much RFI they pump out.

As long as you're using quality coax with quality connectors that are installed properly, you're doing all you can to cut down on any RFI/EMI picked up by the coax runs. Also, try to keep the coax adapters to a minimum, the more connections you have, the more ways for RFI/EMI to creep its way into the system. Some people have good results wrapping toroids around power, CAT5, audio, etc cables to soak up some of that stray RFI/EMI.

You really have to take an aggressive approach to keep your listening post quiet, especially when you're dealing with the lower frequencies.
 

hoser147

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When we had new computers put in our watch room they caused interference. We took some Alum. Screening and wrapped the coax and it took care of the problem. It ended up being a cheap quick fix.
 

izzyj4

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So you blew the polarity protection diode in your 396?
Yah, wasn't paying attention to which DC power plug I grabbed in the car to charge my batteries. Had that lovely "Blue Smoke" smell for a couple of days. I opened it up and it was only one diode blown. I did that a while back and never had the chance to send it out for repair, but it still worked with alkaline batteries so I used it that way for a bit.


Back to my problem, last night I shut down the router and the interference on 46.18 (plus the 46 MHz band) went away. Same thing when I shut the computer down, the 33 MHz interference went away. So now I got to figure out where to move some of the stuff.

hoser said they used some aluminum mess to wrap the coax and me being a guitar player, I have shielded the "pot whole" where the jack and volume controls are with copper shielding to reduce hum. Is there a way or some sort of product to do this with a computer or has anyone done something like this?? What have others with a shack done to cut down the RF?

UPDATE: Well I put a ferrite that I had on the power cable to the router and WHA-LA!!!!! The interference from the router at 46 MHz was cut out of the scanners. I feel like a doofus for not trying that first. Now on to the computer's 33 MHz RF.
 
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hoser147

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Glad to hear you got it figured out. Cant remember what the cost was but it was just Alum Window screen from the local hardware that took care of our problem.
 

slicerwizard

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Yah, wasn't paying attention to which DC power plug I grabbed in the car to charge my batteries. Had that lovely "Blue Smoke" smell for a couple of days. I opened it up and it was only one diode blown. I did that a while back and never had the chance to send it out for repair, but it still worked with alkaline batteries so I used it that way for a bit.
It's easy to fix - any reason you're not just repairing it yourself?
 

izzyj4

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It's easy to fix - any reason you're not just repairing it yourself?
The main reason originally was when I did it it was still under warranty but then I kinda put it on the back burner to get fixed doe to working a lot and not having the spare scratch to get it done. I'm okay with a soldering iron but I rather send it out and have the whole unit checked in case something else is wrong with it. That's all.

Also too as an update, I still get some interference on the scanners even with the ferrite on the power cable to the router, not as bad though, I can squelch it out. I had to put it off for a couple of days because of work but I'm thinking on relocating where my desk is and moving the router as well.
 

slicerwizard

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I'm okay with a soldering iron but I rather send it out and have the whole unit checked in case something else is wrong with it.
If all you did was apply reverse polarity power, the protection diode provided (and continues to provide) a short circuit across the power jack. No other components were affected as the diode is clamping the input voltage well below the levels required to damage any other components.
 

hertzian

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What have others with a shack done to cut down the RF?

UPDATE: Well I put a ferrite that I had on the power cable to the router and WHA-LA!!!!! The interference from the router at 46 MHz was cut out of the scanners.
Ah, the magic of ferrites! I've got a handful of the RS 273-105 snap'ons everywhere. I'll either snap them on the coax, or make *several* loops of power cords through them.

In addition to wrapping loops through them around power cords, I also place at least TWO of them on the coax at the input to the scanner, and also up near the feedpoint of the antennas. This helps cut down the junk being created in the shack from traveling up the outside of the braid, and into the antenna feedpoint and back down. Ideally, up at the antenna feedpoint, about 4 of them are needed in my case just snapped on in series a few inches away from the feedpoint for dipoles, or just about equal to the radial length for ground planes.

Aside from this, the antenna itself may be picking up the rf directly if the chokes aren't helping.

One thing I wouldn't do is tie-wrap both of those feedlines to each other - I'd separate them by at least a few inches to cut down on common-mode current affecting each other. The chokes however should cut down on this problem.

Every interference problem is unique, so these are just general tips that work in my case. Yours may be more difficult or easier to cure.

Just "grounding" your equipment properly is not always the cure from an RF standpoint, and thus chokes are needed, not only to reduce common-mode interference, but is many cases preserve the antenna pattern from being distorted by the braid of the coax acting as it's own antenna.
 
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izzyj4

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Still getting a little Interference, mostly now from my computer in the 33 Mhz band (specifically 33.44, 33.52, 33.56, 33.50) So I know this is coming from my processor, ect. Any shielding ideas for a PC? I've been searching the web and haven't come up with anything yet. I really don;t have the $$$ to buy a new case (which the current one is all metal).
 
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