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Help on interference from nearby tower

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wyldman

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I am currently monitoring mostly 800 Mhz truncking systems at my business.I have a large tower (not sure what it is transmitting) with lots of different antennas and repeaters on it about 1000 ft from me.It seems to cause a lot of interference on my local 800 systems.It makes it almost impossible to receive distant signals.If I drive away from the tower,reception is now normal.My 140 mhz systems come in perfectly,no interference.

I am doing OK with the local systems,I have found a place for my rooftop antenna that makes it bearable,but the distant systems are won't come in.Signal strength when on the control channel for distant systems is good,but it seems as if the receivers lose their sensitivity due to the overloaded signals.

I am using a homemade dipole that seems to work well,have the same antenna at home (about 2 miles away),and it gets evrything perfectly.Is there anyway to shield the antenna from the tower ? Would using a dedicated 800 Mhz antenna reduce the interference from other frequencies ? Any ideas ?
 

Bucko

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I have the simular problem here at home. I have a pager with in eye view on 152.48 and it really plays havic on most scanners. It has been in service for years and have used a notch filter, but then my high band takes a hit using it. Of course you would need to ident the freq from the site that is the offender to notch it out. A directional antenna maybe help you being you are listening to 800mz trunk system, that is if the system you listen to is in the other direction.
 

traumacop

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Try a notch filter. Some are made specifically for the VHF segment for paging systems.

Check out this article about notch filters and interference.

http://www2.arrl.org/tis/info/intermod/intermod.html

There are several manufacturers for them. I would go with a tried and trusted one so you don't waste your money. Use your Close Call or signal stalker to find the exact frequency and go from there.

Here is one with good ratings on eham

http://www.eham.net/reviews/detail/385?page=2

Good Luck,
 

LarrySC

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If you post more details I would offer ideas. Is the scanner for 800 using an outside antenna and what type. Do you have more than the one scanner at work. I monitor LOTs of freqs across the entire band, 39mHz to 869 mHz and can SEE the Mtn where most towers and digital pagers are located. Scanner selection, antenna selection, and programming offer no issues for me. I actually use a 3" 1/4 wave rubber duck for my 800 scanner. All 10 banks programmed and no overload. No RFI and intermod. It can be done.
 

gcgrotz

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Chances are it is 800 Mhz cellular and/or Nextel (also on 800) that's causing it. Cellular has a control channel on continuous xmit and then the voice channels come up and you've got instant overload. Unfortunately if that's the case it will be almost impossible to eliminate. These are the same interference issues that have led to the "re-banding" of 800 MHz public safety and business (read:Nextel) freqs.

You can do a little detective work. Go over and see if there is a sign with the FCC tower number on the gate (It is supposed to be there per regulations) then go back and look it up in the FCC database. You can then search for all licencees on the tower. Or, use a gps and find the lat/long and search for nearby towers in the FCC database. This will at least let you know what you're up against.

Good luck!
 

bwhite

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Although I don't have a scanner with the close-call feature, if you could borrow one it may help to determine what kind of emissions are off the tower, then you could see what options are available to you.
 

kb2vxa

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Hi Wyld and all,

First of all you must find out whether the interfrence is in band or out of band. If out of band a bandpass filter will do the trick. Contact Mrs. Sniva Lee at Tin Lee Electronics Ltd. Toronto, Ontario, Canada via the website. I highly recomend this company having dealt with them before and was amazed at the prompt and courteous service and the unexpected telephone call follow-up floored me. If the interference is in band little to nothing can be done about it.

B. White has the right idea, you must find out what sort of emissions are coming from the tower or perhaps some unexpected source in the area. Obviously the offending signals are quite strong so some sort of RF sniffer is in order. They receive, display and log only very strong signals and are a lot cheaper and easier to use than a spectrum analyzer. Star Tek (or Tech) made one of the best but I don't know if they're still in business so maybe someone reading this can toss out some names.
 
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wyldman

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May 25, 2006
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Well,I've tried close call,and other searches,and nothing comes up.Whatever it is,it stops the scanner from picking up anything distant in the 800 Mhz band on the external antenna.The local stuff comes in,but distorted and lots of static.

When I am on the external antenna,and trying to listen to the control channel,it's just static,with this boom,boom,boom noise in the background.The booms have a pattern to them,that does vary time to time.Sounds like someone beating on a tinny base drum in the distance.

When I listen to the control channel on the local stuff,it sounds fine (like a machine gun),but it gets louder then back to normal,with the same timing and rhythm when I get the boom,boom above).

If I go outside in line with the tower (with the stock rubber duck),I cannot recieve any transmissions at all.I can still hear the scanners inside picking up the locals just fine.

Any ideas what this interference might be ? I can get much in the line of different notch filters and stuff locally,and I don't want to go ordering stuff unless i have an idea on what all will work.

I've looked at cutting the cable going up to this monster tower,but they are too big for my cable cutters. ;)
 

kf4lne

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Jul 31, 2005
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Bristol, VA
Look at the antennas on the top of the tower. Do they look like turnstiles or bowties? if so you probably have a high powered broadcast station on that tower. My Pro 83 gets useless when i go up to Mt Pisgah where the WKSF FM, WXL56 NOAA and WLOS TV trasnmitter is and the whole laundry list of other RF sources. if there is a lot of RF sources on the tower and you are that close you are probably getting a signal as a resulf of mixing in the scanner. The scanner may be receiving several very strong signals that combine in the receiver to create a signal inside the scanner that trashes it. it may be the product of several signals mixing and being very close to the scanners IF. I have seen this happen at hamfests and field day sites where several transmitters are going at once and the signals mix in a receiver and trash that receiver over a certain frequency range. keep in mind that by the nature of the beast a scanner is as wide as a barn door, it has to be to let 29-1300MHz in.
 

Chrome69

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jack103 said:
That might work except he is in canada Brampton Ontario...... I would be willing to pop up their Chris on a saturday and maybe we can do some research on the tower together.. I could also bring up the notebook and record the sounds your hearing so we can share with the rest of the group here...
 
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