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Filter to filter out Sprint/NEXTEL interference

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fmulder13

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Minneapolis, MN
Hello All,

I'm experiencing a great deal of Sprint/NEXTEL interference while monitoring the Oklahoma statewide trunked system in the Oklahoma City area. There are quite a few Nextel towers in the area, and anytime I drive by one the radio goes crazy.

Does anyone know where I might find an inline filter to filter out the frequencies that are causing problems? I'm probably looking for a low-pass filter that would filter out anything above 869.000, allowing the 868ish voice channels to pass through, (still not rebanded) but choking the interference at higher frequencies. I don't care if I'm not able to monitor anything about 869.000. I've looked on the internet and haven't found anything as of yet. Ideas?
 

n5ims

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Jul 25, 2004
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3,676
Normal filters are generally not anywhere near that narrow. To get any real reduction at 869 MHz you'd probably not have much signal on anything until you get below about 850 MHz. What might work is using a cavity filter to try and notch out the undesired frequencies and pass the few you want to listen to.

Cavity filters are often used in a repeater's duplexer to notch out the transmit frequency from the receiver and notch out the receive frequency from the transmitter. You might be able to pick up an 800 MHz duplexer and use the receive section to get close to what you want, if you're lucky. It'll be big, it'll be bulky, and it'll pass only a fairly narrow frequency range so don't expect a miracle.
 

DickH

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Joined
Mar 12, 2004
Messages
4,061
Hello All,

I'm experiencing a great deal of Sprint/NEXTEL interference while monitoring the Oklahoma statewide trunked system in the Oklahoma City area. There are quite a few Nextel towers in the area, and anytime I drive by one the radio goes crazy.

Does anyone know where I might find an inline filter to filter out the frequencies that are causing problems? I'm probably looking for a low-pass filter that would filter out anything above 869.000, allowing the 868ish voice channels to pass through, (still not rebanded) but choking the interference at higher frequencies. I don't care if I'm not able to monitor anything about 869.000. I've looked on the internet and haven't found anything as of yet. Ideas?
It's not really what you consider interference. It is not from a specific frequency. It is overload caused by so much RF spewing out of those towers. I doubt any filter can stop it. Even my system Motorola has problems near similar towers. At certain points it cuts out completely. Move just a few inches and it's better, but not perfect.
 

tbiggums

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Sep 19, 2008
Messages
182
It's not just Nextel sites that are the problem. This desense happens whenever you're within a few hundred feet of most any 800 MHz cellular site.

I bought a cheap used 800 MHz mobile duplexer to use the low-pass side to try and help my PRO-197 cope with nearby cell sites. Although a tracking generator showed it gave a 20+ dB reduction to signals in the 870-890 MHz range, it didn't improve the scanner's performance much. I suspect this was due to the lack of good shielding inside the scanner--that is, the cell site RF is getting into the receiver's front end through the scanner's case (plastic front panel on the PRO-197), downstream of the low-pass filter.

It might be interesting to try a scanner that allows a remote mounted control head, where the scanner can be installed inside an RF tight metal enclosure, and a mobile duplexer or other cavity low-pass filter used on the antenna input.
 

fineshot1

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Sep 17, 2004
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2,478
Location
NJ USA (Republic of NJ)
Hello All,

I'm experiencing a great deal of Sprint/NEXTEL interference while monitoring the Oklahoma statewide trunked system in the Oklahoma City area. There are quite a few Nextel towers in the area, and anytime I drive by one the radio goes crazy.

Does anyone know where I might find an inline filter to filter out the frequencies that are causing problems? I'm probably looking for a low-pass filter that would filter out anything above 869.000, allowing the 868ish voice channels to pass through, (still not rebanded) but choking the interference at higher frequencies. I don't care if I'm not able to monitor anything about 869.000. I've looked on the internet and haven't found anything as of yet. Ideas?
Not sure if these folks can help but give them a call since you have nothing to lose.

PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts
 

Thayne

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May 1, 2002
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2,129
IMHO, and experience, it is mostly because scanners have lower selectivity in the front end RF circuitry, but at least Nextel interference is better than it was.
I always get interference around a cell site by King soopers warehouse with any scanner, within 2 blocks, but when using a Ma/Com M7100 it would not affect it until I drove within 50 feet of the tower---.
 
Joined
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Messages
1,217
Location
Tulsa
What you are experiencing is the reason for the nation-wide 860 MHz rebanding project, all the outstanding rebanding projects should be complete within the next 12-18 months. Currently the ESMR and TRS users are interleaved in the band and a single filter will not eliminate the problem
 

WayneH

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Dec 16, 2000
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7,464
Location
Sitting in an airport somewhere
Does anyone know where I might find an inline filter to filter out the frequencies that are causing problems? I'm probably looking for a low-pass filter that would filter out anything above 869.000, allowing the 868ish voice channels to pass through, (still not rebanded) but choking the interference at higher frequencies.
Sprint/Nextel's iDEN network is not a cellular network so they're using freqs interspersed with SMR, Public Safety, etc in the 851-866 band. Getting a filter to filter out 869 and above will do nothing to resolve iDEN interference. If you're primarily listening to 866-869 see if Par will build you one specific for that range. You'll have to remove it to listen to anything else. Since S/N has not rebanded there is no solution to block them that won't affect anything else unrelated. Once they reband then you can block out anything above 862MHz.
 
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