Band Pass Filters

[cjabass123]

Are there any filters out there that can be used to eliminate cell tower interference?

I mostly monitor re-banded trunk systems between 851 - 860 mhz. Almost every time I drive near a tower, an otherwise strong signal turns to varying degrees of crap, from making the audio scratchy to very distorted to the loss of signal entirely... until I get 1/4 to 1/2 mile away.

I would love to have one antenna set up with an in-line filter that only allows the 851 - 860 mhz band through while rejecting everything else. And have a second antenna set up normally with both going to a switch. When I encounter the tower problem on the un-filtered antenna, I could switch over to the filtered line and get some relief.

Is this possible? If so, where can I get that filter?

 

[popnokick]

What radio / scanner(s) are you using that are experiencing this problem? Part of the problem may lie in the receiver(s).

 

[zz0468]

Are there any filters out there that can be used to eliminate cell tower interference?

Is this possible? If so, where can I get that filter?

That would be a custom filter. Go here:

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

 

[cjabass123]

I've had a 996T for years and just got the 536hp a couple days ago. The 536 still suffers from tower interference. I have not had a chance yet to see if the IF exchange feature helps.

Thanks for the link.

open to more inputs. hoping to save the drudgery of research.

 

[cjabass123]

PAR Electronics a "no go" for 851 -860 notch

That would be a custom filter. Go here:

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

PAR responded to my request very quickly. cudos. Unfortunately, they do not have the filter I want and they are too swamped with milcom contract work at the moment to R&D this particular filter.

I guess I'm going to have to do some research and make some calls, unless UPMAN has anything up his notoriously ingenious sleeves

 

[zz0468]

PAR responded to my request very quickly. cudos. Unfortunately, they do not have the filter I want and they are too swamped with milcom contract work at the moment to R&D this particular filter.

Like I said, it would be a custom filter. The normal filter range is going to be 850-870, and you'll find that range is readily available from several sources.

I guess I'm going to have to do some research and make some calls, unless UPMAN has anything up his notoriously ingenious sleeves

I doubt UPMAN is going to be able to help you. Uniden doesn't do custom filters. Your next best option would be to find a tunable band pass filter, along with someone with the sweep gear necessary to tune it to your requirements. An 850-870 tunable filter could probably be moved down to 841-861 easily enough.

A window filter can be made with a few tunable cavities and critical length cables, but that requires time with a vector network analyzer, and familiarization with Mr. Smith's handy chart.

I don't know who else makes custom filters for the scanner market, but Angle Linear in California and EMR in Arizona will make custom filters. If you have to ask the price for either, you can't afford it.

 

[cjabass123]

850 - 870 might work

Since re-banding, has anything moved into the 860 - 870 spectrum? I realize that these filter outfits usually like to know the "offending" frequencies because most people that get filters often have a strong FM , TV or WX transmitter giving them grief. In my case, I just want my little notch preserved. Thanks for the info again.

 

[nr2d]

Band pass filters will most likely help your problem. It's just finding one made to your requirements. Another factor when using band pass filters is insertion loss. I found that band pass filters can have any where from 1 to 5 dB of insertion loss.

If your having problems in the general location of cell phone towers then you may be have brute force interference. A band pass filter may not help in this case.

Just remember that scanners are wide band receivers. They have none or very little filtering in the front end of the receiver. They will be subject to not only brute force RFI but also receiver intermod. This is when 2 or more close transmitters create a 3rd frequency in the front end of your receiver. This 3rd frequency is either on or very near the frequency that is being affected.

 

[cjabass123]

I agree. In my case, I'm not concerned about insertion loss. Since i prefer to only hear communications near to my current location, my range settings are small so the signals I receive are usually quite strong. I doubt a little insertion loss is going to affect me significantly enough to worry about.

Even with full bar signals, towers still wreak havoc on my listening pleasure so I am willing to try filtration (if I can find one at an affordable price).

Given my limited novice understanding, I think what I need is a "notch" filter that allows my desired freqs thru while rejecting everything else on either end. The hunt is on....

 

[Thunderknight]

Take a look at Filters - Mini Circuits and see if you can find something in that range. Just make sure of the type and connector.

Edit: oh, you have the term notch backwards. Notch removes a particular frequency/band. You want band pass for the freqs you want.
Or, maybe a low pass to block cell above 861 MHz.

 

[cjabass123]

hehehe... like I said, my understanding is weak. Thanks for the info

 

[tbiggums]

I doubt any kind of 800 MHz filter will work very well with a typical scanner. They don't have any shielding to speak of, so the unwanted 800 MHz cellular signals will just get in through the scanner's plastic case or front panel, downstream of any filter you put inline. This cellular interference is usually only a problem when you're within a quarter mile of an 800 MHz cell site, so those unwanted signals are really strong.

The only thing that might work would be a mobile unit that has a remote head on it, where the scanner itself could be installed in a RF-tight metal enclosure, and a suitable bandpass or lowpass filter installed inline.

A while back, I tuned one side of a small 800 MHz mobile duplexer to pass the 760-860 MHz. It actually looked like it would work pretty well, with around 30 dB of attenuation around the 800 MHz cellular band. Unfortunately it hardly made any difference with a PRO-197. I suspect the unwanted signals were getting into the scanner through the plastic front panel, downstream of the filter.

It might be interesting to try with a Uniden 996XT installed in an RF tight enclosure with remote head kit. But since Uniden scanners can't decode P25 trunked system talkgroup patches correctly, I won't be buying any to test out.

 

[cjabass123]

My 996T and 536hp have metal housings. I've tried using a 3 inch 1/4 wave antenna, a 1/4 wave vhf antenna and a 5 db gain 800 mhz antenna on my vehicle. They all suffer from tower interference. As an experiment, what if I wrapped the entire radio in layers of tin foil? Would that eliminate the contention that the unwanted signals are entering through the radio itself somehow and not thru the antenna?

I wouldn't need to see the display while I drive by some known towers that give me problems. I'd be able to tell from the audio quality. I'll report back on my findings.

PS it is my belief that the problem is caused by the strong signals bombarding my antenna.

 

[tbiggums]

You could try the foil plus a filter, but it doesn't sound really practical. Even if it does work, you'll still want to have access to the display and keypad sometimes, and it won't be really easy to open and close the foil while you're driving...

One of my previous employers had a metal enclosure that was designed to provide an RF isolated environment for testing electronic boards or devices, and I performed a quick little test with a VHF portable radio tuned to the local 162 MHz NOAA weather channel. I put the portable in the enclosure with the volume turned up, and found that it would still receive it just fine even if the access door was almost closed--that is, maybe a 1/4" gap along the bottom of the door. Once the door was fully closed, and all of the little copper fingers along the door made contact with the enclosure, the portable radio squelched just as if the transmitter had been shut off. So it doesn't take much of an opening for even a VHF signal to get in (and the NOAA transmitter was a good 8-10 miles away). An 800 MHz signal would have an even easier time getting in through small gaps in the shielding.

 

[w2txb]

Check out this web site: PAR Electronics

The founder/owner/president (Dale Parfitt) is also a native of my original home town and knows his stuff.

 

[jpryor]

Get a BPF-800-N band pass filter 800 MHz narrow (745 MHz - 861 MHz) from scanner master and that will knock down all the stronger 861+ cellular signals.

BPF-800N

 

[cjabass123]

found ths:

Current / Planned Technologies Previous Technologies Band UHF Frequency (MHz)

3G, 4G, MediaFLO (defunct), DVB-H UHF TV 52-69 (698-806 MHz)* 700 698–806
SMR iDEN, ESMR CDMA (future), UHF TV 70-83 (806-890 MHz) 800 806–824 and 851–869
GSM, IS-95 (CDMA), 3G AMPS, IS-136 (D-AMPS) 850 824–849 and 869–894


It seems possible that the offending frequencies could just as easily be below 851 as above 860. The 745 - 860 filter from scannermaster may solve some issues, but not all.

I'll have to decide if spending the money, albeit a reasonable price, to solve potentially only have the problem, is worth it. Good to know. I may ask scannermaster if they can ask the manufacturer if it's possible to modify one to narrow the filtration down to 851 - 860.

I do appreciate the info!!!

 

[zz0468]

You're playing a guessing game when it comes to determining what's actually bothering your scanner. Without a spectrum analyzer it's close to impossible to do anything other than guess. That said, if you must make a WAG on what to do, you're probably safe buying a standard off the shelf 850-870 band pass filter. That would take the vast majority of junk you don't want out of your receiver.

 

[cjabass123]

Found this info:

GSM800 – not exactly a GSM but SMR iDEN technology that uses 806-824MHz and 851-869MHz frequencies. Though soon it may also be used for CDMA

GSM850 – main cellular band in USA used with GSM, CDMA and 3G technologies. Uses 824-849MHz and 869-894MHz frequencies

I found some possible filter choices. Awaiting a price quotes. here's the links.

BANDPASS FILTERS - B01845M1

cf = 845 low stop 0 -824 high stop 869 -2000

This should kill the lower GSM800 freqs and the upper GSM850 freqs.

Another choice is;

AMTI - Search results for keyword: b03882m1

cf = 882 lower stop 0 - 489

This one would kill both the lower GSM800 and GSM850 and could be used in conjunction with the BPF-800-N from scannermaster that kills the upper part of GSM850

But I am really excited about a filter from K&L that is centered at 855 Mhz with a 10 mhz spread. Awaiting a price quote on that one as well. Almost afraid to hear the price!

 

[zz0468]

Found this info:

I found some possible filter choices. Awaiting a price quotes. here's the links.

BANDPASS FILTERS - B01845M1

cf = 845 low stop 0 -824 high stop 869 -2000

This should kill the lower GSM800 freqs and the upper GSM850 freqs.

You don't want that one. You're missing the most important specification, the passband... "Bandwidth at 3 dB 840 - 849 MHz"

Another choice is;

AMTI - Search results for keyword: b03882m1

cf = 882 lower stop 0 - 489

This one would kill both the lower GSM800 and GSM850 and could be used in conjunction with the BPF-800-N from scannermaster that kills the upper part of GSM850

I don't think you want that one, either. The center frequency of the passband is 882 MHz with a 3 db bandwidth of 25 MHz, right smack dab in the middle of the band you want to get rid of.

I wouldn't advise you to get either one of those. You'd be VERY unhappy with the results.

But I am really excited about a filter from K&L that is centered at 855 Mhz with a 10 mhz spread. Awaiting a price quote on that one as well. Almost afraid to hear the price!

K&L makes some very nice filters. I have a few in my junque box. I'd be surprised if they sell a single unit to an individual. Expect Govt/Aero$pace price$.

My suggestions? Get thee to eBay promptly, and buy one of these:

Celewave 800 MHz Duplexer with Motorola PT TDF6980A | eBay

Connect the transmitter port to your receiver, and the antenna port to the antenna. You can leave the receiver port alone, but best if you put a 50 ohm load on it. You'll end up with a tight 850-869 MHz passband filter in front of your receiver that will make the vast majority of your problem go away.

Pssst. It's what I use.