700 MHz public safety bandpass filter

[Tim-B]

Does anyone know of a good high quality bandpass filter that will allow the 763 to 775 MHz public safety frequencies through but block everything else? I mainly want to filter out cellular 4G LTE data in the 700 MHz band so I can install a good high gain antenna in the center of my car's roof and not get cellular overload. I found filters that do the opposite - they let in LTE but filter public safety but I have not found any that filter LTE and let in public safety. Is there one out there? Price is no object.

 

[737mech]

Go ask Dale

Sounds like you need a custom from Dale? PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts He's awesome at filters and his customer service is umatched!! He can probably make anything you need.

 

[Tim-B]

Excellent, thanks.

 

[GTR8000]

FYI the public safety narrowband segment runs from 769-775.

758-763 (D-Block) and 763-768 are the public safety broadband segments, i.e. FirstNet LTE

 

[Tim-B]

Oh yeah, right, thanks. I just want the voice portion from 769-775

 

[GTR8000]

No problem, figured if you're going to have a custom one made, you'll want it as narrow as possible.

If you don't mind, please update this thread if/when you get one, I'm interested in the outcome.

 

[Tim-B]

Sure, I'll report what happens. Right now I am in the planning stages. I will be buying a new car sometime in the next few months and I figured that this time I will go all out and get a commercial high gain antenna professionally mounted in the middle of the roof so that I have no coverage gaps while on the road and get a mobile scanner mounted in-dash. This setup though will be too much antenna for any scanner's receiver so I need a filter to cut out the LTE overload that is really bad in some places along interstates and major highways where there is unobstructed line of sight to cell towers. What I really need is a filter to cut out all frequencies in all bands other than 769-775 so 800 MHz cell towers and VHF paging don't bleed in as well. It's a shame that no one makes a good high end scanner with better selectivity so that we can just use the best antenna setup we can find and not have to worry about cell site overload.

 

[GTR8000]

If you're going to spend that much to do it right, you might just start looking for a decently priced 700/800 MHz radio. Depending on your needs, you could probably pick up a nice XTL on eBay for a few hundred, which would blow the doors off of any consumer grade scanner. If you're monitoring systems that are Phase II TDMA, you'll need something that will cost a bit more than a used XTL.

 

[GTR8000]

So assuming that your primary focus is Lafayette Parish, I see that there is a decent amount of conventional 800 MHz P25 being used, and that the LWIN system is Phase I FDMA.

I see no reason why a nice 7/800 XTL wouldn't be perfect for your situation.

 

[Tim-B]

I would absolutely love to do that and I would gladly spend the money to do it but I don't know anyone who can program it for me on a 700 MHz trunked P25 system and set it so that it never tries to affiliate or transmit ever. I just don't have the time to learn the motorola programming manuals. And can those be programmed to monitor a trunked system without the system key?

 

[GTR8000]

Motorola radios require a system key to program a trunked system. Most radios accept a software generated key. I really don't want to say any more than that, as the subject is taboo. But, where there's a will, there's a way! You might want to check out the austech forum, where stuff like this is more openly discussed.

 

[jonwienke]

This setup though will be too much antenna for any scanner's receiver so I need a filter to cut out the LTE overload that is really bad in some places along interstates and major highways where there is unobstructed line of sight to cell towers.

Not true. I have an ST-2 mounted 30 feet up outdoors, which is about 1500 feet ftom a cell/pager tower with a clear line of sight between the two. Yet I have zero trouble picking up the 800MHz system from the next county, using my 436. And I have no filters on the coax, not even for FM broadcast or pager frequencies.

 

[Tim-B]

A lot of that really depends on where you are or more accurately the quality of equipment used on a given tower. When my hometown used an 800 MHz system you could not go within a half mile of any one of the "A" block carrier's cell sites without the scanner going completely deaf. As the years passed by and they replaced older equipment with newer stuff I noticed that certain sites that badly overloaded my scanners now had no bad effects at all. I could park right under the cell tower and still hear the 800 system that in the past went silent within a half mile of the same tower. When I was working in Southeast Texas last year I could go all over Colorado County and most of Fayette and Washington counties and monitor the TXWARN 700 MHz system just fine. But when I drove south from Columbus to Palacios where there were older rural cell sites on very tall towers my scanners were deaf over half the time with the S-meter wildly bouncing from zero to 5 bars back to zero back to 5 bars like a yo yo over and over. The same thing happened on the highway that goes from Sealy to Rosenburg, TX. The scanners were completely deaf over half the time and I noticed the same type of older rural cell sites. I drove the 40 mile or so stretch along I-10 across the Houston metro area and I remember only one single site overloading my scanner. It was clear and strong the rest of the way. That seemed somewhat counter intuitive since you would expect Houston to be RF hell but nonetheless it was clear except for near that one tower.

I mainly want to use the setup I described to monitor the Louisiana LWIN statewide 700 MHz system but I do know of a dozen or so sites in rural areas in certain places in Louisiana and I can tell you exactly which towers and where they are that will make any unfiltered scanner go deaf. Some will do it on a stretch of two to three miles on the interstate. I guess those have not been upgraded to whatever newer equipment they are using in my hometown.

 

[GTR8000]

Not true. I have an ST-2 mounted 30 feet up outdoors, which is about 1500 feet ftom a cell/pager tower with a clear line of sight between the two. Yet I have zero trouble picking up the 800MHz system from the next county, using my 436. And I have no filters on the coax, not even for FM broadcast or pager frequencies.

Well yay, lucky for you with your particular setup, however making a blanket statement is silly. Anyone who knows the first thing about RF knows that there are many factors that contribute to intermod, front end overloading, RFI, etc. No one who had any real experience in the field would ever make a sweeping statement implying that their particular situation applies to everyone else. :roll:

 

[jonwienke]

It depends on the scanner. The pager transmitter spams some of my Baofengs across the entire VHF band, but my 436 doesn't seem to notice it, even though it is attached to the outdoor ST-2.

 

[Tim-B]

I'm just hoping any praying and waiting that someday at least one manufacturer will wake up and realize they need to make a high end model that is comparable in reciever quality to commercial grade equipment. I would glady pay the higher price tag for half a dozen of them. The fact that so many people buy Unication pagers and commercial Motorola radios to use as scanners proves that the scanner manufacturers are missing a market. Make it. I will buy it. No, wait, I will buy several. And so will others people.

 

[KB7MIB]

A 700/800 MHz public safety spectrum only scanner, with a commercial grade receiver, would probably benefit a lot of scanner listeners. Especially those who deal with LSM issues.

John
Peoria, AZ

 

[jonwienke]

Well yay, lucky for you with your particular setup, however making a blanket statement is silly. Anyone who knows the first thing about RF knows that there are many factors that contribute to intermod, front end overloading, RFI, etc. No one who had any real experience in the field would ever make a sweeping statement implying that their particular situation applies to everyone else. :roll:

I'm simply pointing out that all scanners are not created equal, and that the x36 scanners do just fine in RF conditions that deafen many other scanners and radio receivers. In my case, I have strong local signals in the cellular, FM broadcast, and pager bands, and they have very little impact on my ability to scan, even though I have an outdoor antenna designed to pull in as much signal as possible, and no external filters.

The OP doesn't specify what scanner is using, but given his statement about losing the ability to hear his local public safety system every time he gets near a cell tower, I doubt it's a 436 or 536. I've never had that problem.

 

[Tim-B]

I've had it happen with every model scanner I have owned. On the Texas job that I described last year the scanners were a 396XT and a 436HP. I tried every antenna type I had from the radio shack 800 to short stubby race antennas. The scanners were overloaded with any setup I used. This was inside a car while mobile. I have had it happen with Uniden and GRE scanners and RS models made by Uniden and GRE. The GRE scanners seemed to do a bit better than the Uniden ones unitl they came out with the PSR-500. That thing has a front end like a funnel.

 

[jonwienke]

Given the crappy reception you get with a whip antenna inside a vehicle, I highly doubt your problem was overload.