How to block LTE signals fading scanners

[Anderegg]

I am attempting to protect a news assignment desk 800Mhz scanner system from an AT&T cell phone tower located about 75 feet away from the scanner antennas. :-\

Yes, I know, not an optimal location to setup BCD996XT 800Mhz trunking receivers! A yagi has been installed and aimed directly at our target transmitter site, located on a mountain line of site about 10-12 miles away. The signals from this site are still being faded, even close area signals suffer. The type of interference would be the full signal strength but cutting completely out every second or so, and general static reception kind.

Stridesberg 800Mhz multicouplers with built in filters are not enough to even make a dent in this interference, so I am hoping someone here can recommend a solution, economical, for this 800Mhz ONLY receiving system. The cell tower people tell me that they are blasting out RF on 4 bands on this tower.

Thanks!

Paul

 

[WA0CBW]

Before spending money on "filters" get a spectrum analyzer and hook it to your antenna to see just what might be interfering with the signals you want to receive. Having worked with media gathering agencies I have found interference that was being caused by equipment in the building. Sometimes multiple scanners in the same room can cause interference with each other.
BB

 

[Voyager]

If one of the bands is on 800 MHz (869-894 MHz), your filter will have to be very sharp.

 

[Thunderknight]

Take a look at these folks
Filters - Mini Circuits

you basically have to start any major attenuation starting below 851 and have a lot by 758 MHz.

Assuming the desired system has been rebanded, you want a sharp cutoff above 862 MHz (854 if the desired is all NPSPAC).

 

[cmdrwill]

Paul, sorry to hear that. Is there any other radio that you can put in there? I doubt the poor little BCD996XT has any filtering in it's frontend. So even IF the interference was far from the frequencies you want to listen to, the frontend will overload, and the frequency you want will be obliterated. Pass cavity filters can be very expensive.

And this is just one of the reasons for rebanding to get PS radios away from cell phones and Nextel.

 

[ElroyJetson]

Before spending money on "filters" get a spectrum analyzer...

OK, I laughed at that. Because any spectrum analyzer that can do the job well is way more expensive than a filter.

There is another way....the shadow pole.

What's that?

If you know the location of your LTE interference generator, and your scanning antenna is in a fixed
location, you can trap out the LTE signal by sticking a grounded rod into the earth in a direct line between your antenna and the LTE site. The rod will need to be tall enough to shadow your antenna as well.

Essentially, if you were a bug crawling around on your antenna, the shadow pole would prevent you from ever being able to directly see the interfering source. So the shadow pole has to be placed to block the line of sight view of the source of the interfering signal, as seen at any point on your antenna.

How close you set the shadow pole to your receiving antenna will affect performance. You may want to experiment.

 

[SCPD]

Probably the best solution would be to relocate the scanners to a "quieter" location (an employee's house for example) and operate them by remote control. Depending on how much control you wanted you could just simply stream the audio to Broadcastify or if you wanted more control you could you use the ProScan software package

 

[Project25_MASTR]

Probably the best solution would be to relocate the scanners to a "quieter" location (an employee's house for example) and operate them by remote control. Depending on how much control you wanted you could just simply stream the audio to Broadcastify or if you wanted more control you could you use the ProScan software package

Streaming would be the way to do it on a budget. If worried about ISP issues, use a 5 GHz link.

 

[cmdrwill]

And this is just one of the reasons for rebanding to get PS radios away from cell phones and Nextel.

And it looks a lot like this rebanding does NOT work.

 

[Voyager]

The purpose of rebanding was to give NexTel a contiguous allocation of 3 MHz. They spun the interference resolution as a carrot for the FCC and the public.

 

[Anderegg]

Well, looks like a cavity filter is within the "I can build one of those" of the person in charge of managing these things. The shadow filter is a good idea, except the our news broadcasts may look strange with a new "pole" between the cameras and anchors. We only need to hear 856-869 at the moment, then 851-863 when rebanding occurs in 2054.

The remote streaming scanner idea is being pitched to us by the people who offer those services to people who need to hear things. I am pretty anti that idea, it would not work well with how we do things, and the scanners double as coasters for coffee. The ability to replay would be nice though.

Once we get a cavity filter in place, perhaps we can relocate the scanners in a lead box and remote admin them from a few feet away on a couple of computer monitors? I am relatively sure our roof is not blocking the tower RF which is about 50 feet up and 50 feet lateral of the scanners!

Paul

 

[cmdrwill]

Paul, A filter out of a receiver 800 multi coupler used at repeater sites would work to filter what gets to the scanner. But the lack of shielding inside the scanner may not help.

Be careful with which type of coax cables you use in the high RF environment. Some cables and antennas can generate inter mod. PIM, passive inter modulation.

 

[Anderegg]

Being a TV station, our "splitters" consists of 75 ohm RG59 video cables and about 300 75 ohm BNC T connectors........looks like a rats nest of bike locks. :-\

Paul

 

[ElroyJetson]

At that distance odds are that your scanner will still pick up the signal even if you capped off the antenna connector with a shielded dummy load. In which case, enough signal is getting through the scanner's case to cause problems.

The solution starts with a shielded rf enclosure that the scanner lives in. Which can be expensive and impractical.

I would suggest that if it's possible, you relocate the scanner to a location where it's not getting interference and then connect it via VOIP to your network and monitor it remotely from the desk where the scanner currently lives. OR, simpler yet, if there's already an existing scanner feed for the systems and talkgroups of interest, just monitor them via the web.

 

[prcguy]

I've had to do something similar and WA0CBW has right idea, use a spectrum analyzer to see what your up against then have a custom filter made to fix it. Without a spectrum analyzer you have no idea what your dealing with.

I've used many companies for filters from the inexpensive Tin Lee Electronics in Canada to Daden, Microwave Filter Company, TTE, Lark, K&L, Reactel and others I can't remember at the moment. Once you know the exact range of frequencies you want to get rid of and at what attenuation vs what you don't want to disturb as determined by a spectrum analyzer, you can discuss the rest with the filter mfr.

The rod or pole idea between your antenna and the offending antenna can reduce interfering signals a bit and the rod or pole does not have to be grounded but it does have to be a specific distance from your antenna. It will also alter your antenna pattern in directions you may not want changed. You would also need a spectrum analyzer to even begin to set up and optimize something like this.
prcguy

 

[tbiggums]

A spectrum analyzer would be nice if you have access to one, but I'm willing to bet it's 870-890 MHz cellular that is causing the problem. The 1900 MHz cellular sites don't really de-sense these consumer grade scanners near as bad as the 870-890 MHz ones.

Unfortunately I have to agree with Elroy, though. The lack of any real RF shielding on scanners will really limit the effectiveness of any filter you put on the coaxial cable between the antenna and the scanner.

So you could still experiment with some filters, but don't pay too much for them, and prepare to be disappointed.

I'd love to be proven wrong, though. So if you do try some filters, please let us know what you tried and how well it worked.

 

[cmdrwill]

These cell sites even kill my SAT receiver.

 

[prcguy]

What type/brand sat receive is being affected by cell sites?
prcguy

These cell sites even kill my SAT receiver.

 

[cavmedic]

Get an 800 XTS series and have it programmed non affiliate to the system you are listening to .