246T cellular filter

So, I am fed up with the cell phone interference that jacks up my scanner anytime I get near cell towers while monitoring an 800 mhz Motorola trunked system in my area. It causes either the radio to go completely silent, or freaks out the squelch and causes it to open. Usually it's also associated with really poor reception. I have tried using the ATT feature, but that just help marginally. It seems like it's only getting worse and worse as more towers are popping all over town. And it's irritating, because I tend to listen at work while driving from job to job ( I am a HVAC Controls Technician, so it's all around town some days) and the freeways are the worst! A tower or three every 1/4 mile or so! I gets it really bad when stuck in traffic, must be all my fellow stuck travelers texting, emailing, twit-ing or freind facing their traffic woes to all their concerned friends via their handy mobile distraction devices.

Is there a inline filter that I can add to the antenna that can filter out cellular interference? I tried searching the usual scanner and radio places on line, and came up with zilch. Will such a filter also filter out the TRS I monitor? Its located in the 850-870mHz range. I am using a magnetic mobile mount antenna attached (zip tied, LOL) to a ladder rack on the work van, if that info helps.

I like this old workhorse scanner, missing numbers on all the keys, bashed up case, and all, but are the newer offerings less prone to this? I am not opposed to buying a new scanner, but I would love to just keep this one until I absolutely have to get another one ( either it finally breaks, or the systems I like to monitor go digital, or whatever)...

So, what do you all think?

There are filters out there for what you want. Start with Par Electronics. What you want might cost you several hundred dollars.

For those out there who are a bit more adventurous, go on eBay, and buy an old 800 MHz Motorola Spectra. The go for about $25. There's a good ceramic filter pretuned to 850-870 on the front end board that's easy to get to. Enclose it in an aluminum box with a couple of BNC connectors, and you'll have a good 800 MHz filter for your scanner.

If price is no object, Decibel and Celwave made flat pack trunking duplexers that could be separated. The transmit section has an 850-870 MHz response, steep skirts, and make really excellent receiver filters. I use one in front of a spectrum analyzer when searching for noise and interference, and I can be right next to a cellsite and barely see it. Cost is several hundred dollars on eBay, and worth every penny, if serious filtering is needed.

In addition to what zz0468 said, I'd make sure that what you are experiencing is REALLY from cellular signals and not from other high power in-band signals like other trunking and conventional stuff in the 850MHz to 870MHz band.

If it is in-band a filter won't be of much use as you couldn't filter out the in-band stuff without degrading what you want - about all you could use would be notch filters which would ideally require you to know the exact frequency or frequencies of the undesired interferers and entail compromises such as degraded reception of adjacent frequencies as no notch filter is perfect.

I've experienced major in-band issues from Nextel stuff but that has mostly gone away now that Nextel has dumped iDen. I also get problems from some MDT transmissions which are constantly broadcasting (at least I think that is what they are).

Cell sites by their nature aren't the highest power RF sources individually but they are so ubiquitous and numerous that they contribute a high aggregate RF power density for a given area. Individually, you can find other sources which have much higher power and can even be co-located with your desired transmitters.

Low cost wideband receivers like typical consumer scanners not only may lack adequate filtering but also lack adequate internal shielding and use a sub-standard RF amplifier design which has a poor dynamic range making them really susceptible to in-band and near-band high level signal desense and intermodulation issues.

A well designed filter would ideally help with the above if the undesired signals are sufficiently far from what you want to receive in frequency but care should be taken when using the filter such as making sure to use good quality coax and connectors for attaching the filter between the antenna and scanner. If you make one per zz0468's suggestion I would recommend using short lengths of semi-rigid to internally connect the filter to the external connectors. Short lengths of flexible coax is ok but use as short as possible and expose as little center conductor without shielding as possible when attaching it to the panel connectors and to the filter contacts (use good RF practice for 800MHz wavelengths, etc.).

I've encountered scanners which have such poor internal shielding and antenna connections that they experience issues even with no antenna connected. The problem signals get in via other pathways besides the antenna.

If what you are having problems with are really out-of-band cellular signals (say, in the 824MHz to 849MHz mobile portion [unlikely but possible if you are right next to a mobile transmitter] or in the 869MHz to 894MHz cell site transmitter ["base"] portion) then good quality filtering should help. But I'd try and do some research first and be as sure as possible before spending the money on the filters if at all possible. At least the Motorola route zz0468 suggested is cheap but does require some RF construction knowledge for the best results.

-Mike

zz0468 said:

There are filters out there for what you want. Start with Par Electronics. What you want might cost you several hundred dollars.

Click to expand...

That's probably a bit too much $$ for what need, but I'll give it a looksey...

For those out there who are a bit more adventurous, go on eBay, and buy an old 800 MHz Motorola Spectra. The go for about $25. There's a good ceramic filter pretuned to 850-870 on the front end board that's easy to get to. Enclose it in an aluminum box with a couple of BNC connectors, and you'll have a good 800 MHz filter for your scanner.

Click to expand...

Hmmm.. That's a great suggestion. I just might go that route. I might need some assistance determining which component in the radio is the filter, but I can build it with minimal parts from Fry's or similar. I suppose I could always post a picture of the Spectra (if I can get a good deal on one) in the Motorola Forum, and ask for details there. Thanks!

Mike_G_D said:

In addition to what zz0468 said, I'd make sure that what you are experiencing is REALLY from cellular signals and not from other high power in-band signals like other trunking and conventional stuff in the 850MHz to 870MHz band.

Cell sites by their nature aren't the highest power RF sources individually but they are so ubiquitous and numerous that they contribute a high aggregate RF power density for a given area. Individually, you can find other sources which have much higher power and can even be co-located with your desired transmitters.

Click to expand...

I am pretty sure it's the Cell sites. The interference only happens within 1/8th of a mile from the tower/panels. It's become such an issue, that I know where most towers are located around town. When I get the interference, I simply look around, and sure enough, there is a cell tower with line of sight. It actually took me some time to figure this out. I always thought prior to my "revelation", that the TRS I listen to was a crappy one, LOL.

A well designed filter would ideally help with the above if the undesired signals are sufficiently far from what you want to receive in frequency but care should be taken when using the filter such as making sure to use good quality coax and connectors for attaching the filter between the antenna and scanner. If you make one per zz0468's suggestion I would recommend using short lengths of semi-rigid to internally connect the filter to the external connectors. Short lengths of flexible coax is ok but use as short as possible and expose as little center conductor without shielding as possible when attaching it to the panel connectors and to the filter contacts (use good RF practice for 800MHz wavelengths, etc.).

Click to expand...

Yeah, that's how I have planned to build this filter, if I actually do it. Use as small of an aluminum box as I can and leave very short leads to the connectors. I am planning on mounting this filter directly to the scanner with a female BNC. and then a Male BNC on the other end of the box for the antenna connection.