Figuring out where my interference is coming from (again)

[Kumba]

I know I randomly repeat this topic, but it's such a low item on my priority list, I don't get around to messing with it often. I've mentioned before that I live the shadow of a large radio tower that's got 6GHz & ~25GHz microwave stuff hanging off it it (per antennasearch.com and FCC ULS searching), but something's generating interference somewhere. Not sure how else to track it down. Standing in my living room with the scanner and listening to the control channel while aiming in different directions, or turning the scanner a certain way doesn't seem to affect things much, other than increase the amount of static that I pick up.

I've thought about getting a spectrum analyzer....something that can scan the 800MHz range and help me identify nearby interfering frequencies. Looking on eBay turns up a few specimens at an affordable price range...most are old HP models, and most don't go anywhere near the 800MHz range (As far as I can tell, anyways). Does anyone know a good model to hunt down, preferably for ~$200 or less? Definitely talking about a used spectrum analyzer here...new would be well into the thousands range.

Or can my scanner's (a BCD396XT) bandscope do this, and I just need to find the right software? I had software once that did graph out frequencies, but I forget what it was. It might've been ARC396 Pro.


Anyways, in a feeble attempt to combat this using a portable antenna, I picked up a Radio Shack 800MHz antenna a few days ago, and when I put that on the scanner, the control channel all but gets flooded out by static hiss. An SRH77CA (and a spare RH77CA) that I have will pull in a signal. Yet!, If I use the attenuator with the RH77CA antennas, I lose most of the signal (mostly static hiss), but with the RS 800MHz, the attenuator actually allows it to detect the control channel (but decoding is more spotty than w/ the RH77CA antennas and no attenuator).

I'm targeting the 868.2625MHz frequency, system ID A42E. I also ordered a Larsen NMO800 (has the 5/8 wavelength over a 1/4, I think) for my car (on a Larsen tri-band right now). Even though the NMO800 does 800-866MHz, all the research I've done indicates it'll still suck in 868MHz ranges easily.

I've uploaded audio samples captured from the headphone port of the interference at:
http://drop.io/milgtq0

They are:

• 868-2625-no-antenna-att-wav: No antenna attached (just the BNC stub) with the attenuator turned ON.
• 868-2625-no-antenna-noatt-wav: No antenna attached (just the BNC stub) with the attenuator turned OFF.
• 868-2625-rh77ca-att-wav: RH77CA antenna and the attenuator turned ON.
• 868-2625-rh77ca-noatt-wav: RH77CA antenna and the attenuator turned OFF.
• 868-2625-rs800-att-wav: RS 800 antenna and the attenuator turned ON.
• 868-2625-rs800-noatt-wav: RS 800 antenna and the attenuator turned OFF.
• system-a42e-audio-rh77ca-noatt-wav: ~1min 11sec of normal audio from both Fire/EMS dispatch and Police dispatch using the RH77CA antenna and no attenuator. Signal is weak, as noted by the digital distortion.

I should also add that Squelch is at '0', TGIDs are marked as either digital or analog (but I only ever listen to the digital channels), and the scanner is perched on top of my LCD monitor while making the audio captures (BCD396XT).

Regarding the tower nearby, this is it at AntennaSearch:
http://tinyurl.com/3xx4vh7

FCC Page:
http://wireless2.fcc.gov/UlsApp/AsrSearch/asrRegistration.jsp?regKey=106241

There are two listed antenna sites attached to the above tower (AntennaSearch):
http://tinyurl.com/35b7yv4
http://tinyurl.com/36k7xo2

FCC Pages:
http://wireless2.fcc.gov/UlsApp/UlsSearch/license.jsp?licKey=2976933
http://wireless2.fcc.gov/UlsApp/UlsSearch/license.jsp?licKey=1037583

I know it's got to be something on/near that tower, because once I drive away from my neighborhood, the control channel audio comes back really strong. I've tested this a countless number of times, from both sides of the road and directions of travel. I get the same interference/static hiss on the portable antennas and the larsen tri-band mounted on my car. This makes me wonder if there's any health threat from being so close to the tower (granted, I'm a good 150ft away, but there's some person living in the home in whose backyard this tower sits). Although, at the height of the microwave transmitters, and because I believe them to be fixed line-of-sight, the resultant radiation is probably pretty minimal.

 

[davidgcet]

according to what i see at the FCC site, there are some UHF licenses close to you as well. it could be one of those getting in your scanner, but it would tend to be sporadic so i doubt it. more than likely it is the proximity to the transmitter just saturating the scanner RX. you could even be picking up a local oscillator in one of the stations.

when i pull up close to a tower with my service monitor the spectrum analyzer gets spikes all over it, usually it is not splatter but simple front end overload. just like if you key a radio too close to another it can blank the RX of the 2nd radio, being close to a tower can overload front ends and kill out all signals. i doubt a SA, especially a <$200.00 one would tell you anything due to the same issue at proximity. i used to have a GPS that would not work within 1/4 mile of ANY tower that had a cell/pcs site on it.

i could not listen to your files on this machine, my main laptop is DOA right now and this one the speakers don't work.

 

[Kumba]

according to what i see at the FCC site, there are some UHF licenses close to you as well. it could be one of those getting in your scanner, but it would tend to be sporadic so i doubt it. more than likely it is the proximity to the transmitter just saturating the scanner RX. you could even be picking up a local oscillator in one of the stations.

Yeah, there's two school transmitters for the county nearby, I think a VZW tower about a mile away, and I know there's also a public safety tower about ~2mi away (which I think is the closest one to me). Looks to be a small-time ham operator west of me, Nextel tower 2-3mi southwards. But if any of those were interfering, I'd pick something up, I assume, all over the main road adjacent to my neighborhood. Instead, the interference is limited mostly to about...1/8th of a mile to the left and right of the tower.

when i pull up close to a tower with my service monitor the spectrum analyzer gets spikes all over it, usually it is not splatter but simple front end overload. just like if you key a radio too close to another it can blank the RX of the 2nd radio, being close to a tower can overload front ends and kill out all signals. i doubt a SA, especially a <$200.00 one would tell you anything due to the same issue at proximity. i used to have a GPS that would not work within 1/4 mile of ANY tower that had a cell/pcs site on it.

Pretending for a moment front-end overload is what I am experiencing, is there a way to combat it? Or would I just need to tough it out for another few years until I can afford someplace better?

i could not listen to your files on this machine, my main laptop is DOA right now and this one the speakers don't work.

Got a pair of headphones? Most laptops should have a headphone adapter on them.

 

[davidgcet]

if it is overload, it should get better with attenuation or distance from the tower. since it seems to get better more than 1/8 a mile away, i would almost say it is overload. if your antenna is directional and faces away from the tower you can try putting a corner reflector behind it to act as a shield to block some of the signal from the tower.

far as listening, i have a 6 and a 2 year old headphones disappear like crazy around here! i had 2 sets in my laptop case the other day, now no one has seen them! i meant to listen to it at work today, but it was so hectic i never evne got a chance to get online at all. if i find a set of headphones after while i will listen to it.

 

[jim202]

Have you ever thought that you might be getting hammered by one of the Nextel towers near you?

Why do you think all the public safety agencies are into the rebanding that is going on right now.
Not saying that Nextel is the source, but there is a chance they are. If so, pack up and move
someplace there isn't a cell tower near by. It could be one of the other cell carriers that still
use the 800 band.

There is a slight chance that your getting hit by a 900 MHz paging transmitter. However, normally
you can hear the data and they do go off the air now and then. Even if it is only for a few seconds.

 

[davidgcet]

ok, i finally got to listen to the files and it does seem to get better with attenuation from what i here. that tells me you are getting too strong a signal. this could be the control channel itself is coming in too strong, or a cell carrier is overloading you. if you look at the tower itself, there is a AT LEAST 1 cell carrier on it. see those stand offs with the 3 vertical panels per side, that is a cell carrier. because they hold geographical licenses they do not have to license per tower, so their sites don't always show up on the FCC database. if it is Nextel, that would explain it. once rebanding is done they will go away, but until then not much you can do besides use a lower gain antenna and a possibly an attenuator.

 

[neondesert]

Pretending for a moment front-end overload is what I am experiencing, is there a way to combat it? Or would I just need to tough it out for another few years until I can afford someplace better?

If you are experiencing overload of the front end, I would suggest you look into some notch filtering to help eliminate or reduce the offending transmissions.

A good place to start would be to check out this link: PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

I have read a lot of good reviews about them on this site.

 

[Kumba]

Have you ever thought that you might be getting hammered by one of the Nextel towers near you?

Why do you think all the public safety agencies are into the rebanding that is going on right now.
Not saying that Nextel is the source, but there is a chance they are. If so, pack up and move
someplace there isn't a cell tower near by. It could be one of the other cell carriers that still
use the 800 band.

There is a slight chance that your getting hit by a 900 MHz paging transmitter. However, normally
you can hear the data and they do go off the air now and then. Even if it is only for a few seconds.

As far as I know, there is nothing owned by Nextel hanging off of that tower. Per AntennaSearch, the nearest Nextel tower is a few miles away, and I would doubt that tower is causing the problems. If Nextel does have something hanging off of that tower, then I have no way of knowing with my current knowledge, because nothing is listed in the FCC or on AntennaSearch, and I lack the knowledge to be able to spot the things visually.

All I know is there is 6GHz and 23-25GHz equipment on that tower. This is why I was wondering about a spectrum analyzer to see if it could identify other frequencies that might be emanating (or are at least very strong) and maybe shed some light on things.

And I'd love to pack up and move, but I have to wait at least another two years to avoid paying back the tax credit I got for buying this home. Wanna send me $8k?

ok, i finally got to listen to the files and it does seem to get better with attenuation from what i here. that tells me you are getting too strong a signal. this could be the control channel itself is coming in too strong, or a cell carrier is overloading you. if you look at the tower itself, there is a AT LEAST 1 cell carrier on it. see those stand offs with the 3 vertical panels per side, that is a cell carrier. because they hold geographical licenses they do not have to license per tower, so their sites don't always show up on the FCC database. if it is Nextel, that would explain it. once rebanding is done they will go away, but until then not much you can do besides use a lower gain antenna and a possibly an attenuator.

So this might explain why the RH77CA antennas work better without the attenuator, because they're lower gain, while the Radio Shack antenna, which has about 6db of gain, needs the attenuator just to pick anything up? If there is indeed a cell carrier hanging off of that thing, I'd love to know who it is. It likely is Nextel, because AntennaSearch identifies a Verizon tower a few miles away, so they're set for coverage. Maybe AT&T? I have a small cellphone through them, and it gets a solid six bars at home.

If you are experiencing overload of the front end, I would suggest you look into some notch filtering to help eliminate or reduce the offending transmissions.

Thanks!, this is a good site to keep in mind. I'll have to figure out (somehow) what frequencies are overloading my scanner first.


I also got the Larsen NMO800 in for my car. So far, it's hard to tell if it trumps the tri-band I used to use. I did notice yesterday when leaving home, that as I drove parallel to the tower, the signal improved, but once I turned perpendicular to the tower, the interference came back. So that implies some kind of polarization going on that I can't seem to detect with just the rubber duckies on the unit.

If that doesn't seem to help, I'll try the NMO800B next, which is the small unity gain 800MHz antenna Larsen makes.

 

[jim202]

Have you tried looking on the door of the equipment shelters at the tower to see if there is a large sign
for contact info for emergencies? Most of the cell sites I use to go to were required to post contact
info in case of emergency. Take a pair of binoculars and try reading the signs.

In the info you supplied, one of the holders of the microwave was Verizon. You may have just the
luck that Verizon is using that site as one of their 800 MHz sites. Good luck trying to get out from
under the overload if this is the case.

The other thing is I can't see clearly from the sat picture just how many levels of cellular antennas are
on that tower. Plus the picture is probably 5 or 6 years old anyway. Take a photo of the antennas
and try posting it here for the group to look at.

You may have to look at finding a pass cavity to be able to listen to the public safety trunking system
there. It may or may not help depending on just how far away the cellular channels are.

 

[Kumba]

Have you tried looking on the door of the equipment shelters at the tower to see if there is a large sign
for contact info for emergencies? Most of the cell sites I use to go to were required to post contact
info in case of emergency. Take a pair of binoculars and try reading the signs.

The tower is literally in someone's backyard, so that's a bit out of the question. I'll get a photo of the tower though and post it later tonight.

 

[rfradioconsult]

From what you describe is in-fact interference from Nextel. First Nextel is NOT a cellular company they are an ESMR service provider, which in the present configuration is shared within the same band as Public Safety and other trunked radio systems users. This has created interference with other users especially the PS users; per the Nextel FCC settlement, Sprint Nextel agreed to bear the cost of relocation or re-banding all affected PS users from the 866-869 MHz spectrum to the 851-854 MHz spectrum, this project is currently underway and should be complete by 2012.

If you note your frequency of interest is within that "re-branding" band. In exchange for moving from the shared band Nextel will re-locate to the 866-868 Mhz band. Here is a link to the 800 TA page.

800 MHz Reconfiguration TA: Home

When trying to locate Nextel sites look on buildings for a slim panel antenna about 4'tall by 10" wide with a set of three pointing at an azmuth of 120 degrees between them, the may be mounted on different sides of the building rather than in a cluster.

 

[Kumba]

From what you describe is in-fact interference from Nextel. First Nextel is NOT a cellular company they are an ESMR service provider, which in the present configuration is shared within the same band as Public Safety and other trunked radio systems users. This has created interference with other users especially the PS users; per the Nextel FCC settlement, Sprint Nextel agreed to bear the cost of relocation or re-banding all affected PS users from the 866-869 MHz spectrum to the 851-854 MHz spectrum, this project is currently underway and should be complete by 2012.

If you note your frequency of interest is within that "re-branding" band. In exchange for moving from the shared band Nextel will re-locate to the 866-868 Mhz band. Here is a link to the 800 TA page.

800 MHz Reconfiguration TA: Home

When trying to locate Nextel sites look on buildings for a slim panel antenna about 4'tall by 10" wide with a set of three pointing at an azmuth of 120 degrees between them, the may be mounted on different sides of the building rather than in a cluster.

Yeah, I'm familiar with the whole rebanding thing going on. I was checking around the rebanding websites months ago to try and pinpoint a date for when my county rebands. I should check it again.

Speaking of those tower designs, I've seen those before. Except mounted on poles. I always thought they were the antennas for the public safety system used by various counties, not cellphone towers.

I didn't get a picture of the offending tower near me...forgot to, basically. I'll try again tomorrow or on the weekend.

Outside of a spectrum analyzer, is there some tool I can plug into an antenna line to check the received signal strength in dB?

 

[rfradioconsult]

You are getting into high-end test and measurment equipment that is way over the budget of the typical radio listener. If your scanner has "close call" function you may be able to track it down. If your suspect tower is used as a microwave site as you described I doubt it wouls be the source of your interference; microwave antennas are very directional with a narrow beam width, in a lot of m/w systems they can actually operate both radios on the same frequency with no interference.