Oddball Interference, For Informational Purposes

Wed Oct 20, 2004 9:29 pm

Hey gang. Came across something that I thought the rest of the group may find helpfull. I picked up a new customer a week or so ago and they had a complaint about an intermitent interference, usually on a Friday since last spring. They called me this morning to let me know that it was back, but not as strong as it normally is and that is has been weaker for the last month or so.

I took a ride over to thier facility to check it and it sounded like a SAR-SAT type transponder signal. (ANNOYING beep and a squelch tail roughly every 1/2 to 3/4s of a second) I disconnected the antenna from thier portable and it went away immediately. The signal was stronger outside the building, but still not audible without the antenna, so I borrowed the radio and went back to the shop (About 2 air miles away) to grab some equipment. All the while, I would keep removing the antenna but gave up once I got into town. When I got to the shop, I thought I would try the antenna trick for ha-ha's again and this time it was clear as a bell without it.

I punched the freq. into Google (462.400) to see if anything came up on it. Turns out, this freq is used in some GPS/RTK Surveying equipment. 1 mile up the road (About 3 air miles from the customer's facility) is a new site that is going up for Lowes and I happened to see the surveyors out there as I went by this morning. I did'nt think this would be the source since it was so strong, the surveyors had only been there for a couple of weeks and the customer told me the interference was happening since the spring.

I took a ride up anyway and the surveyor was nice enough to check in the menu on his little computer screen for his operating freq. and sure enough, here was my interference. He changed the channel and it was gone. My only confusion, which I relayed to him, was that the customer stated the length of the interference was for some time. Turns out that the company had been surveying the site across the street from my shop for a new grocery store in the spring and has been back there on Fridays to assist with ongoing site surveys on a regular basis. (Hence, the stronger signal at those times.)

Below is a list of the common freqs. that are licensed to this type of equipment, so if you get an odd birdie, check these UHF freqs first. Beats the hell out of grabbing a Yagi and spending your afternoon on a fox hunt.

461.025 MHz
461.075 MHz
461.100 MHz
462.125 MHz
462.375 MHz
462.400 MHz
464.500 MHz
464.550 MHz
464.600 MHz
464.625 MHz
464.650 MHz
464.700 MHz
464.725 MHz
464.750 MHz
469.500 MHz
469.550 MHz

Don't be fooled by the low power out on these units either. These guys had a gain antenna set up on a fairly high piece of mast (15') which gave them a fair amount of TX distance.

olderookie · Post by **olderookie** » Thu Oct 21, 2004 5:23 am

same thing here in st louis
except they are working on a long term airport expansion, so it will be years . we only hear it when we are off hook and it so happens to be the repeater output on leased freqs. nothing we can do but complain to the comms company and they say we just have to put up with it.

I know it will be years until the airport is done , so my question is this how can they use data in an intinerant mode on this freq and we are using the same freq in LMR?

BTW this can be hears in most of the metro area

RKG · Post by **RKG** » Thu Oct 21, 2004 9:27 am

We have also had problems with the use of unlicensed LMR frequencies by surveying companies. What they are doing is setting up local GPS differential correction systems, and using the LMR channels for backhauling the correction data. Surprisingly (since at least in this neck of the woods the FCC does nothing about interference complaints), the FCC shut down both of the operations we reported to them.

Apparently the equipment they use allows them to select a frequency at will. None were licensed on the frequency they selected.

bcooper · Post by **bcooper** » Thu Oct 21, 2004 8:09 pm

Boy, does this answer my question. We've had this interference on 461.025 for a couple of years and it's strong enough to cover our repeater in some areas. Thanks for the info.

Woody_99 · Post by **Woody_99** » Thu Oct 21, 2004 9:17 pm

Local guys here, an engineering company was showing me their GPS stuff just the other day. They had on them that exact list of frequencies, and this company did indeed have the license. They are cool though, because the reason I was there? They wanted some UHF radios with those freqs in them so they could find a clear channel when they roll into town and not just start transmitting. Good thing too, as the gps box I had in my hand was all heatsink, (had a blue display on the end) and would put out 35 watts!
Explains a LOT of noise...

They had this big 100,000 dollar whiz-bang laser 3d mapper too... the thing spins in a circle sweeping up/down and has such a precise distance measuring laser, it does dot-matrix of everything out to 300yrds or better, trees/people, etc in probably 1 megapixel density... plus, it has a built-in digital cam to colorize the map when done. Amazing to think the photo is not from the camera, but from a laser measuring the distance differences from your nose to your cheek at 100 yrds. Its quite a show once the data is in a computer and you can play with it, do a 360 spin, up/down rotate, amazing.

Fri Oct 22, 2004 3:45 am

olderookie wrote:same thing here in st louis
except they are working on a long term airport expansion, so it will be years . we only hear it when we are off hook and it so happens to be the repeater output on leased freqs. nothing we can do but complain to the comms company and they say we just have to put up with it.

Your people at the airport may not be aware that thier equipment can do multiple channels. I would reccomend contacting the company's president or highest person you can reach before getting the FCC involved. They may just surprise you if they were'nt aware of the machines ability to change channels and will probably be happy to accomodate you. If they don't, then I would reccomend sending a registered letter with return reciept to them first as proof that you did in fact ask them to have some consideration. This way the FCC will see that you have tried to resolve this in a peaceful manner and made the effort to correct the problem on your own before calling them in.

radioshack · Post by **radioshack** » Fri Oct 22, 2004 4:49 am

the very same thing happened to me when I was working in Iowa City...

We had a UHF community repeater that all of a sudden would drop transmissions every second or so. After inspecting the repeater, we found this horrible interference on the RX side. we spent a LOT of time trying to track down the signal.

The next day we found that there was a surveyor 6 Miles away working at the local landfill. He had no idea that he was interfering with anyone and quickly changed the frequency.

What he did NOT know is that the FCC licence they use states that they MUST monitor the frequency for use before using it at 5 watts! Either way, they were very sorry that they interfered and they made a note not to use that freq. again anywhere near us.

No problems since!

-RS

olderookie · Post by **olderookie** » Fri Oct 22, 2004 5:28 am

talked to the git' er done guy yesterday he asked if I could pin it down a little more he would see what he could do to get the interference gone even if I just get it down to the area. I have the time to go out and do a little triangulateing today so as my hero always said "the game is afoot watson!"

Oddball Interference, For Informational Purposes

Oddball Interference, For Informational Purposes

lay of the land....