2 Meter Repeater 'Noise'

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K7IHI, Sep 1, 2014.

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  1. K7IHI

    K7IHI Ham Member QRZ Page

    Most recently our 2m repeater seems to be repeating 'noise'.... and the transmitter does not easily turn 'off'. I am not sure where the noise is coming from; antenna, repeater receiver, or transmitter. How do we troubleshoot this symptom? It is very disruptive to always have the 'noise' repeated. Thanks for any suggestions....

    Larry- K7IHI
     
  2. KM3F

    KM3F Ham Member QRZ Page

    First question is what do you hear on the input?
    Do you hear the noise on your radio at the repeater input frequency?
    If yes, use a beam to get a general direction, then go looking in that area.
    Keep narrowing down till you find the source.
    If no then the repeater receiver has some issue or there is an interfering signal near the repeater input such as a Hospital call paging system.
    Good luck.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You troubleshoot such issues at the repeater site, not remotely.

    At the repeater site, when this problem occurs, shut OFF the transmitter (kill the TX enable line) and listen to the receiver without the transmitter running to see if the noise goes away. If it does, then the transmitter is contributing to the repeated noise.

    That could be lots of things, but as a first step this is always the place to start.

    Repeated "noise" might just be a loose squelch setting, although if CTCSS decoding is being used then the repeater shouldn't be repeating just "noise" in the absence of a tone encoded signal. It could be the transmitter is generating noise in the receiver, and it's repeating that: This would also usually be accompanied by less than ideal receiver sensitivity to incoming signals (repeater going a bit deaf). It could be a mixing product, where the repeater's transmitted signal is mixing with another signal and creating a weak secondary signal the receiver detects and repeats in the form of noise. Noise in repeaters can often come from slightly loose or oxidized connections, almost anywhere including at the antenna.

    It's one of the reasons that repeaters normally use specialized antennas that don't use any clamped connections or dissimilar materials, with all conductors in the RF current path welded, brazed or soldered and not clamped; and it's one of the reasons that repeater systems almost always use solid outer conductor coaxial cable (hardline, Heliax) and not flexible cables with woven conductors that over time can start generating noise.

    I've even seen oxidized connections within the antenna tower itself cause this, including old guy line clamps and turnbuckles and such. When you have a powerful transmitter and a very sensitive receiver using the same antenna at the same time, getting rid of all transmitter noise is an ongoing task for many repeater system operators.
     
  4. K0RGR

    K0RGR Premium Subscriber QRZ Page

    We had a presentation recently from a repeater group in a nearby city. They talked about a serious problem they had that was caused by a metal awning around the building that housed the repeater, which generated noise under certain conditions. They ended up moving the repeater.
     
  5. KV6O

    KV6O Ham Member QRZ Page

    This is why it's a good idea to characterize your repeater setup when it's first installed, or you perform any significant maintenance. Measure the receiver SINAD directly, then thru the duplexer. Then again with an IsoTee into a dummy load. Then again with the antenna connected. Measure power out of the PA, then out of the duplexer. Keep this data, and you'll have a baseline to compare to as things change and age.

    If the noise is a mixing issue or bad connections/feed-line, you can use a signal generator to bring up the repeater at it's minimum receive level, then while listening to the output on an HT, go around and shake, tap, twist all the RF connections, feed-line arrestor connections, ground clamps, tower legs, the antenna itself, etc.

    Steve
    KV6O
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    It could be something as simple as a squelch adjustment. Or a receiver squelch problem.

    Or someone Linking Noise into your repeater. Is your repeater coverage still good ?


    Good Luck.
     
  7. K5GHS

    K5GHS Ham Member QRZ Page

    I overheard some guys talking on the local machine this week about having to go up to the site to recalibrate some things on it. They do it every 6 months, usually in the spring and fall to avoid weather issues (and difficulty driving to the site!).

    I'm guessing you might be experiencing something like that. Or some part is having an issue. But it could just be that stuff needs to be recalibrated.

    Whoever is running the machine should know what that stuff is and how to check it, I'm guessing.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This used to be true in the 60s when everybody built their repeaters and almost all amateur repeater owners were employed in the 2-way radio field and had a lot of experience with repeaters.

    Not so much today. You can buy a repeater, connect it up, flip a switch and keep your fingers crossed. Or in some cases (around here, for sure) someone highly technical built and installed the repeater, but then sold it, and possibly it was sold again and again and the new owner had money and that's about all he/she had. No knowledge whatsoever about how any of it works. That's becoming fairly common as the old 2-way radio guys die off...
     
  9. WB9UDA

    WB9UDA Ham Member QRZ Page

    I was involved in tracking down a buzzing noise in our repeater back in xxxxx, Ill about 30 years ago. The repeater was located in the underground EOC bunker. The bunker was out on the edge of town, in farming country. I traced down the noise with my little Yeasu HT. At the adjoining farm was a stock pen with an electric fence around it. In one spot a tree limb had fallen on the fence thus shorting out the fence. Every time the fence would Pulse, we'd here the buzzing in the repeater. I talked to the farmer and got permission to remove the tree limb from the electric fence. Problem solved. Sometimes it takes a real Technician to find a problem. :)

    So take an HT, tuned to the input of the repeater and with the squelch turned OFF. Then walk around the area till you find the source of the noise. It's kind of a FOX Hunt. Eh?

    WB9UDA
     
  10. KJ4TX

    KJ4TX Ham Member QRZ Page

    I've noded that a number of repeaters in the Portland area all have humm when they transmitt. In the spring I heard people talking about waiting until the snow melted enough so someone could get to where the transmitter was so they could look at it, but it doesn't seem like any of them got fixed during the summer. The humm on the Portland 146.980 repeater is so bad it's annoying when people are talking with other repeaters not quit as bad, but still it seems like there is a lot more noise allowed than should be.

    Mike
     
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