High noise level on 2 meters

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K7DVS, Jun 21, 2019.

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  1. K7DVS

    K7DVS Ham Member QRZ Page

    I have a setup where i run a preamp and 100 watt amplifier at my base station to try and reach some of the distant repeaters from here. Also i like to monitor 652 . Well i am getting a lot of noise when i run the preamp. It shows 20+ on the meter on my ic706. Making those distant signals very tough to copy. I have tried 2 different antennas. A 4 element beam and a 17 foot long verticle. But both the same situation. Now heres the thing. When it rains, the noise goes away. The static is reduced to 1 on the scale. Today it is raining and can copy those distant repeaters perfectly. I also have a ota tv antenna. Today same thing. The one station i get is coming in perfectly clear. But when it dries up again i know it will be back to the noise. What is going on here?
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Are you in an urban, suburban, or rural location?

    It sounds like there is something, nearby, that is arcing when it is dry. Not all that common for the interference to make it up to 144 MHz but certainly possible. You are going to have to do some tracking of the source. Since the interference is bad on FM, it has to be very close and very strong.

    If you have a handheld 2-meter unit, you can make a simple Yagi that you can hold in your hand. Point the antenna in various directions until the noise is the strongest. Then try to locate the source.

    Glen, K9STH
  3. AA5CT

    AA5CT Ham Member QRZ Page

    Glen nails it; when it rains, wooden poles swell up and the hardware becomes tight! No more arcing through an air gap!
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, this has been noted for several decades...when utility service wiring ages and isn't well maintained, it's common that noise levels fall when it rains.

    However, also remember the IC-706 and most other rigs when used on "FM" have extremely inaccurate S-meters; I don't have a 706 to test here, but my little FT-857D, which is kinda similar to an IC-706, has an S-meter indication on VHF-FM that's crazy inaccurate...S1 is about 1/2 microvolt, and "pinning" the meter at S9+40 is only about 20 microvolts.:p Most "FM" receivers are like that, which is why for serious work in land mobile comms nobody uses any and just goes by SINAD measurements. I don't even know of any commercial LMR FM gear that has an S-meter.
    K7JEM likes this.
  5. N8EKT

    N8EKT Ham Member QRZ Page

    I would need to hear the noise to tell what it may be exactly but power line insulators produce that same symptom.
    Micro arcing on dirty insulators goes away immediately after a rain then slowly comes back afterwards.
    This is very common with high voltage trasmission line insulators.
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    I agree with what has been said. There is a chance the problem is on the output side of your service transformer so give this a try.

    If you have a 12 volt battery to power your rig, use it and turn off the Main Circuit breaker in your breaker panel. If the noise goes away, I got good news for you, it is your problem and you can get it fixed by calling a electrical contractor and have him check your neutral circuit from the transformer and inside your main breaker panel.

    If the noise remains, you are pretty much SOL!
  7. K7DVS

    K7DVS Ham Member QRZ Page

    Ok thanks for also the info. I live in a small town. I also have a 80 meter dipole up and noise on HF is not too bad. About 5 s units on 80 which is pretty normal in town i think considering the distant static crashes are 7 s units. So no noise happening on HF just VHF!
  8. W2WDX

    W2WDX Premium Subscriber QRZ Page

    There are many factors not already mentioned. First, where is the preamp located? Is it near the radio or outside on the antenna mast? If it is on the mast the problem is most likely one the several things already mentioned. If it is near the radio, the number of possible intermittent noise causes goes up quite a bit.

    Locating a 2M preamp at the radio means any noise on the coaxial is amplified along with any signals. Some factors that influence this effect are the quality of the coaxial, its length, and how well the connectors are made or attached to the coaxial; to name a few. Sometimes, poor shield contact on the connector at the antenna can produce noise which will be amplified. Also a lack of grounding on the entry point to the shack can also increase noise on the shield which also gets amplified. Corrosion on contacts can rectify broadband RF and create noise on the feedline. These all can be effected by rain, since the water can "make" the connection that when dry is lacking. If the preamp is located on the mast any noise on the coax does not get amplified, just the signals from the antenna. This is why preamplifiers in the shack are generally not a good choice on VHF or above. If you are mast mounted then the problem is most likely an external noise source, like the "swelling wood" issue mentioned by others, as one example.

    As a general rule on 2m and above, I use mast mounted low-noise pre-amplifiers mounted as close to the antenna as possible, use high quality coaxial (LMR-400 minimum) and use water-tight coaxial connectors.
  9. W6KCS

    W6KCS Ham Member QRZ Page

    I'm pretty sure a sledge hammer will be mentioned soon :)
  10. AI3V

    AI3V Ham Member QRZ Page

    It's been my experience that modern receivers on the 2m band are allready noise limited in most locations.

    Adding a preamp usually just causes overload from other off frequency radio signals.

    Then come the many, many unintentional radiators.

    Things like switch mode power supplies, and digital circutry of all types.

    Then brush type (commutating) electric motors, things like vacuum sweepers, sewing machines,blenders etc.

    Less common is the preamp oscillating.

    I would not expect "wood swelling" or noisy power lines.

    For the op, with the noise coming and going with rain, I'd put money on oscillating.


    P.S. one other thing, forget about "grounding" on 2 meters, it's not happening.
    Last edited: Jul 9, 2019

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