Newbie Questions

Discussion in 'General Technical Questions and Answers' started by K3HMG, Mar 8, 2021.

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  1. K3HMG

    K3HMG Ham Member QRZ Page

    Evening all......I am a fairly new ham and I finally got all my gear in order and I have noticed a few things that have puzzled me. I was on 2m today hitting a couple repeaters to get a couple signal reports. It was about 3 or so in the afternoon and the noise floor was reading s3 with an occasional quick spike (like a blip) to s9 and immediately back down to s3. I double checked all my connections switched from my antenna to my dummy load to see if the floor would drop and it went immediately down to zero.

    As soon as I switched back to my antenna the floor came back up to s3 again. I didn’t worry too much thinking it was the antenna picking up the rfi. A few hours later I went back to my closest repeater for a NET and I noticed the The noise came up to s9. I cycled back to the dummy load with the same result as before. I jumped to another repeater about 40 miles away where I was able to bring up the repeater earlier and I got a report back as barley readable. I notice the s9 noise floor reading on that freq as well and also noticed a higher SWR of @2.1. I Went back down to the local repeater freq and had a more respectable SWR of 1.47:1. However the noise floor was still reading s9. I went over to 70cm and scanned thru several freqs to see if I was getting the same S meter readings and on 70cm the S meter reading were all near or at zero. Here is a rundown of my set up.....

    Icom 7100
    Jet stream PS
    DBJ-1 (ham) dual band base antenna (144-148 MHz and 440-450 MHz)
    All equipment on bench is grounded individually to a single point on a copper ground bar with copper strap directly behind the equipment. From there the grounding bar connection runs out and ties into the buried ground rods that are bonded together.
    I buried 2 additional 8 foot ground rods that are bonded together and then bonded to the house ground rod under the house electrical meter. They are spaced approx 10 feet apart and bonded with #6 solid copper wire.
    The antenna in mounted to a metal mast (mast is grounded to the grounding system at the original ground rod under the electrical meter.

    If this was common mode noise wouldn’t it be present on any band or freq no matter where I go?

    why does it seem to go up and down? Could it be rfi directly from the antenna? I understand if it’s common mode maybe something came on inside my house that caused the higher reading. For a brief moment I thought it might be the HW heater or garage fridge they were both running but after throwing the HW breaker and putting a mix 31 ferrite bead on the fridge cord it was still present.

    Does rfi or common mode noise effect SWR?

    Could this be a problem with the antenna?

    any and all help would greatly appreciated and I apologize in advance for the rookie questions.

    73

    Jim KO4MOO
     
  2. AI3V

    AI3V Ham Member QRZ Page

    Ok,first off you do not in any way shape or form have any sort of a "rf ground" on 2m or 70cm.

    Whoever convinced you to buy a "buss bar" and multiple rods pounded into the ground wa$ted your money.

    Second, "common mode" has nothing at all to do with your rx noise.

    Neither will VSWR.

    You are picking up some sort of externally generated radio frequency signal.

    What is it?

    One of the several hundred billion things that run on electricity . :)

    The challenge will be to build a directional antenna (I suggest a Quagi) and start hunting.

    Good luck, and let us know when you find the offending emmiter.

    Rege
     
    Last edited: Mar 8, 2021
  3. AI3V

    AI3V Ham Member QRZ Page

    A short explination of the hopelessness of trying to get a "rf ground" at vhf.

    When the wire - of any thickness- going to your rod pounded into the ground, is more than a fraction of a wavelength long, it is, in fact, a antenna.

    And acts just like any other purpose built antenna.

    Rege
     
    Last edited: Mar 8, 2021
  4. K3HMG

    K3HMG Ham Member QRZ Page

    I may have found the issue. For the hell of it I disconnected the feed line at my coax switch and pulled out my trusty multi meter. I tested from the switch to the radio for a short. Passed no short. I then tested the feed line from the antenna it was reading a short somewhere in the line.


    Now I double and triple checked all of my PL connectors after I installed them on the LMR 400. There were no shorts anywhere. I was sure of this. But I continued on with my search.

    I then decided to go outside to where the antenna was and I disconnected the feedline from the antenna. I went back inside the shack and tested the feed line where I initially discovered a short reading and the second test indicated there was no more short. I then went outside and brought the antenna inside and tested the feed point of the antenna. When I placed the multi meter leads on the So 239 outter jacket and center conductor it indicated that there was a short some where inside of the wiring on the antenna. Going to contact the person who made the antenna for a remedy.

    if this was it I am shocked it worked at all.....back to being puzzled I guess.
     
  5. K3HMG

    K3HMG Ham Member QRZ Page

    Now I am totally confused. Don’t rf ground my shack? Or simply do not rf ground the vhf/uhf portion? If you don’t rf ground then what happens when operating on Hf?
     
  6. AI3V

    AI3V Ham Member QRZ Page

    That is normal, and will not cause noise while the repeater signal is unchanged.

    You will find there is a dc short in the matching section in the base of that antenna for the purpose of providing a low vswr at vhf/uhf

    Rege
     
  7. K4JDH

    K4JDH Ham Member QRZ Page

    Most VHF and UHF antennas are a short circuit to DC. They are not a short at their operating frequency but resonant.
     
  8. AI3V

    AI3V Ham Member QRZ Page

    Nothing.

    "Grounding" has very little to do with noise, and everything to do with preventing certain electrical faults in the house wiring from becoming a shock hazard.

    And a secondary function to provide some extremely limited protection from lightning burning down your house.

    Rege
     
  9. N3PM

    N3PM Ham Member QRZ Page

    If the antenna uses an internal shunt inductor for matching, you'll probably read a short at the connector.
    Mike N3PM
     
  10. K4JDH

    K4JDH Ham Member QRZ Page

    I hurt my back pounding a ground rod adjacent to my tower. Not for protection but to show the insurance man if he has to come.
     
    AI3V likes this.

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