Window line and coax in the same chase.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K0LEA, Apr 14, 2020.

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  1. K0LEA

    K0LEA Ham Member QRZ Page

    TL;DR: Am I going to have issues if i run 35 feet of window line in the same 4" chase as my coax feed line?

    Hi I live on a small lot in a cookie-cutter subdivision in an HOA-controlled neighborhood. So I have installed my antennas in the attic. I don't have many other (permanent, convenient) options.

    I'm feeding a fan dipole (Alpha-Delta DX-CC) and an Arrow U/V J-Pole in my attic with two separate 75-foot LMR-400 coax feed lines. Both feed lines run from the attic to my shack in the basement via a straight run of 4" PVC, a vertical drop of about 35 feet. The lines then run across the basement ceiling to my shack.

    The main issue that I'm having is with common mode on the HF feed line and a lot of RFI /noise at the radio. I've installed chokes at both ends of the HF feed line and that has helped somewhat. The dipole is installed at least 6 feet away from any electrical feeds in the attic. Still the RF interference is bad and I'm continuing to work through finding and eliminating sources, but at this point most of what remains is coming from my neighbors / outside my home. (I turned off all electrical circuits coming in my house and ran on battery to check this.)

    I would like to leave the U/V antenna feed line as it is and pull the line that feeds the fan dipole and replace it with an 80-foot run of 450-ohm twin lead / window line. I'm going to run the window line balanced all the way from the dipole to a Palstar AT4K tuner and then connect that to my rig with a short coax patch cable. The most convenient way to run the window line is in the existing chase, with the coax feed line.

    My question is whether having the window line and the coax run parallel for 35 feet within 4 inches of each other is going to to create problems. I'm moving to the window line to try to eliminate common mode on the feed line and to reduce loss, etc but if I'm going to have the same problem, I may pull the U/V antenna out.


    - troy / K0LEA
    Fort Collins, CO
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I would leave the coax run for HF, and use a remote, automatic tuner in the attic. Individually choke both ends of both coax runs, or get a bigger cores with a larger aperture, and wind both coaxes together through common cores at both ends of the runs. If you run a DC twisted pair to a remote tuner, be sure to choke those wires as well.

    Running balanced line parallel to, and within a fraction of an inch away from coax is not going to work, besides it is difficult to choke the balanced line to prevent it from conducting noise from your house upwards toward your antenna. Choked Coax is hugely better at preventing RFI from being conducted to an antenna than balanced line is, period!
  3. AI3V

    AI3V Ham Member QRZ Page

    No, don't try and run parallel line in conduit with or without coax, the parallel wire line needs to be several times its spacing away from any other object.


    Plus, you have to stop thinking of signal as some magic wave that only is picked up by the antenna, amd noise as a similar magic wave that is only picked up by the "common mode" on the feedline.

    If fooling around with your system caused a difference in either signal or noise it is only because you changed the directivity of your system.
    Last edited: Apr 15, 2020
    AG5CK and WA0CBW like this.
  4. K0LEA

    K0LEA Ham Member QRZ Page

    Thanks, that's what I was worried about. Appreciate the responses!
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Of course a feedline carrying common-mode current changes the directivity of an antenna. The reason you choke the feedline is to suppress the common-mode along the feedline and thereby restore the natural pattern and directivity of the antenna.
  6. AI3V

    AI3V Ham Member QRZ Page

    And of course you have a 50-50 shot at one of 3 things happening :)

    1- nothing.
    2-pattern changes to make the noise louder.
    3-pattern changes to make the noise weaker.

    Since it's a complete crapshoot, and seldom successful, most stations will find it a waste of time and money.

    Time and money that could be much better spent moving the antenna farther away -read higher- from the suspected noise,

    Or building a small, directional, steerable rx antenna to null the noise.

  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    I agree with #1. Sometimes nothing happens when you put a Common-Mode Choke on a feedline. You are out $10, or you have a new item in your junkbox. There will likely be a future use for the choke.

    I have never seen #2 happen. Even if it were to happen, take it back out and see #1.

    #3 works for me and others... It cost me $10 and a bit of effort to find out.

    The way the pattern changes for the better with a choked feedline is because without it, the antenna starts inside your hamshack, goes all the way along the feedline to the "real" antenna. That puts part of your receiving antenna only inches from a computer, monitor, modem, wall-warts, etc. Choking the feedline between the ham shack and where the "real antenna" starts effectively prevents the feedline from being part of the "real antenna". In most installations, hopefully the "real antenna" is 50 or more feet from the RFI producers inside your own house... The un-choked one is only inches from the noise sources...
  8. AI3V

    AI3V Ham Member QRZ Page

    Except that "common mode" and all your posts that go with it are a Sammy Hammy issue.

    Like I keep trying to explain to you, these "chokes" are unknown in the professional world.

    You simply cannot grasp that the "half antenna" you are so proud of guarantees that the house wiring will be the rest of the antenna.

    And instead of using a whole antenna, you continually post a whole series of band aids in a futile attempt to "correct" the uncorrectable.

    It is my hope, that some day when you step away from the theoretical world, and have a fraction of my real world experence that you will begin to understand the error of your posts.

    You should start by stop recommending poke and hope "solutions" :)


    P.S. still waiting for some links to commercial companies that recommend "chokes" to "cure" "common mode" ,the equipment to measure it, and how you know when you have eliminated it.
  9. K4SAV

    K4SAV Ham Member QRZ Page

    You will be going in the wrong direction replacing coax with twinlead. If most of your noise is being picked up from your neighbors it is likely picked up by your antenna not the feedline. The only solution for that is a different antenna for receiving or a noise cancelling unit that uses separate antenna for receiving the noise and phases the signal from both antennas to reduce the noise.

    Every computer monitor I have ever bought has had chokes on the cables to the monitor.

    Jerry, K4SAV
    WB5YUZ, W1PEP, KI8DJ and 2 others like this.
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    For the first 35 years I was a sammy hammy, all power supplies were brute-force linear and lighting was incandescent by Edison, and I never needed a choke. It has only been the past 15years (since switching power supplies, CFL and LED ballasts) that they have been able to improve HF receiver SNR.

    Unlike you, I understand the theory of why chokes improve the situation. Obviously, the 1950s Navy Tech Manuals you learned from didn't cover advanced topics...

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