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Common mode currents on an OCFD

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M0ZAI, Dec 28, 2011.

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  1. M0ZAI

    M0ZAI Ham Member QRZ Page

    Hi all,

    I've recently bought an OCFD for 20m-6m excluding 17m. Running it barefoot into my FT857D is producing very high amounts of feedline radiation even at 20m where it exhibits a SWR of 1:1. The common mode currents are enough to reset the rig even at 20w. A few turns of coax helps with the common mode problems but raises the SWR.

    Is high common mode current on the braid a feature of OCFD? I would have though the balun would stop this.

    Has anyone heard of this before or have any suggestions as to how I can cure it?

    73

    Matt M0ZAI
     
  2. VE3EKJ

    VE3EKJ Ham Member QRZ Page

    First off. let's assume it's not a Carolina design with a vertical radiator portion of the feedline.

    With a traditional OCF dipole, you absolutely need to be using a current balun or you will have RF on the feedline. Also with any OCF it's important to bring the feedline away trom the antenna at right angles, for as far and as best you can. If' it's dressed too close to on side element or the other you will also have RF on the feedline problems.

    The "few turns of coax" ugly balun trick is NOT wideband enough to cover all bands! It tends to only work well on one or two bands at a time. You can google "ugly balun" and you'll find lots of stuff.

    In your case, things are bad enough that I would consider a true choke balun, where the feedline is run through large ferrite beads. A sleeve of about 10-12 " long of such beads will usually block RF from flowing any further. It's a bit heavy but you could mount it outside, before the feedline enters the shack.

    Some hams have reported success when using smaller feedlines like RG58 or RG8X by using a large toroid a couple of inches in diameter, big enough to wind a few turns of the feedline right on the toroid, a few inches to a foot below the feedpoint balun. Such a trick can be easily weatherproofed with tape and doesn't have any connections to corrode anyway.

    Still, you seem to have an excessive condition. I would be looking at my initial couple of tips first, like how the feedline is dressed, if I were you.
     
  3. KU3X

    KU3X Ham Member QRZ Page

    You need a good 4:1 Guanella Current Balun. Here's one that will do the trick.
    Search, " gu4-hf-5kw" in Google. You not only will find the balun but you'll find a video of it on
    youtube.
    I would not bank on your OCF20 to be resonant on 17 meters. The OCF40's are not but the OCF80's are.
    Barry
     
  4. K1DNR

    K1DNR Ham Member QRZ Page

    Just to say it again - SWR and transmission line radiation have nothing in common...

    Transmission lines radiate because of current imbalance. When you feed an *off center* antenna, you instantly have an issue with balance.

    The issue may get better or worse depending on a variety of factors including the length of your transmission line, etc.

    This is a very, very common problem with off center feed dipoles. The common mode issue may or may not be easily correctable to your satisfaction.

    I thought these things typically came with 4:1 current baluns. Maybe its a voltage balun. If you already have the 4:1 transformer at the feed point, then you might try putting a second 1:1 choke balun in line as well. Do not put two 4:1 baluns in line. Then your SWR really will get bad!

    If you had no transformer you would not be getting a 1:1 SWR. So I'm going to assume you already have the 4:1 transformation and need a good choke that does not transform the impedance any appreciable amount.

    A good 1:1 current balun might solve the problem.

    Or replace the existing 4:1 with this current balun, which has two cores and a very high common mode impedance http://www.balundesigns.com/servlet/the-75/OCF-balun-4-cln-1/Detail
     
    Last edited: Dec 29, 2011
  5. N0AZZ

    N0AZZ Ham Member QRZ Page

    On my 270' coax fed OCFD I have a 6:1 balun and do not have that problem only bands need tuner on are 15,30m and band edges on 160m. The rest if any VSWR is present < 2:1 the internal tuner in my amp takes care of it.
     
  6. K4SAV

    K4SAV Ham Member QRZ Page

    What model antenna did you buy? Most OCF antennas have a high SWR on their fundamental frequency, however there is at least one commercially made antenna that intentionally uses common mode currents on the feedline to obtain a low SWR on the fundamental frequency. It has no choke at all but only a 4 to 1 voltage mode balun.

    Jerry, K4SAV
     
  7. K1DNR

    K1DNR Ham Member QRZ Page

    Just out of curiosity have you ever measured the common mode current anywhere on the coax? The op is complaining about RF in the shack which has nothing to do with SWR, although that seems to be a commonly held belief.

    As K4SAV mentioned, sometimes the coax is intended to be part of the radiating system. This is usually how end fed designs work - then they place a choke at some point down the coax from the antenna. If its a long length of coax, and mostly vertical it might be part of your success!

    How bad the RF is in the shack depends on many factors, which likely differ from your particular setup.

    I believe the standard OCF design presents ~200 ohms on even harmonics - which would match 50 ohm coax with a 4:1 transformation.
     
  8. KA4DSK

    KA4DSK Ham Member QRZ Page

    HI I have a homebrew OCFD 133 feet long with a 4-1 homebrew current balum. Can use 80 to 2 meter and top end of 160 with out a problem. The tunner in my icom746 PRO will bring all to 1-2 or less. I found that by moving the feed point several inches toward the long or short side of the antenna made things much better. In others words the long side was not one third of the overall length which was 133 feet and placing the balum feed with the correct side of the coax (the center not the out side shield) to the long side of the antenna shield to the short side worked best Using a 50 foot coax feed straight to rig.
    Check out this site " The Windom Antenna Handbook,by Buck Rogers K4ABT"
    Get some wire and build your own all band windom, i used #12 solid insulated home depot house wiring.
    Best luck have a great new YEAR and i hope to hear you on the air Frank
     
  9. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    OCFD Madness

    Yes ,CM is the main drawback to the OCFD.
    Many times the balun supplied at the feed point will be a 4:1voltage type balun. These are less effective at reducing CM on the feedline.
    A 4:1 current balun at the feedpoint would be more effective,but you still may need another 1:1 current balun further down the feedline.Both of these need to be high impedance baluns over the bands used.
    Something else to consider may be to ground the coax outer shield outside the shack. This may or may not be a help,you could try it and see.
    What-ever you do ,make only one change to the antenna system at a time, then test thoroughly and take notes.This way you can identify what works,and then later learn why it worked.
    Hope this helps.
    Brendan

    Here is some links for some theory and practical solutions.
    http://www.w8ji.com/core_selection.htm
    http://vk1od.net/balun/index.htm
     
  10. N7WR

    N7WR Subscriber QRZ Page

    I have a homebrew 270 foot long OCFD (180 on one side and 90 on the other) Fed at 90 feet on the tower, ends at 55 ft. 6:1 current balun and an 8 inch diameter coax choke balun just below the 6:1. Outstanding performer on all bands and no CM on the feedline.
     
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