Feeding the OCF

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KA2AIC, Jun 11, 2021.

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  1. KA2AIC

    KA2AIC XML Subscriber QRZ Page

    I need to lengthen the feed line to my multiband (40, 30, 20, 17 meter bands) ocf antenna 25 feet more.

    Choice 1. Add more rg8x to the 4:1 balun so239 connector.

    Choice 2. Add open feed line from the 4:1 balun to the antenna wires.

    My radio is a Kenwood TS 450 SAT.

    Please advise
    Thanks
    KA2AIC, Jim
     
  2. KU3X

    KU3X Ham Member QRZ Page

    You NEVER want to feed and off center fed antenna with parallel feeders. You are feeding an unbalanced antenna with balanced transmission line and the transmission line WILL RADIATE.
    If your OCF is feed about 1/3 of the way in from the end, it is NOT resonant on 30 meters and should not be used on that band. The actual SWR on that band, at the antenna's feed point is around 18 to 1 SWR. You see less in the shack only because of its losses in the transmission line.

    If you want to operate on 30 meters, erect a 30 meter dipole or another resonant antenna like a half square of a delta loop.

    Barry, KU3X
     
    AK5B likes this.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I understand OCFs enjoy a vegetarian diet, I'd stick with that.
     
  4. K1LH

    K1LH XML Subscriber QRZ Page

    Choice 1: Add more coax.

    The length of balanced line is possibly part of the overall design and contributes to the bands it covers.

    As noted above the balanced line will radiate due to the unbalanced nature of OCFDs, it's generally accepted now that using lengths of balanced line is not favorable - but you can still find articles that show the use of balanced line. The preferred way is to use a dual core balun which includes a very good 1:1 current choke with a 4:1 stacked on-top such as those made for OCF use by Balun Designs, DJ0IP has a lot of detailed info on this subject too, see https://www.dj0ip.de/off-center-fed-dipole/balun-selection/. I have my 80m OCFD setup with a dual core balun, and it works well.
     
    WA7ARK likes this.
  5. AK5B

    AK5B Ham Member QRZ Page

    May be vegetarians but certainly not vegan---some commercial OCF's are downright cheesy!
     
    WB5YUZ and KU3X like this.
  6. W6MK

    W6MK Ham Member QRZ Page

    That's from another antenna design!

    The G5RV and variants are doublets (balanced antennas) fed with balanced line of a certain length which is then attached to a balun and to coax of any length.

    The length of balanced line is determined so as to provide appropriate impedance transformation at
    the balun/coax transition.

    As already noted, all you have to do is add more coax.
     
    AK5B, WB5YUZ and WA7ARK like this.

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