80M off-center-fed dipole - best feed point? - feedline type? - compared with center fed?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K1HL, Dec 9, 2019.

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  1. K1HL

    K1HL Ham Member QRZ Page

    I wish to erect an off center fed dipole on a small lot. My available land is a rectangle 35 ft by 96 ft, with the usable space (for neighborhood reasons) on only two of the boundaries. My house is approximately centered in the lot. I have a Johnson KW matchbox. Because I already have antennas for 40 thru 10, the feed-point does not need to be manipulated to make it resonant other than for 80M. I cannot run 135 ft of wire in a straight line and therefore will need to use the entire 96 foot boundary and, at right angles to it, the full length of the 36 ft boundary. I will somehow figure out where to find the last three feet. My thought is that I would feed the OCFD with 450 ohm ladder line connected at a point 33% from one of the ends. The result would be a 90 ft leg and a 45 ft leg. The antenna would of necessity be about ten feet high at the end points, and 40 feet at the center; an inverted vee, off-center-fed dipole with a 90 degree jog at the 96 foot point.

    OPTION: I could manage to center feed the dipole with either 52 ohm coax or ladder line from my antenna tuner.

    Antenna guys: Would a regular center fed dipole work better than one OCFed? How do you think the right angle bend will affect the impedance at the feed point? Overall, will this contraption work? How would this work with a different feed-point, and how might that affect the impedance? I believe I need to have maximum RF current at the feed point. Will my Johnson Matchbox do that? If center fed, should I use coax or ladder line? Thanks for putting up with this long query!
    Harry K1HL
     
  2. SM0GLD

    SM0GLD Ham Member QRZ Page

    For 80m only I think center fed is best.
    At 40ft and 90 degree angle the impedance will be around 30 ohm.
    Lowering the ends down to 10ft will not change the impedance much according to simulations.
    Try raising the dipole ends if you can because the low height is not good for efficiency.
    If you use ladder line, one half electrical wavelength is a good choice.
    If that is to long for you you might use coax.
     
    Last edited: Dec 9, 2019
  3. SM0GLD

    SM0GLD Ham Member QRZ Page

    I have to make a correction.
    The impedance will be higher when the 90 degree bend is not in the center of the antenna.
     
  4. W5DXP

    W5DXP Ham Member QRZ Page

    I could have any wire antenna and I have chosen to erect a ZS6BKW - a 96 foot dipole fed with 40 feet of ladder line through a 1:1 choke-balun with coax the rest of the way to the shack. With a couple of simple modificati0ns, it works well on 80m, 40m, 20m, 17m, 12m, and 10m. Since you are short on room for a 120 foot dipole, why not consider a 96 foot long ZS6BKW? More information at:

    http://www.w5dxp.com/ZS6BKW80/ZS6BKW80.HTM
     
  5. KF4ZGZ

    KF4ZGZ Ham Member QRZ Page

    IMHO .... I would go diagonally across the property with an 88ft. dipole .... or 96 ft. either/or ..... and let the matchbox do the work.
    At 88ft. with the matchbox you can do 80m-10m, don't see why 96ft. wouldn't do it.

    One thing people fail to consider with OCF, G5RV, ZS6BKW antennas and the like is they were developed by very experienced, knowledgeable
    hams, usually for a very specific purpose. They just happen to do one or more things as well.
    Stick with the basics until you find a need for something else.
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Using the layout shown below, fed in the center using 50ft of Wireman #552, the transformed impedance seen by the tuner would be:

    (freq: R ,,,, jX)
    3.5 : 106, -530
    3.8 ::: 53, -304
    4.0 ::: 42, -217
    which will be easy for a tuner.

    k1hl.png

    Average Gain is 0.42, which isn't bad for a low antenna.
    Elevation pattern shows that it is a cloud burner because of its low height:
    k1hle.png

    Azimuth pattern is omnidirectional.

    I saved the model, so can easily re-run it with changes.
     
    Last edited: Dec 9, 2019
    K0UO likes this.
  7. W1VT

    W1VT Ham Member QRZ Page

    DX or local? A dipole should work just fine for local ragchewing. An Inverted L or top loaded vertical is likey to be a better 80M DX antenna.

    Zak W1VT
     
    WB5YUZ likes this.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I doubt that... He could only run ~40ft vertically, and a max of ~60ft horizontally to create the L wire, but he would have only two radials (along the fence line) to feed against. Because of the low resistive feed impedance of a shortened Marconi, with such a crummy radial field, the losses would eat the Marconi's low-angle pattern gain.
     
    Last edited: Dec 9, 2019
    K0UO likes this.
  9. SM0GLD

    SM0GLD Ham Member QRZ Page

    I modeled the antenna using these limitation, one 96ft side and one 36ft side.
    WA7ARK's antenna is nice and symmetric but will not fit according to the limitations.
     
  10. W1VT

    W1VT Ham Member QRZ Page

    Google shows a picture of the house with the ocean in the background. I'd think a vertical or Inverted-L would do great where he lives.
     

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