Impedance change in vertical dipole vs. horizontal

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0EYJ, Jul 13, 2017.

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  1. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Is there a different formula for a vertical dipole. I have hung vertical dipoles that were resonant as horizontal, and they changed a lot. It has been a while. Would like to get close on a vertical dipole, from the start. Does it come out longer? Shorter? What %, appx? Thx.
     
  2. NH7RO

    NH7RO Ham Member QRZ Page

    I suspect the change is due more to the surrounding objects close to the elements than anything else.
     
  3. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Could be, as I am hanging along a spiderbeam HD fiberglass pole.
     
  4. NH7RO

    NH7RO Ham Member QRZ Page

    I would think that the fiberglass wouldn't make a large difference but then again, it is a significant amount of dielectric right next to your elements. Best to cut long and trim, rinse and repeat until you're happy. Let us know if you figure out what the approximate difference is once you can; maybe it'll be a lot more than the usual 3-5% as with insulated/bare wire.

    73, Jeff
     
  5. N8CMQ

    N8CMQ Ham Member QRZ Page

    Height above ground also enters into the formula with detrimental results if it is too low.

    N8CMQ Jeff
     
  6. IK3UMT

    IK3UMT Ham Member QRZ Page

    I suppose the major affecting elements are ground proximity (affecting also an H dipole anyway) and the feedline approach to feedpoint (feedline ineraction wth antenna element)
    How much did it change ?
     
  7. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Well, i cut a 20m vertical dipole at 4.74m, and ended up 1:1 resonant at about 14.350..... a bit high. I figured for about 6% velocity, on the cheap 20ga wire with heavy plastic shield. Not bad. 4 to 5% velocity trimming should be about right, for next trial, as I overcut. Didn't see a problem being way off, but also did not try the antenna horizontal.

    Was aiming for 14.200 to 14.250 at 1:1
     
    Last edited: Jul 14, 2017
  8. KD0CAC

    KD0CAC Ham Member QRZ Page

    A thought , free-space in one thing , reality is another .
    Coupling with its environment , horizontal / vertical ?
     
  9. VE3DXK

    VE3DXK Ham Member QRZ Page

    Just curious, how did you route the coax?
    With horizontal dipole it's easy to get the feedline perpendicular to the antenna, but with vertical dipole you have to be creative...
     
  10. SM0AOM

    SM0AOM Ham Member QRZ Page

    The major reasons for the influence on the resonance frequency are the different near-field distribution geometries with respect to the mirror image of the antenna below the ground.

    The mutual impedances between the above-ground antenna
    and its image become different between the horisontal and the vertical cases.

    73/
    Karl-Arne
    SM0AOM
     
    KE0EYJ likes this.

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