A 40m Bent half-wave Zepp dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7ARK, Jan 9, 2020.

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  1. WA7ARK

    WA7ARK Ham Member QRZ Page

    Most of us are familiar with the classic Zeppelin antenna, where a quarter-wavelength section of ladder-line (LL) is used to feed the end of a half-wavelength wire. Some of us know this as a J-Pole.

    I created the following model for a friend who has two existing supports about 60ft apart at about 35ft agl, and wants to get on 40m (single band). He prefers not to have a coax hanging from the middle, as it would with a loaded dipole..., and he has some 5inch-spaced home-made (open wire) LL.

    I used AutoEz to find the required length of the vertical LL and the length of the horizontal wire to create a perfect 50 Ohm feed impedance at the bottom coax-to-LL transition. Here is the model and results:
    J1.png

    1 Amp at the Source produces most of 1A between the two vertical LL wires. The current at the top in the open-ended wire of LL goes to zero. The current that exists at the top of the other LL wire is continuous around the corner at the top. The effective "horizontal dipole" length starts approximately where the currents in the "feed" and "open" wire are equal, about 10% of the way from the top of the LL.

    You can see that the LL has CM current (proportional to the difference between current in the "feed" and "open" wires that comprise the LL). The LL radiates, so the net effect is somewhat like a ground-independent inverted-L. This will allow the vertical LL to pick up local vertically-polarized RFI, and it will be worse in that respect than a center-fed coax-fed dipole with a CM choke at the feedpoint.

    The impedance at the source is 49.92 + j0.12, for an Swr50 of 1.003 at 7.140MHz. The Average Gain of this antenna is 0.54 (-2.7db), which means almost half of the applied power never gets radiated. By comparison, a 66.6ft long center-fed dipole at 39ft agl has an AG of 0.77 (-1.2db)

    The Swr bandwidth is about what you would expect (just a bit worse than a center-fed dipole):
    j2.png
    The patterns:
    J3.gif

    I would use 50 Ohm coax of any length to the rig. It matters not if the coax center-conductor goes to open LL wire or the other one. I recommend a CM Choke be placed at the coax to LL transition.
     
    WB5YUZ and AK5B like this.
  2. BILLYBOB560

    BILLYBOB560 QRZ Member

    Never gets radiated? Where does it go?

    Dissipated as heat, I presume. But WHERE?

    BB
     
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    In the earth, within a circle of about 25 to 50 wavelengths around the antenna. And that presumes that area is relatively free of houses, garages, fences, powerlines, and trees...
     
  4. KM3F

    KM3F Ham Member QRZ Page

    The power is radiated but is absorbs by surroundings. A transfer from one condition to another.
    It doesn't just disappear unaccounted for.
     

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