Modified Fan Dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC3PJ, Mar 25, 2020.

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  1. KC3PJ

    KC3PJ Ham Member QRZ Page

    I currently run a fan dipole for 80, 40, and 20 meters. The elements are running in different directions as viewed from above. Just imagine looking from above and seeing elements going off at the 1, 3, 5, 7, 9, and 11 o'clock positions. I am wanting to do a bit better on 20 meters being the leg angles are rather steep. My question is.....could I add another half wave of wire to the 2 ends of that dipole and form a full wave 20 meter loop under the feedpoint? Feedpoint is up about 40 feet. It seems like the same principles of how the fan dipole works would also allow this configuration to work. RF would flow to the resonate portions of the antenna on each band with the loop not effecting the 40 and 80 meter dipoles. I would appreciate any insight. Thanks.
     
  2. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    I’m confused. When you say steep angle, is that the angle from horizontal, - so is it a fan inverted V dipole, rather than a flat top?

    There would be interaction when using the lower bands but how much I don’t have a feel for since I’ve never tried to combine the two types of antennas. Without modeling I’d think that a full wave loop for 20m would have appreciable effect when using the antenna on 40m because then the loop would be 1/2 wave.
     
    Last edited: Mar 25, 2020
    AK5B likes this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    A square, vertical loop, fed in the center of the top wire, where the top wire is 40ft agl, is resonant at 14.15MHz when the circumference of the loop is 73.21ft (side is 18.3ft).

    It's feed point impedance is 132 Ohms. That same loop also resonates at 27.33 MHz where its feed point impedance is 244 Ohms.

    How well that would play when paralleled with other dipoles will take a while to figure out....
     
    Last edited: Mar 25, 2020
  4. KC3PJ

    KC3PJ Ham Member QRZ Page

    Thanks for the input. Yes, it is technically a fan inverted v. The 2 lower bands have a more reasonable angle between the legs but 20 meter legs are probably about 60 degrees apart. Probably lots of cancellation of signal at that angle.
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Have you considered running the 20 meter legs in parallel with one of the lower frequency legs like a more typical fan dipole such that the 20m inverted-V has a wider apex angle? All you need is some spreaders to hold the 20m elements away from the lower frequency elements.

    FWIW, my current favorite way to build fan dipole spreaders is what might be called torsion bar spreaders. You only need one of these at the end of each 20m element with the lower frequency element running above it. Basically the upper wire is the element for a lower frequency band (longer) and it makes a jog in the spreader that's drilled with a hole about a third of the way down from the top, the shorter element (20m in your case) ends just before the spreader. These work well and don't tend to spin or tangle which makes the antenna easy to raise or lower. I used sections of 3/8" fiberglass rod sold as electric fence posts for these spreaders but any sufficiently strong and light insulating material would work. They look like this:

    upload_2020-3-24_20-53-5.png
     
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  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, that 20m angle is not good. I agree with K7TRF and would try to make it much flatter, at least 90 degrees as a minimum and preferably 120 or more, before giving up and going to the loop.
     
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  7. KC3PJ

    KC3PJ Ham Member QRZ Page

    Thanks. I'll try to open up the angle before going any further.
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Droopy inverted V's may not be as bad as you think. I just did this yesterday.....
     

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