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Fan Dipole Construction Details

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WT4W, Nov 27, 2019.

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  1. WT4W

    WT4W Ham Member QRZ Page

    I got one fan dipole (40/30/20) deployed and tuned, and while it works well I'm not totally satisfied with it structurally. This one is oriented NE/SW and I'll be putting up a second one oriented NW/SE in the near future.

    Before I put the next one up I'd like to come up with a more robust design. Let's see what everyone else has done.... photos and sketches with dimensions would be most helpful. (I only have the option of supporting it from the ends, a support in the center is not possible.)
     
  2. N2PQW

    N2PQW Ham Member QRZ Page

    In my most humble opinion, Alpha Delta is the current state-of-the-art for fan dipoles. Their construction methods may-or-may-not appear sophisticated, depending on your perspective, but they are robust and very effective.

    You may find that buying on from Alpha Delta isn't all that expensive, but if I were planning to build one, instead of purchase, I'd copy their design.

    Obviously, not supporting the center feed-point would require the wire elements to carry all the load, so be sure to use sturdy wire. Additionally, a strong non-metallic cord could be bound to the longest element, and extended beyond the ends of the antenna to provide all the support, sort of a backbone, and also serve as the hanging method from the ends. This way, the entire wire antenna is hanging from the cord, like clothes on a clothesline.

    Have fun!

    Cheers,
    David / N2PQW
     
    NG0L likes this.
  3. KU3X

    KU3X Ham Member QRZ Page

    I use 1/2" gray electrical PVC for spreaders, cut them 12" long and space the wires around 5" from one another. The least spreader is only 6 inches long since it only holds two of the dipoles together. Put a good balun in the middle and away you go. I've been doing that for 20 years and it works great.
    PVC is cheap and the wire is just plain old #HHN from Home Depot.
    Barry, KU3X
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    FWIW, I used a 'torsion bar' spreader design with my most recent fan dipole. It's basically a set of spreaders with holes drilled in both ends and one about a third of the way down from the top. The longer wire in each wire pair passes through the top holes and back out the offset middle hole and the shorter wire element terminates on the bottom of the spreader. The wire spacing at the center insulator/balun was about six inches.

    Here's a look at the torsion bar spreaders:
    [​IMG]

    I find this method works pretty well, creates a clean antenna and resists spinning and twisting better than more conventional end spreaders.
     
    K1VW, WT4W, WU7A and 3 others like this.
  5. AJ5J

    AJ5J Ham Member QRZ Page

    Glad Dave posted his "torsion bar" method above---clever and just plain cool.
     
  6. WT4W

    WT4W Ham Member QRZ Page

    That looks interesting. What material is the torsion bar made from?
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    A piece of fiberglass rod. It's apparently used for electric fence installations, I picked it up at a local ranch supply store. It's about 3/8" diameter but that's not critical it's just what they had.

    The duct tape just helps keep the rod from splintering when drilling the holes and I suppose I could have removed the tape after drilling the holes. I suspect the colored fiberglass rods that they sell here at local hardware stores to mark driveways in the snowy months would work just as well though the gray has low visual impact.
     
  8. WA4SIX

    WA4SIX Ham Member QRZ Page

    I used CPVC pipe with 3/16 dacron rope from each spreader to tension them. Also each spreader was the termination of a wire. Similar to the torsion bar, but 2" spacing.

    Ed
     
    K6CLS likes this.
  9. N7WR

    N7WR Premium Subscriber QRZ Page

    Exactly the same approach I have used for years
     
    KU3X likes this.
  10. KM6BSQ

    KM6BSQ XML Subscriber QRZ Page

    I also don't have a way to support the center, but I run an additional rope from tree to tree to support it i.e. center insulator with 1:1 balun is tied to it directly, while antenna legs have their own ropes. I found that raising the center first helps significantly when maneuvering all these spreaders.
    I cut 1/2" pvc pipe in half hoping to reduce the weight. In retrospect, it was a mistake, this created edges that were catching gutters and tree branches. The longest stretchers are 36". Center insulator makes wires 6" apart.
    I added a stretcher in between otherwise wires were tangling over each other when I'd raise the dipole.
    I found that any fancy stretcher attachment is not necessary, a single hole for the wire is OK, on each side of the stretcher I added electric tape to make wire thicker than the hole to prevent stretcher moving in either direction.

    I 3d printed the center as long as my printer bed allowed. https://www.thingiverse.com/thing:4008547
    I found that dipoles coupled little with each other, but the 20m couples with everything and can only be realistically tuned when raised.

    I'm thinking now that I should probably print an insulator with a balun box to protect it from weather, but it's actually harder than it sounds - there should be weeping holes in the box and it will prevent air cooling in summer.
     

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