Doublet Questions

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3HMG, Mar 4, 2021.

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  1. K3HMG

    K3HMG Ham Member QRZ Page

    I am a fairly new radio operator and I am about to start exploring the HF world. I am wanting to build a multi band broadside doublet covering 10, 12, 15, 17, 20, 30, 40 meters. The length calls for 44’ (13.4m), however my space in limited to approx 30’. Going to be feeding the antenna with 450 ohm balanced line into a manual tuner.

    Would it be possible to use loading coils so I can shorten the length to say 28’? If so is there a calculator out there to build the coils? I know there are coil calculators out there, but I haven’t found one that recommends a certain size based on going from a certain length to a shorter length and that would be compatible at multiple freqs. Any help would be greatly appreciated.

  2. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I would just let the ends of the doublet hang down, or zig zag them a bit.
    M0AGP, G5TM, N7EKU and 4 others like this.
  3. K3HMG

    K3HMG Ham Member QRZ Page

    I thought of that as well was a bit concerned about performance. Maybe an easier route would be to linear load it which IMO is much easier that coil loading.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

  5. KI4IO

    KI4IO Ham Member QRZ Page

    An interesting problem
    I'd be inclined to fabricate some wide-spaced open-wire feed (instead of the window-line) and see how things
    work with your manual tuner. I use my 40 meter dipole on 80 meters with 4" spacing OWL and seem to do
    quite well. Your manual tuner might have a hard time with the feed impedance on 30 and 40. My coupler is
    a reversible L-network and I'm able to switch from high to low-Z loads. Of course, YMMV!

    Good Luck
    Warrenton, VA
  6. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    I did some modeling on your 30' doublet and a few others to see if there is a good compromise antenna. All antennas were modeled 30' above real, medium ground. The 450 ohm SWR was swept 7-28MHz. The best looks to be the ends hanging down doublets.

    #1, 30' horizontal
    #2, 30' horizontal with ends hanging down 18'
    #3, 30' horizontal with ends hanging down 12'

    #3, 30' horizontal with center loading coils for 7MHz resonance
    #4, 30' with loading coils near the center for 7MHz resonance

    #5, 30' linear loaded using a folded dipole with ends not joined
    #6, 30' linear loaded at the center

    My experience with doublets used below their design frequency is that when shorter than 70% they just do not load and work well at all. For example, I have found a 33' doublet to work okay on 30 meters but not on 40 meters.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What kind of multiband doublet design is that? Do you have a plan or a link to a plan that's published on the web somewhere?
  8. K8JD

    K8JD Ham Member QRZ Page

    Shortening with loading coils may not work well due to use on several bands ! That works ok for a single band ant.
    Getting it up as an inverted "V" would be a better idea.
    Center high and ends lower takes up less linear space in your yard.
    BTW the 44 Ft length will be hard to match on 30 M and REALLY hard on 40 m !
    MY 20 M EDZ is 90 ft long and uses 450 Ohm Line . tunes OK on 5-18 MHz, tuner arcs and sparks on 80 and 160 M bands !!! :eek:
    M0AGP and N7EKU like this.
  9. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    I should have added that the loading coil designs present the worst match. I can post the 450 ohm SWR plots of the seven antennas if anyone wants to see them.
    N2EY likes this.
  10. 2E0LPL

    2E0LPL Ham Member QRZ Page

    As somebody else said....just let the extra hang down or go off at an angle....if its a bit shourt it will still work.....just try maybe please.....

    Mine is well short for 160m....but gets out real well on that band.

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