Parallel Dipoles in the Attic

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WD9DWT, Jun 1, 2021.

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  1. WD9DWT

    WD9DWT Ham Member QRZ Page

    I have a 20m dipole in my attic. As a stand-alone antenna it is resonant at about 48 ohms. I installed another antenna that runs parallel to it. The second antenna is a compact 40m dipole, where one leg is a 1/4 wavelength wire and the other leg is a 3-foot 40m mobile hamstick. It is resonant at 54 ohms. The two antennas are not connected to each other - they each have their own feedpoint. I don't have any feedlines or baluns connected to them yet. Their separation varies between 8 and 14 inches. With the 40m antenna installed I connected the analyzer to the 20m antenna again. I was shocked to see that now the 20m dipole was not resonant anymore, anywhere! It is as if the parallel 40m dipole has de-tuned the 20m dipole. Can these parallel antennas be made to work together? If so, should their spacing be closer together or farther apart? Should I use spacers between them to keep them apart at a consistent distance? If so, how close should they be?
  2. W4HWD

    W4HWD Premium Subscriber QRZ Page

    Short of turning them 90 degrees to each other I don't see how you can avoid mutual coupling in an attic.
    KU3X, AK5B and WA7ARK like this.
  3. K7MEM

    K7MEM Ham Member QRZ Page

    Yes. Parallel dipoles will interact with each other, but you should be able to trim them to work fairly well. Hams use "Fan Dipoles" all the time. When they are installed, each of the dipoles in the fan need to be trimmed. Starting with the highest frequency dipole and working towards the lower frequency dipole. If you then add another dipole, for another band, you will probably have to adjust all of the dipoles again. Hopefully, you analyzer will help the adjustment.
  4. WD9DWT

    WD9DWT Ham Member QRZ Page

    Thanks for the suggestion. Unfortunately, I am not able to configure these two antennas as a fan in my attic, which is why I had to take the parallel route. Looks like I may have configure them orthogonally as the first reply suggests.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Start over. What are you trying to accomplish?

    Have an attic antenna that works on both 40m and 20m?
    SM0GLD and W4HWD like this.
  6. WD9DWT

    WD9DWT Ham Member QRZ Page

    I'd rather not start over. I designed these antennas to fit within the constraints of my attic. How big is yours?
  7. AK5B

    AK5B Ham Member QRZ Page

    I'll suggest getting one of those out of the attic, despite condo restrictions. If done well, you could camouflage the 40m hamstick and use it as an elevated vertical instead. The two radials you'd need could be made with 26 gauge DavisRF Polystealth that is both strong and essentially invisible to the naked eye above ground.

    I paint all of my antennas with either Rustoleum Camo paint (spray can) or mix matte acrylics from Walmart (good stuff for 50 cents a bottle you apply with a brush---once dry it lasts well outdoors)---you can, too, to achieve stealthy installations "hiding in plain sight.";)


    Lord Stealth
    17th Earl of Camouflage
  8. K7MEM

    K7MEM Ham Member QRZ Page

    Sorry, I probably didn't make myself clear.

    I am not saying that you have to configure your antennas as a fan. They are already configured as a fan. Fan simply describes a system of two, or more, parallel antennas. The distance between the antennas will vary. Sometimes the dipoles fan out at the ends, sometimes they don't. The point is, when you put two dipole antennas parallel to each other, they interact. You can straighten them out, or space them a bit more, but they will still interact. You need to compensate for this interaction.

    By compensate, I mean lengthen or shorten the 20 Meter antenna, to put it at the frequency range you want. Then, check the 40 meter antenna. Adjust it as necessary. If you made any adjustments to the 40 Meter antenna, go back to the 20 Meter antenna and check again. If you only have two antennas, this should be a simple process. If you add another antenna, in parallel with the others, the same process would be used. Start with the highest frequency antenna and work towards the lowest frequency antenna.

    Once you connect feed lines and baluns, it is likely all your adjustments will need to be repeated.
    AK5B likes this.
  9. SM0GLD

    SM0GLD Ham Member QRZ Page

    One leg of the 40m dipole is 1/4wl witch is resonant on 20m and it will surly affect the resonance on 20m. That element is not terminated since you dont have anyting connected to it.
    NE4ME likes this.
  10. KI8DJ

    KI8DJ Ham Member QRZ Page

    How about something like the ultimax 100 antenna. 24 feet of wire and a 9 to 1 unun. Works 80 through 10 M usually just with radios built in tuner. There are reviews on eham.

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