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

    Thanks so much!
     
  2. WD9DWT

    WD9DWT Ham Member QRZ Page

    I began with that. I tried in the yard and in the attic with a 20m and a 17m dipole both connected to the same feedpoint, analyzing them with no balun and no feedline, fanning them a couple feet from each other at the ends. There was too much interaction between them. Perhaps my problem was the analyzer itself, expecting to always see a 50 ohm system.
     
  3. N3DT

    N3DT Ham Member QRZ Page

    Probably not, fan dipoles are notorious for interaction although some seem to get them to work. I never had much luck. The CR section will hardly be noticed on the fed antenna.
     
    WD9DWT likes this.
  4. SM0GLD

    SM0GLD Ham Member QRZ Page

    Those two are very close in frequency so they will interact a lot. It is easier with 40/20/10m.

    On a 40/20/10 fan dipole injecting 28MHz only the 10m dipole will have low impedance so it will "steal" all the power.
    All the other dipoles will have high impedance so almost no effect will be consumed by those dipoles. And this goes on for all the different frequencies.
    This is the reason a fan dipole will work.
    Trying to add a dipole to the fan that doesn't behave as either low or high impedance will be problematic.
     
    AK5B, WD9DWT and WA7ARK like this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    If you put a coax common-mode choke (aka 1:1 current-mode balun or coax-line-isolator) at the feedpoint of a fan dipole, there is almost no interaction between bands. Trimming one wire to move the resonant point on a given band has almost no impact on the other bands.

    If you leave off the choke, then depending on total coax length and how/if the rig is grounded, there can huge interactions between the wires.
     
    K9UR, AK5B and W1PEP like this.
  6. WD9DWT

    WD9DWT Ham Member QRZ Page

    That's the best answer I've heard to solving the delemna I had with analyzing my fan dipole. By the way, I've appreciated everyone's comments. It has been good learning. Thanks to all of you!
     
  7. WD9DWT

    WD9DWT Ham Member QRZ Page

    Where could I find something like this?
     

    Attached Files:

  8. K3UJ

    K3UJ Ham Member QRZ Page

    If you are analyzing with no feedline, are you standing at the middle of the antenna? You are now part of the pattern and can through things off.
     
  9. AK5B

    AK5B Ham Member QRZ Page

    Office supply or computer/electronics store, most likely.
     
  10. WD9DWT

    WD9DWT Ham Member QRZ Page

    Agreed. The main reason for an initial analysis directly at the feedpoint is just to get an idea for what ratio of a balun I would need to get. Then I would install an appropriate balun, hook up the feedline and properly do my successive analyses from the radio-end of the line.
     

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