Dipole Length Annoyance

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD2CMJ, Mar 24, 2021.

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  1. KD2CMJ

    KD2CMJ Ham Member QRZ Page

    Hey y'all,
    So I've come to the annoying conclusion that the EAntenna 40/80 fan dipole I bought is short on 40m.
    My question is, is there a secret to lengthening a dipole outside of re-stringing this thing?

    For reference, see the spectrum images from my VNA.
    Antenna: EAN-R2010049
    40m.jpeg
    80m.jpeg
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    How high above ground is it installed?
     
  3. KD2CMJ

    KD2CMJ Ham Member QRZ Page

    Feed point is 55' up a hanging from a side arm mount.
     
  4. W9WQA

    W9WQA Ham Member QRZ Page

    did you check with a radio...?
     
  5. KD2CMJ

    KD2CMJ Ham Member QRZ Page

    Yes, the SWR meter on my 756Pro indicates similar readings.
     
  6. KD2CMJ

    KD2CMJ Ham Member QRZ Page

    The radio will tune 40m without an issue, as you can see, the antenna has a wide bandwidth. However, I can't tune 80m due to the narrow bandwidth.
    I obviously don't want to tune the 80m side until I have the 40m side fine tuned.
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    How long, and what type of coax?

    The bandwidth on 80m is typical. All you can do is move the swr min to a different frequency.

    I cannot find that antenna on their (or DXE's) website. Is it discontinued?

    I modeled it with insulated wire, wire ends of the 80m dipole at 30ft agl (inverted-V) over average dirt, with wires space 4 inches?

    To be resonant at 7.15MHz, the short dipole should be ~67.33ft.

    To be resonant at 3.8MHz, the long dipole should be ~123.6ft.

    To be resonant at 3.9MHz, the long dipole should be ~120.5ft.
     
    Last edited: Mar 24, 2021
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    At 55' earth influences still count, but mostly on 80m and not as much on 40m.

    80m full-sized 1/2-wave dipoles generally cover only about half the band with a reasonably good match. Wide band, percentage-wise.

    To "add length" to a dipole without re-wiring the whole thing is possible, of course. You can splice on more wire, and good splices are transparent to RF; I've often just used "tag" wires that hang down from the ends of the original dipole wires and are secured in place with wire clamps (SS). That the very ends of the dipole are hanging down instead of in a straight line doesn't seem to matter at all, as long as the whole structure is high above ground.
     
  9. KD2CMJ

    KD2CMJ Ham Member QRZ Page

  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    That shows a trapped dipole with a different part number, not a fan.

    Make sure you read post #7, again...
     

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