$300 mistake antenna, on multiple levels.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4VBB, Sep 10, 2019 at 7:30 PM.

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  1. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Not my problem and not the point of my post. The OP started with an antenna that was too short for his intended band of 80m, by about half. I reviewed the math that he should have done to arrive at his original goal of an 80m dipole. That's all.
    NH7RO likes this.
  2. N3PM

    N3PM Ham Member QRZ Page

    Maybe a Double Extended Zepp? Now that you mention it, If I were to take some measurements I bet that's EXACTLY what I wound up with.[/QUOTE]

    Double extended Zapp.
    Mike N3PM
    K4VBB and NH7RO like this.
  3. NH7RO

    NH7RO Ham Member QRZ Page

    Good one!
  4. K4VBB

    K4VBB XML Subscriber QRZ Page

    Nope. This antenna was going to be for the center of the phone band on 80. Math say's it should be a little over 60ft per leg for a total of just over 120 feet. Had I fed it as I intended, it would have been fine.

    Now that I have my final measurements in front of me, here are the final dimensions of the antenna:

    57.5' per leg for a total of 115' total
    31.9167' 450 ohm Window Line.

    It resembles an Extended Double Zepp in appearance, but not dimensionally. Since it performs well I think I'll keep it up for now, but as soon as I get my coax in the mail I'm taking it down and making a proper dipole for 80.

    I've attached an image of the last output I ever got from the RigExpert. This was before final trimming, but it looks pretty good on paper.

    Attached Files:

  5. AA7EJ

    AA7EJ Ham Member QRZ Page

    Thanks for posting this calculation "in cubits" as a classic / typical UNNECESSARY mixing of measuring in cubits and metric system.


    Now tell us ( the math!) how you are going to physically realize / make " .7 " of a cubit.

    Now compare same task in building same length in meters

    300/3.5 =~ 85.71 METERS
    85 meters
    71 centimeters

    And if you insist in including "end effect "
    85.71 * .95

    Any decent hardware supplier will sell you measuring tape marked in BOTH cubits and meters.

    73 Shirley

    Just for fun - show how to calculate side of the diploe in cubits - from 133.7 cubits.
  6. N4MU

    N4MU Ham Member QRZ Page

    Okay with all that tech stuff. Let's get serious here: Did you get 5T5PA in the logbook (at the expense of your analyzer)? All America wants to know!
    W9FL and NH7RO like this.
  7. KB1CKT

    KB1CKT Ham Member QRZ Page

    115' of radiator, 32' of 450ohm line. Sounds like a quarter wave matching section for 40m. Not too far from a G5RV actually. That uses 102' and thus explains why it's down at 2.6MHz.

    Anyhow. Better luck next time. All I know is, when I go to make stuff, no matter how many times I cut it, it's always too short.
    NH7RO and KT5WB like this.
  8. K4SAV

    K4SAV Ham Member QRZ Page

    Actually no math is required for this. Since he has 4NEC2 all he has to do is model it and trim either the wire length or the feedline length to get to the right frequency. When you do that you will find there is apparently interaction between this antenna and the other one that is only 18 inches away because the model doesn't agree with the measurements. So if you want a more accurate answer, the other antenna needs to be included into the model.

    Math guys can try to develop the equations that describe this system. Report back in a couple of years when you finish.

    Jerry, K4SAV
  9. W4KJG

    W4KJG Subscriber QRZ Page

    K4VBB's story sounds like one of my many adventures that go deep into the night. It's not usually good when a project has gone awry and then reaches way past midnight. :confused:
    NH7RO likes this.
  10. K4VBB

    K4VBB XML Subscriber QRZ Page

    During initial trimming the first antenna was not attached. I did the final trimming after putting them both up for the very reason you state. Either way, it is strange, and I think I need more education when it comes to modeling. Maybe when I get home I'll upload the model with under a new thread--hopefully you smart 4nec2 guys can point me in the right direction.

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