Feed 40m wire vert with open line?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AK3W, Nov 17, 2009.

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  1. AK3W

    AK3W Ham Member QRZ Page

    Its a little chilly outside but I am going to put up a 40m wire vertical (33'),
    with 3 or 4 elevated wire radials (8 feet off ground). Is this an unbalanced antenna? Will it cause unbalance of homebrew 600 ohm feed line?
    Will balun at feedpoint work for open line?
    I am primarily concerned about radiation off feedline and rf in shack.
    I will use an MFJ 949 tuner.
    Thanks
     
  2. AI3V

    AI3V Ham Member QRZ Page

    This antenna will have a feedpoint resistance between 35Ω and 50Ω

    It will be an unbalanced antenna.

    Simple direct coax feed is the preferred method.

    EDIT: you should not need a tuner to load this antenna with an 820 (The 820 already HAS a tuner built in!)

    Rege
     
  3. G3TXQ

    G3TXQ Ham Member QRZ Page

    I agree with Rege - use direct coax feed. But use a 1:1 current balun/RF choke at the feedpoint to stop the braid becoming the 5th radial and coupling RF back into the shack.

    Steve G3TXQ
     
  4. AK3W

    AK3W Ham Member QRZ Page

    Thanks Rege,
    Yes coax would be best, however I have none and
    do have a bunch of free wire to build an open line.
    And I will try to operate the vert on other bands.
    Matt
     
  5. AI3V

    AI3V Ham Member QRZ Page

    Steve, How do you propose that he route the coax, or open wire, or whatever he kludges up so that it won't pick up the rf that has been radiated by the antenna?

    And any kind of balun, or does he need a particular mix of ferrite?

    Of course, he's going to "use the antenna on different bands", Mix 73?

    That shoud be FB 160m-70cm.:rolleyes:

    Rege

    EDIT: What kind of choke impedance will the o/p need if he operates this antenna on it's second harmonic?
     
    Last edited: Nov 17, 2009
  6. R3BU

    R3BU Ham Member QRZ Page

    In case of 35ohms we are lucky in soil conductivity and efficiency.With 50ohms -to think to move all up, away from poor soil.
    Usually in 35ohm case I move the feed point along the antenna to 50ohms place.Vertical itself has about 0.27wl, radials about 0.23wl.Only with RF choke on the cable.SWR=1.0000 :)
    6-7 turns with coax on ferrite ring on 40m.
     
    Last edited: Nov 17, 2009
  7. AK3W

    AK3W Ham Member QRZ Page

    Thanks again Rege,

    Question: Will a 1:1 or 4:1 balun at the feedpoint keep the
    currents balanced on the open feed line.
    Matt
     
  8. AI3V

    AI3V Ham Member QRZ Page

    Alex, the o/p is trying to build a ground plane style of Marconi antenna.

    With 4 x 1/4 wl radials @ .06 wl above ground, there will be very little antenna interaction with the earth, especially from a earth conductivity standpoint.

    Rege
     
  9. AI3V

    AI3V Ham Member QRZ Page

    NO.

    Rege
     
  10. R3BU

    R3BU Ham Member QRZ Page

    I have made the calculation of Marcony antenna. In worst case with bad soil at 0.06wl it may have 13% of loses due to the ground.
     
    Last edited: Nov 17, 2009
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