Feeding cubical quad antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK8HW, Nov 23, 2019.

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  1. VK8HW

    VK8HW Ham Member QRZ Page

    G’Day, Desiring to feed a 2 element cubed Quad antenna with 450 open wire ladder line ( .88 Velocity Factor, and My intention is to use multiples of Half wavelengths) connected to good old bt1500 balanced tuner, then coaxial cable to transmitter. However at the Antenna feed point IS WHERE I AM UNCERTAIN exactly what to do. Should I simply connect the 450 Ohm line directly to the Quad driven element, OR would it be better to have a 4:1 Balun/ balbal there. Most Quad. Antennas show approx. 105 Ohms or so at the desired feed point. Indeed, if I decide to use no feedpoint transforming device, probably 300 ohm ladder line would be better. Alright, if anyone can clear the waters on this, please go a ahead,

    Thanks de Charles.
    Last edited: Nov 23, 2019
  2. AF7ON

    AF7ON Ham Member QRZ Page

    A cubical quad with typical dimensions could have lower impedance than 100 ohms. I have fed quad loops with open wire line (650 ohm) directly from a balanced tuner, in which case impedance doesn’t really matter. Line SWR may be high, but losses are low. I don’t think baluns will buy you much.

    Beware, though, if you intend to rotate the quad. An easy job with coax, more complicated with ladder or open wire line.

  3. VK8HW

    VK8HW Ham Member QRZ Page

    That is a good report from your experiences there, Thanks de Chuck.
  4. KH6AQ

    KH6AQ Ham Member QRZ Page

    A 1:1 choke balun is not needed when feeding a 1 wavelength loop with ladder line. Even coax does not need a 1:1 choke balun, although it doesn't hurt to use one.

    I would use whatever length of 300 ohm ladder line physically fits. The antenna tuner will take care of the transformed impedance.
    K0UO likes this.
  5. K7JEM

    K7JEM Ham Member QRZ Page

    I think the idea of using multiples of half wave length for the feedline is good. If you have a 100 ohm feedpoint impedance, that will make it close to 100 ohms at the tuner, which should be pretty easy to match. If you had an odd QW length of feedline, then the impedance presented to the tuner would be around 2000 ohms, assuming 450 ohm line. This may be hard for the tuner to match, or even be out of range of the tuner, depending on the particular frequency. The specs for the tuner show 2500 ohm max on 160-20M, but 1000 ohms max on 17-10M.

    Switching to 300 ohm line would make the match better, with 900 ohms being the result of a 100 ohm antenna and a QW feedline.
    WA7ARK likes this.
  6. VK8HW

    VK8HW Ham Member QRZ Page

    Reply to Kh6aq, Yeah, thanks, again you folks have helped my many thoughts on this “feed the Quad” job so much clearer and tangible. Mmm yep, three hundreds Ohms lines at 1/2 wavelength line lengths( or multiples of 1/2 wave) is on the board.
  7. VK8HW

    VK8HW Ham Member QRZ Page

    also Mike thanks for mentioning the tangles and rips that happen when rotating. Happily this will be mostly anchored in one direction. Armstrong but weak. ( Weak but Armstrong).
    Last edited: Nov 23, 2019
  8. VK8HW

    VK8HW Ham Member QRZ Page

    reply to k7 jem; Thankyou for your time and sharing your info Dear Friend. Yes, now I can go ahead with this little quad beam all assured.
    Got all I need now thanks everyone for your supporting commentaries.
    Wishing you all well, de Charles Vk8hw.
  9. AI3V

    AI3V Ham Member QRZ Page


    K0UO likes this.
  10. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    What he said

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