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Full wave length Round Loop antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KA9JLM, Mar 18, 2012.

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  1. KA9JLM

    KA9JLM Ham Member QRZ Page

    Has anyone used a full wave Round Loop antenna, on 20 meters ?

    I have used a 4 sided full wave, but never a round one.

    What would be the best way to match it to the radio , for the best power transfer ?

    And what is the best feed point location ?

    I was going to use 1/2 " PVC to keep it close to Round and a 12ga wire inside of it, and build it for 20 meters.
    Would that be a good Idea ?

  2. N7WR

    N7WR Subscriber QRZ Page

    Generally round loops are hard to construct requiring more supports than a square loop....and likely not worth the effort. If you have access to a good antenna modeling program check out the round loop you propose and compare it to a square or rectangular loop.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Thanks Jerry,

    Can You recommend a good antenna modeling program ?
  4. N7WR

    N7WR Subscriber QRZ Page

    EZNEC is the best program to use. I recently asked a fellow, very tech savy ham to model my recently constructed 80 meter horizontal loop (square shape requiring only 4 supports). I was amazed at the results. He ran some theoretical comparisons and no other shape had any real advantages over square.
  5. MI6KAK

    MI6KAK Ham Member QRZ Page

    I looked at a square and round on a modeling program and they appear to behave the same. As Jerry has said square would be a lot easier to constuct no real gains to making a round one.

    73 Carl
  6. KO6WB

    KO6WB Premium Subscriber QRZ Page

    In theory the round shape will have a slight gain over the square, diamond and delta configurations. The round shape has a greater square area then the other shapes but not much and it is doubtful you would notice the difference. The shape that offers the least amount of gain is the delta but again it isn't much lower than the others.
    Feeding the loop is accomplished by using a matching section of 75 ohm coax that is fed to the loop at the open ends. The matching section is 1/4 wave long (234/F)*VF= feet in length. F is the frequency is Mhz and VF is the velocity factor of the coax. The remaining distance to the shack is 50 ohm coax of any lenght.
    If you have the feed point at the very top or bottom and your loop is perpendicular to the ground you would have hortizontal polarization. If you fed the loop on the side (either side) then the loop will have vertical polarization.
    A good reference for these are in the ARRL Antenna Book or you can download this It's very lenghty and packed full of good information. Unlike the ARRL Antenna Book this book uses the 468/F = feet formula a lot. In the recent ARRL publication I could find that formula just once.
    Good luck
    Last edited: Mar 18, 2012
  7. KH6AQ

    KH6AQ Ham Member QRZ Page

    Think of a one wavelength loop as two halfwave dipoles connected end-to-end and spaced 1/4 wavelength apart.

    For horizontal polarization feed the loop at the bottom and for vertical polarization feed the loop at the top. The free version of EZNEC, which is limited to 20 current segments, is enough to model your antenna. Start with a square loop having five current segments per side. Feed it at the bottom then on a side and see how the radiation pattern changes. Remember to use REAL ground and place your loop at the height it will be used.
  8. N7WR

    N7WR Subscriber QRZ Page

    My 80 meter loop is fed at one corner with 50 ohm coax through a 4:1 current balun and works great 80-6 meters
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Thank You Gary, and the others for the reply.

    That is the info I was needing, I am new to round, always used a square.

    The reason I wanted to go with the Round was so I could use a rotor and get some sort of directionality path. Not sure how well that would work, without some experimentation.

    So feeding it from the top would be a different polarity than feeding it from the bottom ?
    What does feeding it on the side do ?

    I did want to slope it for the range of the contact. I normally use a Cloud Warmer, But wanted something different.

  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    You're obviously talking about a vertical loop. Very different from a horizontal loop.

    Horizontal loops on HF are mostly 1WL in perimeter (or just a bit longer).

    A 20m loop would be 23' in diameter, that's a lot to rotate. Is this what you mean?
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