Yet another biquad antenna

Discussion in 'Wireless Internet' started by Jeff Liebermann, May 29, 2005.

  1. I needed a waterproof 2.4GHz antenna, fast. So, I built my own.
    http://802.11junk.com/jeffl/pics/antennas/biquad2/
    This was suppose to be a fast and cheap construction job. It wasn't.
    Everything went wrong including trashing the memory card in my digital
    camera with photos of my mistakes in progress. Argh.

    The 5"x5"x2" PVC box in the photo is made by Stahlin, but I can't find
    the part number. About $11/ea. The inside dimensions at the bottom
    are just shorts of the 120mm needed for a real 1/2 wavelength. So, I
    ran the aluminium tape up the sides slightly. Not ideal, but close
    enough.

    I used LMR-400 UltraFlex coax cable. It was really tricky getting the
    rubber outer jacket through the cable clamp and crimp on N-connector.
    The smooth jacket standard (stiff) LMR-400 is easier to work with.

    The center cable feed thru is a 1/2" Liquid Tight Cord entry.
    Arlington Industries LPCG50-1. About $2.50. The LMR-400 fits
    perfectly around the clamping end. However, to keep the coax from
    flopping around, I wrapped the coax at the antenna end with electrical
    tape until the diameter gave a snug fit inside the cord entry.

    The coax cable was trimmed as in the photo. A small "tab" of the
    shield braid was left intact and tinned. This is where the ground
    connections for the biquad antenna elements were connected.

    Bending the antenna elements is the only critical part of the puzzle.
    See:
    http://martybugs.net/wireless/biquad/
    for instructions on how to cut and bend the elements. The only
    difference is that Martin suggests cutting the ground end of the
    elements to accomidate the increased coax cable (copper pipe)
    diameter. Bad idea. Leave them long, even if they are odd looking.
    That basic idea is for each loop to be exactly 1 wavelength long. The
    locations of the corner bends are non-critical. It could be circular
    shapped if that were easier. Only the length is important. Also,
    each quad antenna should be identical, and the ground points soldered
    as close together as possible.

    The 1/2" nylon plus 2ea 3/16" washers in the pictures were suppose to
    support the ends of the biquad. That's un-necessary as the assembly
    is self supporting thanks to the thick coax cable.

    Cost on this model is more than I would like.
    Stalin box $11.00
    cable feed $2.75
    3ft LMR-400 (scrap)
    N-connector $5.00
    ===================
    $17.75

    Methinks I'll try a cheaper box on the next model.

    I haven't done much testing with the finished antenna. It certainly
    has more gain than any of the rubber duckies or small panel antennas I
    have floating around. VSWR with my home made directional coupler is
    tolerable at under 2:1. Resonance ended up at 2380Mhz which is way
    too low. Cut and tweak comes tomorrow. I'll make some real
    measurements when I have time.
     
    Jeff Liebermann, May 29, 2005
    #1
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