Best com antenna for Glasair III

Discussion in 'Instruments / Avionics / Electrical System' started by marcmccalmont, Sep 6, 2019.

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  1. Sep 6, 2019 #1

    marcmccalmont

    marcmccalmont

    marcmccalmont

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    I restored a Glasair III using Bob Archer folded dipole com antennas
    The performance is not great, wondering if anyone has had good luck with a particular antenna in a glasair or other fiberglass aircraft?
    Thanks in advance
    Marc
     
  2. Sep 8, 2019 #2

    proppastie

    proppastie

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    did you have a metal ground plane .....attach to glass without one and it will not work well
     
  3. Sep 8, 2019 #3

    BJC

    BJC

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    The standard Glasair com antenna is a dipole made from 1/2” copper tape, connected with RG 400.


    BJC
     
  4. Sep 8, 2019 #4

    Derswede

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    But RG-58U or "Mini-RG-8U" will also serve fine for feed. Both of those coax's are common and inexpensive, as well as fine at VHF frequencies. Make sure the dipole is not pointing against any metallic items, and if you use metallic paints, it can have some effect. You want any antenna away from metallic objects as those can detune the antenna and cause poor performance. West Mountain Radio shows the following for a homebrew dipole.
    Frequency: 126 MHz
    Total Antenna Length ("L"): 3 Feet, 8.5 Inches
    Element Length ("E"): 22.3 Inches
    Feed from the middle, each element connected to one of the two conductors of the coax. Make sure neither end touches a ground on the airframe. The calculator is at: http://www.westmountainradio.com/antenna_calculator.php?frequency=126
     
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  5. Sep 8, 2019 #5

    pictsidhe

    pictsidhe

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    No balun? It's actually fairly easy to make a co-ax balun. Slip a 1/4 wave of braid down the outside and solder to the ground braid at the start of the balun section. This works best if you use a low loss insulator between ground braid and balun sleeve.
    Mounting onto fibreglass will detune the antenna compared to air. It would be worth making it tweakable and finding someone to lend you a VSWR meter.fe
    A coaxial antenna is much like the sleeve balun, but has the outer braid connected where the braids terminate in the middle of the dipole that it forms. This one would suit internal mounting. That relies on low losses through the skin.

    Aircraft band is not far from VHF radio. If you have a receiver with a signal strength meter, you could use that to play with ideas, then scale to suit aircaft band.
     
  6. Sep 8, 2019 #6

    wsimpso1

    wsimpso1

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    These are calculated using thin wires and may be pretty specific as to tuned frequency - we need to cover a decent band of frequencies for our COM antenna so this might not work as well as you would hope.

    For antenna with wide use in plastic airplanes, go to RST http://www.rstengineering.com/rst/products/plasticplaneantenna/plasticplaneantenna.htm. Jim Weir, a frequent presenter at OSH, is the company and he talks in his booklet about how to have enough band width to cover the range of frequencies used in our COM and NAV radios - he recommends using 1/2" copper tape and reduce the length a little to compensate for the width. The booklet gives dimensions for what works in little airplanes. He also recommends ferrite bead baluns and sells a kit with the booklet, tape, and beads. $30 well spent.

    Billski
     
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  7. Sep 8, 2019 #7

    Pops

    Pops

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    Been using Jim's antennas on wood airplanes since about 1981. They always work better than the high dollar antennas.
     
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  8. Sep 8, 2019 #8

    BJC

    BJC

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    The standard Glasair com dipole used two 1/2” X 20” elements.


    BJC
     
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