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Thread: Crosshair - 10 dbic Circularly polarized antenna tutorial!

  1. #111
    ScratchCrasher Sfakias's Avatar
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    Thank you Alex for the quick reply.

    Angle is indeed 20deg, I made a typo in my original post, which I edited while you were typing ! When you say to put it closer to the edge of the roof, I suppose you mean that most of the roof should be at the back of the antenna, right? Is it better to just leave it on the ground? Would you suggest to use a CloverLeaf or a SPW on the plane?

    Best regards,
    Stavros
    Last edited by Sfakias; 7th March 2012 at 04:24 PM.
    To the skies !

  2. #112
    Engineer for Jesus Christ IBCrazy's Avatar
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    You have the right idea. Put it so the back of the antenna faces the roof of the car. I'd mount it on the roof rather than the ground. A cloverleaf or a skew wheel will do. I'd go for a cloverleaf, but if I only had a skew wheel, I'd use that without a second thought.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  3. #113
    Navigator Hemmi's Avatar
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    Can I use ca-195rw coax for this antenna?
    I think I'm gonna smash this VTX with extreme prejudice!! KK4HBG

  4. #114
    Engineer for Jesus Christ IBCrazy's Avatar
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    If it is 50 ohms you can. If not, you can, but shouldn't.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  5. #115
    Navigator Hemmi's Avatar
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    Quote Originally Posted by IBCrazy View Post
    If it is 50 ohms you can. If not, you can, but shouldn't.

    -Alex
    Yes its 50ohms. its the same cable Lcom uses on all thier patch antennas. The white stuff
    I think I'm gonna smash this VTX with extreme prejudice!! KK4HBG

  6. #116
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    Aaaand... here we are, just finished! The Toothpicked Crosshair

    RHCP, 1010mhz, 250mm reflector side made of wood with alumnium foil glued with white glue.

    I gives a Vf of 1.98v and Vr of 0.22v thus giving an swr of 1.25 !! Too good to be true??

    Still untested "in the wild"
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by merengue; 8th March 2012 at 07:58 PM.

  7. #117
    Engineer for Jesus Christ IBCrazy's Avatar
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    Quote Originally Posted by merengue View Post
    Aaaand... here we are, just finished! The Toothpicked Crosshair

    RHCP, 1010mhz, 250mm reflector side made of wood with alumnium foil glued with white glue.

    I gives a Vf of 1.98v and Vr of 0.22v thus giving an swr of 1.25 !! Too good to be true??

    Still untested "in the wild"
    That's about right, actually. More or less perfect. It looks great, too! I think you will be very happy with this one.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  8. #118
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    Quote Originally Posted by Sfakias View Post
    Hello all.
    I just finished my 1280MHz crosshair, and I attach pics of it. I also made a simple wooden support, since I plan to have the antenna placed on the roof of my car. The FOX800 rx is mounted at the back of the reflector, using magnets.

    I would be grateful if you could answer the following questions:
    - Is it OK to place it on the car roof, or can the big metal mass (car roof) cause deterioration of the antenna performance?
    - The antenna is tilted 20deg from the vertical. Does this angle seems right for distant flying, without a tracker?
    - Can the small radious bend of the coax cable from the balun to the vid rx, deteriorate the reception quality?

    Thank you for your answers, and a special thanks to Alex for taking the time to write this tutorial.

    P.S. I am looking forward to trying this in the air, but it may be a while until the next flying session for me...
    Do you want to share all your measurements. I run on the same frequency and would also like to make one.

  9. #119
    Engineer for Jesus Christ IBCrazy's Avatar
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    @mickeshobby - It's really simple:

    For 1280MHz:

    Copper tube - 2.29"
    long element length: 2.3"
    Short element 1.931"
    Reflector - 12X12"
    Wire diameter: .035"

    Easy. Just plug your frequency into the formulas on the second post.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  10. #120
    ScratchCrasher Sfakias's Avatar
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    I did exactly what Alex suggests, plug in the numbers in his equations.

    I give below the dimensions for 1280MHz, quoting Alex's original posts, where calculations and unit conversions are applicable, for the metric folks : the figures with the three decimal digits are the dimensions in mm.

    ... Copper tube either ( or 3/8) 6.350 or 9.525
    ...Wire diameter (in inches = 42/f in MHz to 60/f in MHz) 0.833 to 1.191
    ...Height over reflector in inches = (2350/f in MHz) 46.633
    ...Length of copper tube = (2933/f in MHz) 58.202
    ...Size of reflector panel (in inches= 8800/f in MHz to 17500/f in MHz (larger reflector increases gain and F/B ratio)) 174.625 to 347.266
    ...Short leg length (in inches = 2490/f in MHz) 49.411
    ...Long leg length (in inches = 2945/fin MHz) 58.440
    ...For this tutorial I am building a 1280MHz Crosshair. I am using (.035") 0.889 Welding wire and RG316 cable
    ...Make a cross across the panel. Drill the center out on solder in your copper tube such that only the height calculation protrudes (2350/f in MHz) 46.633 if you plan on using a balun
    ...Now drill 4 holes on the X to accommodate your nylon bolts. I placed mine at (1800/f) in MHz 35.719 away from the center, but this is not critical.
    ...Now measure down from the outside INSULATION wavelength (2933/f in MHz) 58.202 and remove the outer covering at that point
    ...The measurement from the top of the tube to the base of the reflector should be about (1/16) 1.588 less than your height (hence=45.045mm) so your elements can clear it.
    ...Now strip as much insulation off the center element as possible without it shorting out to the shield. You should have about (1/16) 1.588 of insulation sticking out of the cable at this point. Tin both the center element and the shield. Trim the shield back very short so that only about (1/8) 3.175 or less is sticking out the side.

    Regards,
    Stavros
    To the skies !

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