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Thread: A high gain CP OMNI finally! Parasitic Lindenblad

  1. #1
    Navigator Kondor-FPV's Avatar
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    A high gain CP OMNI finally! Parasitic Lindenblad

    Ok, as promised I will post pictures of this high gain Lindenblad, I will call it
    Lindy5.8

    It is a parasitic Lindenblad built in the easiest way possible, with a vertical dipole in which part of the connector and the outer shield work as the ground plane and the inner conductor works as the active element. This will avoid problems with matching, in fact the SWR is quite good at 1.4:1 on 5705.

    The design is actually LHCP as you see in the picture since the "circulator" uses parasitic, not driven elements.

    I have flown this antenna for 2 days and I can honestly say it outperforms the SPW for low angles, it gets easily 6-7dbic gain, if you fly level, and at 30 you get about 3dbi, that's my estimate based on flight path, altitude and distance from base compared to video degradation.

    I think this antenna would have no problems blowing away the spw for LR records and multicopter flight, but it needs a bit of modeling to understand well what the effects of the connector are, being so close to the circulator.
    Of course some better looks would also help, but that is for later.

    If someone is willing to model it to understand the effects of the connector being so close to the circulator, I am willing to give out measurements of the g10 cnc cut circulator, and everything else.
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  2. #2
    Banana Drone Flying Monkey's Avatar
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    Looks dangerous! Lol, cool antenna! I'd make one but I don't have 5.8ghz. Can it be made at other frequencies? 900 or 2.4?
    "You can't take the sky from me"

  3. #3
    Navigator Kondor-FPV's Avatar
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    Quote Originally Posted by Flying Monkey View Post
    Looks dangerous! Lol, cool antenna! I'd make one but I don't have 5.8ghz. Can it be made at other frequencies? 900 or 2.4?
    yes, it can be made for all frequencies, but at this moment I don't have actual measurements, and the antenna needs to be tuned by chopping it down once the circulator is installed.. 1 week and I will have all frequencies down.
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  4. #4
    Banana Drone Flying Monkey's Avatar
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    Oh okay cool!
    "You can't take the sky from me"

  5. #5
    Just dog tired. Wearyman's Avatar
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    I noticed that you have it listed as LHCP. Since pretty much ALL the other CP antennas out there are RHCP, can this be made RHCP as well so that it is compatible with existing systems?

  6. #6
    Engineer for Jesus Christ IBCrazy's Avatar
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    Now that's a killer! How did you come up with such a thing? I will admit I have not seen anything like that. How is the bandwidth?

    I have to admit, I tried to build one of these and failed... Good to see it worked out for you. If you give me the measurements for 1.3GHz, I will see if I can model it in 4NEC-2. For some reason 4NEC-2 doesn't like frequencies up around 5.8GHz.

    And another question: Can I try it?

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

    videoaerialsystems.com - Performance video piloting

  7. #7
    Navigator Kondor-FPV's Avatar
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    Quote Originally Posted by IBCrazy View Post
    Now that's a killer! How did you come up with such a thing? I will admit I have not seen anything like that. How is the bandwidth?

    I have to admit, I tried to build one of these and failed... Good to see it worked out for you. If you give me the measurements for 1.3GHz, I will see if I can model it in 4NEC-2. For some reason 4NEC-2 doesn't like frequencies up around 5.8GHz.

    And another question: Can I try it?

    -Alex
    Sure,
    you can try it...
    I will give you the starting measurements but than you have to optimize it, either by trial and error or through a nec program. This 5.8 one flew so I know it works, with the 1.3 you'll be testing new grounds.

    Basically make a vertical dipole with a brass tube and coax going through it, or just use the outer shield of the coax as the ground leg. Normally the ground leg has to be a bit shorter than the other pole since it's bigger diameter. Tune it for 1.2ghz, so you have room for chopping it down to get best SWR later.
    At this point make a square block or something with a hole in the middle to distance the parasitic elements from the outer shield 0.15 Lambda, which is about 30.1 millimeters at 1300mhz.
    At this point cut 4 pieces of 3mm thick copper 102mm long and glue them at 30 from horizontal on the edge of the standoff.
    The center of the standoff should be perfectly in the middle between ground element and active element.

    Let me know if you have other questions.
    The impedance will get lower once the circulator is placed in the middle of the dipole, about 30 ohms give or take so you need a choke or simply make the dipole use up all the length of the coax.

    As far as LHCP goes, I just thought I was making it RHCP but once I hooked up a 4 turn RHCP helix , performance was terrible, than I realyzed it actually reverses polarity, I switched to a LHCP helix on the RX and BOOM! cristal clear picture, and great performance.

    To reverse polarity just reverse the incline on the parasitic elements.

    This could turn any stock whip into a CP antenna, but with a lot of fiddling, because of impedance mismatch problems.

    EDIT: I forgot to answer your question: bandwidth is VERY good, just like a dipole, depends a lot on material and thickness.

    Thick copper : NICE!
    Last edited by Kondor-FPV; 26th July 2012 at 03:08 PM.
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  8. #8
    Navigator Kondor-FPV's Avatar
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    for nec modeling I suggest trying to optimize the parasitic element which should be between 98 and 102mm, and also thickness, 2 or 3mm would be good.
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  9. #9
    Shedding light over FPV HugeOne's Avatar
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    Good work Kondor!

    This is a model I made last year, never could get gain above 2.5db with acceptable axial ratio. This one bear 0.88, not bad. Never finished it as I got caught with antennas production, sale, support...



    If you don't mind, you could send it over to get gain and axial ratio measured on my analyzer.

    -Hugo
    Last edited by HugeOne; 26th July 2012 at 04:12 PM.

  10. #10
    RDWHAH bgarber's Avatar
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    Quote Originally Posted by Kondor-FPV View Post
    At this point make a square block or something with a hole in the middle to distance the parasitic elements from the outer shield 0.15 Lambda, which is about 30.1 millimeters at 1300mhz.
    Kondor, please forgive my "noob-ness" here but I have a question about the element distance. When you calculate 0.15 lambda to get 30.1 mm, are you using a velocity factor of 0.87?

    I saw an article about Parasitic Lindenblad antennas for 70cm in an AMSAT (Radio Amateur Satellite Corporation) publication... www.stalad.it/iz4bqv/antspec/2006ParaLindy.pdf

    Very interesting! When can we expect to see some production models for sale???

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