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Thread: Turnstile UHF RX antenna tutorial

  1. #1
    Engineer for Jesus Christ IBCrazy's Avatar
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    Turnstile UHF RX antenna tutorial

    In this tutorial I will show you how to build your own dual polarized turnstile antenna for improved performance from your LRS. The antenna should be laid flat along the bottom of your wing for best performance. Your TX antenna should be pointed to the side instead of upright. This boosts performance by about 8db.





    I give you this tutorial in the name of my Lord and Savior, Jesus Christ. I wish you the best of luck in your FPV ventures.

    -Alex
    Video Aerial Systems LLC
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    Last edited by IBCrazy; 23rd March 2011 at 11:31 PM.
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  2. #2
    Engineer for Jesus Christ IBCrazy's Avatar
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    How to build the antenna

    This antenna is basically two out of tune dipoles fed in parallel.



    Take 2 pieces of 18-22 AWG wire 15 (40 cm) long and strip the insulation off of the middle exposing only about or less of bare copper. Bend this 90 degrees and tin it with solder to hold the bend.


    YOU MUST USE 18-22AWG WIRE! THE THICKNESS OF THE WIRE IS CRITICAL! You may use stranded or solid wire. Your preference.



    Now strip your coaxial cable (I used RG316) and split the center element and the shield. Solder the center element to one of the L shaped elements and the center conductor to the other. You want the feedpoints to be as close as possible, but not touching.


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

    videoaerialsystems.com - Performance video piloting

  3. #3
    Engineer for Jesus Christ IBCrazy's Avatar
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    Tuning the antenna

    You now have a parallel dipole with 4 elements about 7.5 long. You need to trim this down to the proper length. The best way to do this is to mark a sheet of paper with a cross with the required dimensions:

    Short leg of the cross: 11 3/4
    Long leg of the cross: 13 1/2

    Make sure the lines cross right in the middle. Now place your antenna on this drawn cross so that the feedpoint sets right on the intersection. Clip off the ends of the 4 wires at the edges of the cross. Add a drop of glue (I used hot glue) on the feedpoint to keep the antenna from shorting out.

    You now should have a cross with two legs 5-7/8 and two legs 63/4" long.

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

    videoaerialsystems.com - Performance video piloting

  4. #4
    Engineer for Jesus Christ IBCrazy's Avatar
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    Mounting the antenna

    Mount the antenna flat to the bottom of your wing. One of the best ways to do this is to run one element down the fuselage and the other out the wing. If using a flying wing, simply mount it underneath the plane.

    Aim your transmitter antenna to the side rather than upright. This makes it horizontally polarized. Horizontal polarization has shown longer range than vertical. You can also build a turnstile for your transmitter if you like.


    -Alex
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    Last edited by IBCrazy; 27th March 2011 at 03:48 PM.
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

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  5. #5
    Engineer for Jesus Christ IBCrazy's Avatar
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    How the antenna was designed and how it works

    I need to thank MR RC-CAM (Thomas Black). He came up with this antenna for his 900MHz system. I knew I needed two elements of 100 Ohm impedance in parallel phase shifted 90 degrees. When I asked him about how he obtained the phase shift (I asked if he used a 90 degree phase stub) he responded “I used a reactive coupling system”.

    At the time I had no idea what he meant and thus the antenna remained a mystery. I tried a phase shifting stub anyway – FAIL. Then while researching the quarafiliar helix antenna by Walter Maxwell (W2DU) I ran across the line that explained that the phase shift in the quad helix was obtained by the reactive components determined by the thickness of the wire.
    I then ran a frequency sweep of a standard dipole and watched the reactive impedance change from lagging to leading crossing over the resonant length at the 0 reactive impedance point. I now had it! All I needed to do was make two dipoles and adjust the lengths so that they had 100 Ohm impedance and their phases were 90 degrees apart. Ideally this would be 75+J75 and 75-J75. This would give me a perfect 50 ohm load. This is of course not possible. However I found the optimal (real) impedances were: 54+j87 and 57-J84. This gives a 91 degree phase shift – close enough for me.

    So I went back to my frequency sweep and realized that I got these impedances at 405MHz and 470MHz. So all I needed to do was make two dipoles, one for 405 MHz (6.75” legs) and one for 470MHz (5.85” legs), set them in parallel and it would give me my antenna.
    I then tested this theory in 4NEC2. After many attempts to fix the “segment errors” I gave up and just let them be. Analysis showed that I had the desired impedance almost perfect. When I ran a frequency sweep, I noticed the antenna had a very wide bandwidth. Unfortunately I could not set it such that the resonant frequency hit the center of this band. Realizing the toleranes weren’t nearly as tight as they were with my 2.4GHz antennas that always seemed to work great, I decided to stop there and just build it. Later analysis showed that adding 1/16” to the element lengths pushed the resonant frequency more to the center.

    So now I had a choice. Build a whole bunch of these and sell them on my website keeping the details to myself and making a bunch of money off of it (since I’d be the only one who understood it other than a select few), or write a tutorial and give it all away free. I did neither. I made a video instead. Yes, I know I’m a terrible business man… but hey, I’m an engineer. I continue to give the information away free and business continues to come. Granted, not as much as if I developed everything in secret, but enough that I am happy. Money isn’t everything.
    I describe the value of my life as such: “Life is a journey, not a destination. It’s value is in the times we’ve had, the memories we keep, and the lives we touch.” – A. Greve

    FPV and the research I have given away has made me many memories and many friends World wide. So if I have made a positive impact on your life, this is all worth it. There is no need to repay me for anything. Just pay it forward.


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

    videoaerialsystems.com - Performance video piloting

  6. #6
    Fly FPV, sleep; repeat
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    Awesome!! I definitely will be making use of this antenna with my DragonLink system. This world needs more people like you. Couple of questions...

    - What's the gain of this antenna?

    - How does it compare to a typical dipole antenna? (range capability, radiation pattern, nulls, etc.)

    - What does the vertical radiation pattern look like?

    - Is it ok to feed the antenna with a long coax (18-24")?

    BTW, I've bought a couple of your antenna kits so far (Vee & BiQuad) and will continue to do so to help fund all your great work on this!

    Thanks!

  7. #7
    Penguin flyerDisco buildr mike20sm's Avatar
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    So is this supposed to outperform the Sander style antenna?

  8. #8
    FPV Legendary Loser Mark Hitchman's Avatar
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    ALEX, you are a legend

    These will be perfect on a wing where you don't have anything sticking up to attach a dipole to.

    Thanks

    Mark

  9. #9
    Engineer for Jesus Christ IBCrazy's Avatar
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    What's the gain of this antenna? - 2.2dbic

    - How does it compare to a typical dipole antenna? - It is a near perfect sphere. It has no nulls which makes it excellent to avoid dropouts.

    - What does the vertical radiation pattern look like? - Spherical.

    - Is it ok to feed the antenna with a long coax (18-24")? - Yes.

    @Mike - The Sander style antenna doesn't even compare in performance to the turnstile.

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

    videoaerialsystems.com - Performance video piloting

  10. #10
    Fly FPV, sleep; repeat
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    Quote Originally Posted by IBCrazy View Post
    - How does it compare to a typical dipole antenna? - It is a near perfect sphere. It has no nulls which makes it excellent to avoid dropouts.

    - What does the vertical radiation pattern look like? - Spherical.
    I thought a perfect sphere was impossible to create. But I guess that's why you say "near" perfect.

    So does this mean diversity antennas are no longer needed? No nulls to worry about. But do you still have polarization issues when the airplane is banking?

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