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Thread: Using circularily polarized directional antennas for sending

  1. #1
    Join Date
    Jul 2013

    Using circularily polarized directional antennas for sending


    First of all, thanks heaps Alex for all your work on antenna design
    I'm a very happy user of your antennas for almost 2 years now. I built CL/SPW/Helix for 5.8ghz myself according to your tutorials and also bought the ones you built for comparison

    Recently I started working data links over WLAN in the 5ghz band. Since one of the stations will be flying I thought using CP antennas would be a good idea. My initial setup was a Mad Mushroom on the plane and a helix (12.5 dbic Black Cannon) on the ground. The problem I'm facing with the helix is, that it is not so efficient apparently. I remember that there were quite a few (hot) discussions on that topic here as well.

    In my tests I compared the CP setup to a linearly polarized setup with regular WLAN antennas (3dbi omni with 12dbi directional). To achieve the same signal quality with the CP setup I had to increase the output power on the helix side by at least 10dbm. The used frequency might also not be optimal, I used 5680mhz in my latest test, but since the helix is pretty wideband I figured this shouldn't be much of a problem.

    What is the general opinion on sending with the now widely used directional antennas for FPV? What are the options? (maybe different type of helical, parabolic dishes, etc.)

    I'm also wondering if I should try building a Crosshair?
    I believe this has been done successfully for 5.8ghz as well, I could even build one for a lower frequency if that simplifies it a little (e.g. for 5500mhz since I won't operate it in the US).

    Any help, ideas, suggestions are very appreciated

  2. #2
    Engineer for Jesus Christ IBCrazy's Avatar
    Join Date
    Mar 2011
    Amherst, VA
    Helical effieicncy is lower than many antennas due to side lobe radiation. A helical has sidelobes which allow an aircraft to fly in close without signal loss. These take away from the main beam of the antenna making it narrower. This is the efficiency reduction we speak of.

    A crosshair is an excellent antenna, but it is hard to build. It has no sidelobes and thus the efficiency is quite high. I have used it and helicals both for WiFi communications and they worked spectacularly.

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

  3. #3
    Join Date
    Jul 2013
    Hi Alex

    Thanks for your answer. I think I'm going to try the Crosshair approach then

    Btw, I completely forgot to mention the distance in my tests: it was 6km, this should put the 10dbm power increase into perspective.
    (EIRP for one side can be 27dbm max. so we don't require a ham license to operate it)

    Last edited by ada; 27th June 2014 at 06:01 PM.

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