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Thread: An idea of how to decrease RF interference when using 2.4 ghz for video

  1. #1
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    An idea of how to decrease RF interference when using 2.4 ghz for video

    Hi guys,

    Just wanted to share and discuss with you my idea of how to decrease RF interference from wifi and similar devices when flying using 2.4 ghz for video.

    In general, most of us good FPV guys put a circular polarized antenna on a quad and use a high-gain linear polarized antenna at the receiving end, thus having a 3 db loss from cross-polarization. On the RX end in most cases without thinking on it twice we put our patches or yagis so that they actually work in vertical polarization...

    The idea is to use horizontal polarization on the receiving end in order to significantly decrease the interference, especially when flying in urban areas.
    As far as I know most part of the devices that could generate interference on 2.4 ghz band transmit in vertical polarization. Just take a look at your router! So when you try to receive a vertically polarized signal with a horizontally polarized antenna, cross polarization losses theoretically can be infinite, but in practice can easily reach 10-20 db. It's a pure gain in signal to noise ratio

  2. #2
    Co-Pilot rlage's Avatar
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    You can build a simple dipole, installing it horizontally on the quad.
    Share your results .
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  3. #3
    Good idea. Any testing / real world experience of this? - presumably it would be quite easy - receive interference twist your Yagi by 90 degrees to see if it goes away.....

    Do your reckon this would be better than using a circular polarised receiver (e.g. Crosshair or pepperbox)? I guess the maths and logic would say yes as though you would be removing the cross polarisation loss of 3db, that would boost the reception of the interference as well.

    I was looking at an old thread on this kind of topic by Sircana http://fpvlab.com/forums/showthread....PV-Video-Link& In there it was suggesting that the ultimate link could come from transmitting on two separate bands in 2.4 and using diversity pick the best signal. I guess it may be battery drain, but not sure why that never took off

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    Quote Originally Posted by rlage View Post
    You can build a simple dipole, installing it horizontally on the quad.
    Share your results .
    Build a dipole, install it horizontally... and then enjoy huge dropouts of video trying to maneuver That's why they use a circular polarized antenna on a plane or quad.

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    Quote Originally Posted by FlyThis View Post
    Do your reckon this would be better than using a circular polarised receiver (e.g. Crosshair or pepperbox)? I guess the maths and logic would say yes as though you would be removing the cross polarisation loss of 3db, that would boost the reception of the interference as well.
    My method should be better, or correct me if I'm wrong. When using circular polarized antennas on both ends, you not suffer from cross-pol, but at the same time the interference will be attenuated only by 3 db. In this case signal to noise ratio will be better only by the same 3 db. However, using circular pol on TX and horizontal pol on RX could give us let's say 20 db (a rough approximation of vertical to horizontal cross-pol loss) minus 3 db = 17 db of increase in signal to noise ratio. That's much better!

  6. #6
    I Like Waffles... SENTRY's Avatar
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    Yes - but contrary to your thoughts - MOST commercial sources of interference (VHF / UHF) are vertical, so you'd wanna go horizontal in urban areas. Companies with coin to spend are just now moving to CP. I did some short testing in this in Atlanta. The issue will be when you bank to turn - lights out.

    I have an even better deep urban solution - but I haven't flown it but once or twice so the results, while promising, are immature to discuss wholly at this time.
    Last edited by SENTRY; 28th September 2013 at 08:40 PM.
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  7. #7
    I Like Waffles... SENTRY's Avatar
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  8. #8
    The main flaw in this scheme is assuming that the sources of 2.4Ghz interference are all vertically polarized.
    Every Wifi network is a combination of the Wifi router and all the various other devices connected to that router.
    They are laptops, USB Wifi dongles, smart phones, tablets, smart TVs, and so forth. Many of them
    are thin and flat, and often oriented horizontally, not vertically. My last Linksys router was
    flat, and had no vertical antennas.

    BTW, experienced FPVers don't put CP on the aircraft, and linear on the Rx, if they can help it.
    It's sometimes done the other way around, just to have a smaller lighter antenna in the air.

  9. #9
    FPV Addict Scotttu's Avatar
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    Quote Originally Posted by SENTRY View Post
    EDIT - I'm drumk.
    Waffles?

    How drunk are you?


    Yes.
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  10. #10
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    Quote Originally Posted by Daemon View Post
    The main flaw in this scheme is assuming that the sources of 2.4Ghz interference are all vertically polarized.
    Every Wifi network is a combination of the Wifi router and all the various other devices connected to that router.
    They are laptops, USB Wifi dongles, smart phones, tablets, smart TVs, and so forth. Many of them
    are thin and flat, and often oriented horizontally, not vertically. My last Linksys router was
    flat, and had no vertical antennas.

    BTW, experienced FPVers don't put CP on the aircraft, and linear on the Rx, if they can help it.
    It's sometimes done the other way around, just to have a smaller lighter antenna in the air.
    Well, that's correct, I assume that most of the 2.4 Ghz interference source are vertically polarized. The difference as usual is hidden in details. You're right when you say that every wifi net consists of a router and other devices, however the most powerful part is exactly the router and when talking about domestic sources of interference (when you fly over a village, for example) this is what we should consider. Other devices such as laptops and dongles are too weak to be seriously taken into consideration. But most of the commercial and enterprise sources of interference as far as I know are vertically polarized and only some companies go for CP or HP antenna systems...

    All in all, we talk about a theory, a concept which should be proven in practice. As soon as the time and weather permits, I'll try to do that and share the results.

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