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What it takes for radio interference?

Discussion in 'General Discussion' started by voodoo, Dec 23, 2014.

  1. voodoo

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    Guys,

    I've been in several situations, whether they be literal or planning purposes, where I am unsure about the stability and reliability of the radio connection between my Phantom 2 Vision + and my transmitter. I fly mostly VLOS only, but sometimes am bold enough to fly FPV when panning around a building or video target. How much physical separation can the radio waves take? I am sure that flying on the other side of a skyscraper is an unstable situation for radio waves? No I am not doing any such flying, but I think about if it is possible. A similar and more realistic situation would be flying on the other side of a large farm structure, FPV only. In this case, VLOS is broken, along with the direct line of radio wave transmission between the Phantom and transmitter.

    What about sometime as simple as a tree line? I would imagine this is fine. The radio wave probably have nearly zero issues with a hedge line being in the way of your transmitter and Phantom.

    Within the vicinity of power lines? How much do these interfere with radio waves such as 2.4 or 5.8Ghz? frequencies that a P2VP or standard P2 use?

    Sorry for being a radio noob. Cheers! Any comments of advice is greatly appreciated.
     
  2. noiseboy72

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    Don't confuse interference with lose of signal as you can have interference cut the link, but still have a good signal path.

    5.8GHz is pretty much line of sight, with little scatter or reflection. 2.4GHz is a little better, with control still possible without LOS. Different frequencies are absorbed at different rates and are also reflected by different materials. Lower frequencies will pass through solid objects much better - get down to 20KHz or so and they will pass right through the entire planet! I have sent a 2.1GHz video signal almost 2 miles from 1 building to another, reflecting the signal along glass fronted buildings down a road! No line of sight, loads of local interference and we still managed a full resolution HD broadcast signal :) To be fair, this was with a 10W Tx and 30dB directional antennae, but it proves that you don't always need LOS when there are shiny things available to bounce off. Tarmac and concrete reflect 2.4GHz equally well and even hard packed earth and water work to some degree.

    Air absorption becomes an issue at 5.8GHz, with the range for a given power level being about half that of 2.4GHz. All in all, it is not a great frequency for radio control, but is one of the few available world wide, hence why DJI chose it. Had they gone for 400MHz or 900MHz, the range would have been much better, but the reduced bandwidth can make this more susceptible to interference as frequency hopping becomes confined to a smaller baseband.

    Most loss of control interference is caused by high power signals on a different frequency swamping the receiver and causing it to lose selectivity. This means the receiver stops only "listening" to the frequency it is tuned to and starts to respond to a much wider range of frequencies - in fact, given enough power, pretty much any frequency! This is why if you fly close to a cell phone or TV station tower, you can lose control. The frequency may not be similar, but the high power basically prevents your signal from being separated from the other transmissions and control may be lost.
     
  3. Hughie

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    I would be interested in determining a safe distance to keep from power lines. This would help a lot with route planning. I am a bit paranoid at the moment and dont go within 200m of them. I am sure it is safe much closer than that, but how close can one go before it becomes unsafe?
     
  4. Khudson7

    Khudson7 Guest

    Sorry noiseboy72, we posted about the same time so I did not have the advantage of seeing your post. It appears you have a very good handle on this subject. I am certainly no expert in any sense of that word. But I have a question for you. regarding the above.

    A couple of programmable radios out there using either FASST or ACCST protocols(some Futabas and FrSky Taranis) appear to try and mitigate this loss of control interference by avoiding a lock on a set frequency, rather spreading the data packets across the 2.4 spectrum, therefore re-binding very fast as someone else stated below in this discussion of radio protocols:

    ACCST and FASST seem to be the most complex, and possibly the most bullet proof.


    If you look at these on a frequency scanner, they appear pretty much like snow. Packets of data across the whole 2.4 spectrum.

    The others like DSM and FHSS tend yo use fixed channels. They choose 2 or 3 frequencies, and stick on them until such time as interference occurs - then they hop.

    I guess that's why FASST and ACCST have such fast rebinds!

    In a of noisy environment - I would think that FASST and ACCST are superior - just because they never lock to a frequency!


    What are your thoughts on that...
     
  5. Khudson7

    Khudson7 Guest

    NOW, this was REALLY weird...I submitted a post with link as below about same time as noiseboy72, I know I saw it posted there, but now it is gone....**** it REALLY SUCKS getting old!

    "For those with too much time on there hands(very long article) and a proclivity to want to learn more about radio waves...
    http://www.tapr.org/ve3jf.dcc97.html
     
  6. ianwood

    ianwood Taco Wrangler
    Staff Member

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    5.8GHz is definitely LOS only. A couple pieces of cardboard can make it attenuate noticeably. Interference, as mentioned above, is something different.
     
  7. noiseboy72

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    I don't claim to be a subject matter expert on RF, but I have spent a lot of years setting up point to point video links and using up to 30 radio mics on touring shows, so have some experience of trying to keep stuff working!

    The issue with any receiver - analogue, digital, frequency hopping etc. is that if it is overloaded with RF, it simply cannot work out what signal it is supposed to be listening to! You can actually get to the point where the controller itself becomes a radio receiver, but this is much more of an issue with analogue systems, not digital.

    All of the common digital R/C systems use spread spectrum or frequency hopping technology of some sort, as this makes it possible to fit a lot more channels into the space. An old fashioned single channel (35MHz) radio set will occupy the channel the whole time, even though the limited amount of information it needs to send could be sent in a 1000th of a second every 100th of a second or so. With all of the modern radios, the TX and RX switch channels continuously and in a pseudo-random order. This means that they never occupy a particular channel for very long, so the chances of repeated clashes and corrupted data is very low. By repeating the information and adding in checksums, the receiver can be assured it has good data. It should be noted this technology is more to do with fitting in more channels, not avoiding interference from other sources.

    However, if a powerful - or close transmission covers the entire band that the TX can use, there are no gaps for the system to use and the level of data corruption increases massively. This is the issue with WiFi on the Go-Pro and the 2.4GHz R/C.

    I think the DJI flyaways and loss of control issues are only partly related to interference in the receiver and also in the way that the controller deals with corrupt data. It is almost as if the controller locks up if too much bad data is detected and instead of going into a safe hover, it commands a high power forward flight!

    There are lots of other RF nasties as well, including intermodulation, beat frequency oscillation and re-transmission, but these are more of an issue with fixed frequency analogue links - including fpv video links than radio control.

    As I said, I don't think 5.8GHz is a good frequency for radio control. It simply does not have the propagation and penetration required for anything more than line of sight over a few hundred metres at the power levels we are allowed.
     
  8. Khudson7

    Khudson7 Guest

    Thanks, noiseboy72, that is VERY informative...