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Math Behind Range and Transmitter Power and Antenna db

Discussion in 'Standard/4k Discussion' started by roymoon, Sep 28, 2016.

  1. roymoon

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    Firstly it's not an exact science. There is a bit of black magic when you don't expect it.
    Modulation methods can make a big difference in range vs interference.
    But I am comparing apples to apples.

    On our Phantoms:
    CE = 16dbm = 40mw Transmit power
    FCC = 20dbm = 100mw Transmit power
    At first you see in the US we have 2.5 times as much power as the CE countries.
    But that doesn't equate to a guaranteed 2.5 times the actual distance.

    It comes down to the inverse proportional laws of physics.
    The signal strength our bird sees at 200 feet 8 times weaker than at 100 feet.
    Not half as you might think.
    To get the same signal strength at twice the distance you have to increase the transmitter power by a factor of 8.

    Our antennas are rated in db's.
    Typically an antenna with 3 db of gain will double the signal strength whether its receive or transmit.
    But to get that magic twice the range you have to use an antenna with 9db of gain.

    Now for the voodoo part of things.
    In transmit you really do need a 'tuned' antenna.
    It has to do with SWR (standing wave ratio).
    As the transmit power heads down the coax into the antenna, the antenna has to be the correct length for the frequency of the signal.
    Shorter antenna for higher frequencies and longer for lower.
    If you don't have the right length part of that power gets reflected back the opposite direction into the final output stage of the electronics.
    That's bad.
    If the tuning is really off and the power is high enough you can damage the transmitter.
    You want the ratio of the power leaving the antenna vs reflected to be as low as possible.
    Notice how some antenna manufacturers say to use channel 'x' for best results?
    That's because theirs is tuned for that one frequency.

    Now for the part where some are going to disagree with me.
    The receive antenna does not need to be tuned.
    The longer the better.
    On a helical I might eat a few of those words a bit.
    But in general I'll stand by it.
    The longer receive antenna the better.

    You may throw the tomatoes now.
     
    Philltayl likes this.
  2. Mark The Droner

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    Ya know, just the other day I typed out that I thought FCC mode, or to put it another way, the max power allowed in the US, limited us to 100 mw, and I was corrected that it was more like 1000 mw and the poster provided a link with regards to ham radio operators.

    So what is it?
     
  3. roymoon

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    The law may say 1000mw (1wt) but DJI decided 100mw was enough.
    To pull 1 watt from that little 3.7 volt battery would be a real problem.
    The controller run time would be 1 or 2 flights only.
     
  4. With The Birds

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    I haven't looked at the battery in the RC for a while but I'm pretty sure it's 1.8Ah, if not more. Capable of powering a 1w output stage for significantly longer than 2 flights.

    As to power I suspect it is total radiated power necessitating that the antenna gain is factored in. 1w output stage would be outside standards.
     
  5. With The Birds

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    A tuned RC antenna will give better receive performance. This is not speculation.