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Why there is large range variation...

Discussion in 'Phantom 2 Vision Discussion' started by gpauk, Apr 26, 2014.

  1. gpauk

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    I had 3 PVs around lately, and measured the 5.8GHz tx out on them all..
    FCC mode is supposed to be 125mW or 21dBm
    The 3 I measured in FCC mode with fresh batteries (using peak hold on an avantek calibrated 20GHz spectum analyser) varied from -3dBm (!) to +15dBm. All well under spec -- and a colossal variation!

    No wonder we all get different results....
     
  2. gunslinger

    gunslinger Moderator
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    Seriously? WTF? Anyone else have a spectrum analyser available? This is a ridiculous variation ! :shock: :shock: :shock:
     
  3. OI Photography

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    I'd bet that poor antenna lead connections in the Tx may to blame for a lot of that.
     
  4. PVFlyer

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    Sounds almost impossible to get -3dBm. 0dBm is 1mW, -3dBm is only 0.5mW and basically no output power at all.
     
  5. gpauk

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    0.5mW ia a lot more than no power! With a decent rx (and the dji one seems to work quite well) it'll work better than you think. I work with low power microwave radio a lot.
    It's be quite easy if there's an assembly error, bad soldering or failed part... I see this a lot in cheaply made rf power stages.
    That unit manages about 300 feet range, despite being clearly faulty.

    We connected directly to the ufl on the tx board with a phase calibrated cable to the analyser.
    So the loss will be no more than the lead to dji's antenna. (mind you, ufls are pretty crap rf connectors).

    I've seen reports of people flying only a couple of hundred feet and wondering why, and some managing a couple of thousand out the box. I'd expect a fair variation in tx performance from this sort of device, we see big variation in mass produced stuff that isn't end-of-line calibrated - and there's no requirement to do so with ism band stuff like there is with, say, cellular.
    We'll try and fix that bad tx today, got some suitable mmics in the spares box, as the weather has gone drizzly again!
     
  6. PVFlyer

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    Well, it's hard to determine what is "a lot". $1M is lot to me, but probably is nothing to those billionaires. If I flew my P2V/Vision+ to 3KM away, then 0.5mW of 5.8G is probably nothing for the aircraft to receive the control signal.

    I think that's either a malfunction 5.8G transmitter or the measure pigtail cable/u.fl plug.
     
  7. Seahorse

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    Is there a simple, effective and cheap way to measure this, that would then guide how to fix/improve if required... :?:
     
  8. gpauk

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    In theory you could use the immersion RC power meter to measure it...

    Curious -- on the back of the TX on the certification sticker it says output 11dBm...

    We fixed the duff tx. Interestingly it's actually a transceiver board, but the receive path is not - at the moment - used.

    The tx / rx switch device at the front end was acting like an attenuator.. So we bypassed it... Range now > 1500 feet...
     
  9. helixaviator

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    I suppose it is the same for the wifi rf power as well..I have tried everything but I cant seem to get over 800 ft or so.
     
  10. J.James

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    are you even really sure its actually in FCC mode????? cause just turning the trim pot Does not do any thing at all to change the mode from ce to fcc mode all that trim pot on the back does is vary the out put power to fine tune the power up or down in either mode. and changes the beeps.

    To actully get the tx in fcc mode you have to also plug it in to the rc assistant and select FCC mode then power the tx off and back on before it will actually be in the fcc mode.

    If in ce mode still turning the trim pot will just vary the power up or down from the 25mw and increase or decrease the ce range.

    IF set in the rc assistant soft ware threw the usb cable. then the pot will vary the power up or down from the stated 125 mw range.

    also them trim pots are not the best and all have some slight variations in them esp when at the very ends of the its limits. my trim pot actually if turned all the way to the end of the stop clock wise actually makes more resistance when checked with an Oh meter then if its backed off a tiny tiny hair or a fraction of a degree from the end of how far it can turn clock wise.

    I also suspect after dissecting the stock antenna and finding one pole being off by an entire mm from the other due to the soldering job they did could make for variations in output from one to the next. To who ever made it, close enough to perfect was considered close enough to work.