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Thread: 7.3km on 5.8 with DUMMY LOAD on the tx - No Antenna

  1. #1
    Navigator Kondor-FPV's Avatar
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    7.3km on 5.8 with DUMMY LOAD on the tx - No Antenna

    Hello,
    I am a bit puzzled here, as I was testing for the first time my 16 turn 5.8 helical. I didn't want to go far, because I had different solutions available to compare for a future record attempt. So I installed a 50ohm sma dummy load cap on the foxtec 5.8 400mw tx...

    I was expecting 1 maybe 2km but I was actually able to track the plane up to 7.3km before turning back...
    This is really strange, because it means either that the 16 turn helix is a killer rx antenna or that the Dummy Load bought on ebay is actually radiating quite a bit...

    Is it possible that the the tx itself has a trace or something that is radiating before getting to the connector?

    This will make things much slower for me.. as I can only test 2 antenna solutions per battery charge, I was expecting to do a whole afternoon of going 1-2 km and swapping at least 10 antennas on the rx side... but I could only do it 2 times, with the other one going 5 km before turning back...

    This is the video, it's unedited so please skip to 2:50 unless you want to watch 2 minutes of dry grass!


    http://www.youtube.com/watch?v=CXXyF...ature=youtu.be
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  2. #2
    I see you... Derrick's Avatar
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    that is pretty crazy... really makes you wonder. I'm sending you a PM about a similar topic.

  3. #3
    Navigator Kondor-FPV's Avatar
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    Quote Originally Posted by Derrick View Post
    that is pretty crazy... really makes you wonder. I'm sending you a PM about a similar topic.
    Derrick... I'll take care of that
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  4. #4
    I can't breathe Hans's Avatar
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    I am thinking two things:
    - The atmospheric conditions are excellent judging by the video
    - I always thought that the amount of radiation depends on the surface of copper used to radiate the proper output. Thus really really thin wire for, lets say a cloverleave, won't work that well. I always thought it had something to do with the skin-effect (Electrons travelling on the outside of a conductor. The same reason why highpowerlines are hollow, saves weight too) So maybe coincidently your dummy has the proper amount of surface to radiate optimum forma? (And visa versa, too much copper adsorbs instead of radiating)

    edit* Interesting read about skin-effect
    Last edited by Hans; 1st August 2012 at 02:45 PM.

  5. #5
    Navigator Kondor-FPV's Avatar
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    very very strange, I measured the swr with the dummy on, at 1.1:1 so there should be very little power excaping the tx...

    And that's not counting the 3dbi loss for polarization mismatch.

    Today conditions were superb, I could have gotten the record with any antenna mounted...
    Too bad my battery was dead after testing a 16 turn and a 12 turn... helices
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  6. #6
    Navigator Airborne's Avatar
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    A 50 ohm resistor would present a 1:1 match, and probably radiate quite a bit of signal if not contained inside of a shielded enclosure.

    I had an old novice class study guide from the early 1990s that mentioned the author making a long distance tropo contact on 2m using a lightbulb as his dummy load.

  7. #7
    Navigator Kondor-FPV's Avatar
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    obviously the dummy load was radiating some, there is no way you can go 7.3km on 5.8 if you are not... it just is really strange to be able to get that distance with a RHCP antenna on the ground, and no proper antenna on the tx...
    This is all good for my future record attempt at that frequency
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

  8. #8
    Shedding light over FPV HugeOne's Avatar
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    Free space loss over 7.3Km @5800 span 125db, let's say you have -90db on the Rx and guessing 16-17db on the antenna.
    That mean the Tx with dummy load is spewing 18db or so (63mw)
    With a CP antenna, your back at 23dbm, theoretical range of 25km.

    -Hugo

  9. #9
    Engineer for Jesus Christ IBCrazy's Avatar
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    I have found most dummy loads do in fact resonate at 5.8Ghz. It just depends on the particular load. The cheap dummy loads work better then the more expensive ones with a cap on them at cutting the signal to nothing.

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

    videoaerialsystems.com - Performance video piloting

  10. #10
    Navigator Kondor-FPV's Avatar
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    Yes, I noticed that... and the dummy load antenna was born! hahahahah...

    just kidding! I will soon try with the lindenblad on the tx to see how much further I can go with it.

    Tony.
    www.fpvlr.com - Home of your favourite Custom FPV antennas.

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