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Question About Parabolic Antenna Attachments and "SWR"

Discussion in 'Pro/Adv Discussion' started by Bigfootbuilt, May 30, 2016.

  1. Bigfootbuilt

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    I've watched several videos and read articles about various parabolic attachments for the DJI transmitters but it got me to thinking about what effect this would have on the transmitter itself. If any of you are old enough to remember the CB radio days, care was taken to lower, as much as possible, the SWR (Standing Wave Ratio) of the transmitter. The higher the number, the more of the signal was being reflected back to the transmitter causing it to overheat and burn it up eventually. My question is, does the same apply to RC transmitters? Does anyone know? Would using parabolic attachments like the Skyreat copper reflectors shorten the life of the RC since half the signal is being reflected right back to the transmitter?
     
  2. Pheonix7

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    I'm a ham operator and I wouldn't think this would be the case as the rf has already been transmitted via the antenna. I see the windsurfers as just a reflector of transmitted rf. SWR mainly comes into play if an antenna isn't properly tuned for a specific frequency which restricts the transmitters ability to emit rf appropriately via the antenna resulting in energy return to the transmitter itself.


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  3. Bigfootbuilt

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    But isn't reflection coefficient taken into account when measuring SWR? I remember being able to adjust radio antennas to lower the SWR but it had to be in an open area, because as soon as you get near a metal building or another vehicle, the SWR would rise again.
     
  4. WetDog

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    Yes, SWR is important. For a reflector like a Windsurfer it's likely not going to change much (yes, SWR changes with near field but if the Windsurfer is properly designed it should be OK). Hopefully the commercial antennas like the FPVLR and ITELITE flavors have had their base SWR checked.

    Unfortunately, it's pretty hard for all but dedicated microwave enthusiasts to determine SWR for a given antenna system. They do make commercial SWR meters for than range but they tend to be expensive. They're actually pretty easy to make if you have any electronics background (look them up). Might be a fun project.
     
  5. Pheonix7

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    This. Yes it's taken into account but at the low rf levels the remote is producing I can't see how it would be an issue.



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  6. WetDog

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    You certainly would not get heating and you are probably not at much risk of transmitter damage but you would lose efficiency which could be important especially given the low output.
     
  7. Bigfootbuilt

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    Thanks for the replies guys. Around here it is difficult to get distance because of dense trees (I live in South Carolina) and I was looking at the Skyreat parabolic attachments in particular, but either way, I think as long as obstacles like trees are between me and the P3A, it probably won't make much difference in most places no matter what mods I have on the RC. It would still be nice to have if I had a clear line of sight and a distant target to fly to.
     
  8. WetDog

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    Yeah, trees are a *****. They're just water and 2.4 Ghz signals love to interact with water. What we need is a skyhook system. A small UAV with a pair of narrow angle antennas - one pointed down and one horizontal. A radio set up as a 'bent pipe repeater' which takes the signals from one antenna and stuffs them out the other. Lots of battery and GPS so it will hover in place. Then rig it to talk to the Phantom and the RC.

    Probably take a set of competent engineers about a year to put it together and the FAA (and FCC for that matter) about an hour to shut it down.

    Tall ladders might be an option.
     
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  9. wa5tef

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    Good questions BigFoot. The SWR is a measurement of antenna matching, not the transmitter. True that a high SWR can damage a transmitter but at these power levels I doubt it would do damage to the RC unit. I work with 2.4 ghz units designed to attach to a dish, vertical or panel antenna and I do testing most of the time with any antenna or other load attached to the transmitter. I have not had one unit be damaged by this yet. The added reflector should not change the antenna impedance or SWR provided it is mounted at the focal point of the reflector. A good way to check it is to use a laser pointer and direct the beam straight into the reflector surface at several different points and the beam should be seen on the antenna in all cases. If it is not aligned properly the beam will not shine back to the antenna and thus won't work properly.
    I received my set of Skyreat Aluminum Parabolic reflectors in today from Amazon. To test them, I flew the Phantom 4 out to 1850 feet at 145 feet altitude, I think, and had a 100% signal reading. I then turned the array to the left and at about 30 degrees, the signal started to drop. At 90 degrees, it dropped to 80% and at 180 degrees it was 68%. Thus when pointed away from the drone, the signal dropped that much! I repeated the test to be sure and got almost the same figures. They do work! But do the laser test and be sure the parabolic surface is free of dents or wrinkles and adjust the array so it can be pointed at the drone.
    Good luck,
    Jim
    WA5TEF
     
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