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Long i2c, antenna cable and GPS 6 pin cable for large aircraft

Posted by Anand Swaminathan on April 30, 2015 at 11:17am
Hi,
I am looking to use the pixhawk for a large aircraft (3m wing tip to wing tip) and am looking to place the telemetry module, antenna (spektrum antenna) and GPS/Magnetometer module at one of the wing tips. Would there be any issues with having 1.5m long cables for the magnetometer(I2C), spektrum antenna and 6 pin cable for the GPS? It will also cross close to a 60A ESC. 
My tests so far have indicated that the ESC itself should not cause significant coupling with the cables themselves but I am unsure as to the losses and/or RF radiation from the 1.5m cables. Thoughts? 
Regards,
Anand

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Replies

  • Ricardo Bencatel April 30, 2015 at 1:23pm

    Hi,

    I am not sure about the I2C and GPS cables, but as they carry digital signals, there probably is no problem. Adding an external shielding (wire mesh or other) connected to the ground should avoid major interferences in long cables.

    The antenna cable is another case entirely. There is attenuation in the cable, which depends on the cable shielding and length. You can get cables with low loss, but they will be heavier. In terms of the length, it should be a multiple of the radio frequency wave-length to minimize the losses, and of course longer the cables have more signal attenuation.

    The ESC might increase the noise level in the cables. I had a yaw gyro on my T-Rex that would display different bias, depending on the current passing in the cables from the battery to the ESC. A common mitigation solution for the ESC pwm cables is to loop them a couple of times around a magnet, but I don't think that would solve the issue from the power cables.

    The structure of the aircraft around the cables can also provide some shielding against radio noise, depending on the material it is made of.

    Cheers,

    Ricardo

    • Anand Swaminathan > Ricardo Bencatel April 30, 2015 at 2:36pm

      Thanks Ricardo!

      I agree that digital signals should be fine as long as ohmic loss isnt large. The antenna cable I talked about is actually the remote receiver extension cable :http://www.spektrumrc.com/Products/Default.aspx?ProdID=SPM9012

      I misspoke in that regard and this extension should also be digital and therefore OK as long as the length isnt too long for significant attenuation to be a problem.

      With respect to shielding, I was thinking of using twisted cables to minimize RF as much as possible and then apply wire mesh in areas that may be problematic.

      Regards,

      Anand

      Remote Receiver Extension 12-inch (SPM9012): Spektrum - The Leader in Spread Spectrum Technology
      • Anand Swaminathan > Anand Swaminathan April 30, 2015 at 3:00pm

        Or even use aluminium foil to wrap around sensitive areas and then run a ground wire to the receiving end.

        • Ricardo Bencatel > Anand Swaminathan April 30, 2015 at 3:24pm

          Yes, that seems like a good approach.

          Cheers,

          Ricardo

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