REFLECTOR: Intercom question

[Dave T Nelson]

Subject: Re: REFLECTOR: Intercom question


  Ground loops will invariably create electrical noise.  In particular in
  fiberglass airplanes, since, as an example, the absolute potential of the
  ground at, say, the instrument panel will be slightly different than the
  absolute potential of the ground at, say, the firewall, due to the
  resistance of the ground wire going from the instrument panel to the
  firewall.

  So - A ground loop is created by having the shield on a shielded wire
  connected at both ends.  Because the ground potential (the voltage) will
  always be slightly different at each end of the shield, there will be a
  current induced in the shield. Shields should *never* have a current.
  But, because of the ground loop there is now a current in the shield, any
  electrical noise picked up by the shield will change the voltage
  difference between the shield and the center conductors (mike or audio).
  Since electrical noise is generated all over the place (think of strobes,
  or someone keying the mike, the processor in your GPS, etc.), you will
  for sure get some of that noise into your intercom.  Some intercoms are
  better than others at noise rejection, but, the real solution is
  simple... connect the shield at one end, but not at both ends.  Leave one
  end open.  If you do that... no ground loops.

         On 2/17/2013 7:04 PM, Scott Derrick wrote:
               agreed

               Glenn Falkenberg <airmech2 at gmail.com> wrote:



               I think the best way would be use a shielded twisted pair.
               Float the shield at the Jack and take shield to ground at
               intercom panel.


               On Feb 17, 2013 4:25 PM, "Dan Schindler" <
               danpilot91 at gmail.com> wrote:
                 My friends airplane intercom is wired in such a way that
                 each jack only has 1 wire running to it. Utilizing the
                 shielded portion of the wire as the common ground. Is this
                 an acceptable way to do this. He is getting feedback in
                 his headset when he pushes the PTT.

                 Thank you,

                 Dan Schindler
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