Posted for "terrence o'neill" <troneill@charter.net>:

 Brent,
 Many thanks.
 Very electrically challenged Terrence N211AL




  From: Brent Regan
 
"""
  Shields Up!
 
...snip...

  The shield will work with only one end tied to ground because, like an
antenna, the circuit is partially copper and partially an oscillating magnetic
field. However, energy intercepted at the far end of the shield must
transverse the length of the cable, and all its DC resistance, before finding
ground. If there is a ground at both ends then the energy only has to travel,
at most, half as far, and there are two parallel paths to take.
 
  Now imagine you have a device that draws 3 amps at 14 volts and is connected
by a 24 gauge 25 foot power cable. The DC resistance of 25' of 24 gauge wire
is 0.66 ohms. Multiply this times 3 amps and the voltage drop in the wire is 2
volts. 2 volts in the power wire AND 2 volts in the ground wire. This means
that if you put a volt meter between the aircraft ground and the case of the
device you would see a 2 volt potential. Now imagine you need to pass a
digital signal from the device and that signal is referenced to ground.  When
the device sets the signal to "low" it may be ground at the device but it will
be at 2 volts at the other end of the cable so the receiver may not interpret
the signal as "low". Remember that the signal path does not have significant
current flow so there is not the voltage losses that are present in the power
cable.
 
...snip...

  Many engineers do not sufficiently understand or appreciate the issues. For
example, Hamid and I designed a tablet computer for Canadian Marconi
Electronics (CMA-1100 Pilot View) and as part of that effort, took the system
to a test facility to verify the electromagnetic radiation of the system was
within limits.  The closest the system got to busting the radiation limit was
3db (1/2 the allowed power) at about 150 MHz. When CMC repeated the test with
a CMC built system they reported their system was over the limit by more than
3 db, 4 times the emissions we measured.  Many phone calls and thousands of
dollars in travel and chamber time later we discover that CMC did not follow
our recommendation for terminating the power and serial communication shields
directly to the connector body at both ends. Rather, they terminated using the
CMC standard method of soldering the shield to a 22 gauge wire and then
grounding the other end of the two inch long wire. Replacing the cable with
one that had properly terminated shields fixed the problem.  CMC did not
believe this to be true until we did side by side tests at an independent
facility with a CMC engineer present.
 
  The above is a very brief review. I have omitted and simplified much for the
sake of brevity.
 
  Regards
  Brent Regan
"""