A wire categorization and separation scheme is sometimes compromised at the
connector. A good example was discovered on some E-6 and E-3 AWACS airplanes.
There, two or more cables requiring separation were brought together into one
connector at a display console.
During installation, the wire crew bound the two cables together as they
penetrated the floor from the lower lobe. The cables continued together for
several feet to a single connector. A designer had joined them for convenience
of installation. The proximate cables permitted low impedance audio signals in
one cable to couple efficiently into a high impedance microphone line in another
cable. The result was crosstalk between channels in the audio distribution
The solution was to separate the two bundles, up to the connector The
crosstalk was reduced to a satisfactory level even though the bundles shared the
same connector This experience illustrates the need for EMC personnel to examine
installation drawings in detail, and where customer furnished equipment is
involved examine more than one sample!
A peculiar and interesting interference problem between a VLF receiver and
400 cycle power arose on an E-3 AWACS airplane During installation and check out
of the airplane early in the program, system test personnel discovered that a
harmonic of the power frequency interfered with one Omega channel.
Discovering the coupling mechanism between 400 cycle
power and the Omega system was difficult. The first clue came when EMC
engineers found that all cabin fluorescent lighting used the airplane
structure to return 400 cycle power.
Those familiar with the Omega system suggested that interference coupling was
through the antenna behind the ah bulkhead E-3 AWACS EMC personnel finally found
that 400 cycle current on cables' shields penetrating the aft bulkhead radiated
enough energy to disturb the Omega receiver
The solution was to avoid grounding cable shields or equipment outside the
bulkhead dead-ended shields could not support induced 400 cycle current EMC
personnel have since enforced a strict policy of "no net cable current" outside
the aft bulkhead, and consequently have been successful in eliminating
interference to the Omega system