**4.1 Cable
Coupling EM Field to Cable Coupling**

**Cable to Cable
Coupling**

Cable to cable coupling can cause an interaction between two systems on the same
aircraft when the high level signal in one cable is coupled to a cable operating
at a low signal level. A low interfering signal is then conducted along
the victim wire to the sensitive devices attached. Cable to cable coupling
normally occurs at close distances. Consequently, both magnetic field and
electric field coupling must be evaluated. Normally at low frequencies, the
magnetic component of the diced will couple predominantly while the electric
field coupling will be predominant at higher frequencies.

Magnetic Field Coupling

The source of the magnetic field may be a transformer, a solenoid, or any
current-carrying wire. Power frequencies, 60 or 400 Hz, for example, are
particularly a problem for video and digital control circuits. Magnetic coupling
will occur as the current Owing in the source cable generates a magnetic field
around the victim wire. This results in a current induced in the victim wire as
the field changes with time.

Interference voltages induced in the victim cable can be estimated with the
simple formula shown below. The voltage induced in a loop by an adjacent wire
of infinite length carrying current as represented in Figure 4-3 will
be:

V=K*f*L*I*In((r+s)/r) Eq. 4-2

Where:

f = frequency, hertz

L = length (inches)

I = current, amperes

V =
induced voltage, volts

r = distance between source and victim wires (in.)

s = distance between wires in the victim cable (in.)

K = 3.19*10 to the
-8power (when dimensions are in inches)

The induced voltage is directly proportional to frequency, source current,
and length of parallel wires. Figures 4-4, 4-5 and 4-6 represent examples of
worst case cable coupling for electrically short cables in free space. The
induced voltage also increases with the area enclosed by the pickup loop.
Therefore, increasing the value offs" increases the coupling. This induced
voltage will cause a current to flow through loop and will be determined by
the loop impedance. At low frequencies, the impedance of the pickup loop
consists primarily of wire resistance, and maximum current will be delivered
to a load of low resistance. Conversely, voltage will be maximum when the loop
impedance is high. For this simplified description, it is assumed that the
source cable is a low impedance circuit since the most significant
interference will result from a high current.

In the presence of a ground plane, the coupling between two parallel wires
can be calculated by the formula:

V=3.12* 10 to the -4power * L * I * F * E/R

Where:

L = Length in Feet

V = Millivolts

I = Source Current

F = Frequency
in Hertz

E = Elevation above Ground in Inches

R = Distance between Source
and Victim Wires in Inches

Figure 4-7 depicts typical cable coupling parameters at different
elevations above the ground plane at 400 Hz for two parallel wires. Spacing,
length of run, and elevation will also produce the same 6 Db/octave coupling
relationship. The nomograph in Figure 4-7 (see page 14) was constructed from
this data so that wire separation could be calculated from different
elevations above the ground plane. The frequency of 400 Hz and a cable run of
3.05 m (10 fl.) were each selected as a zero dB reference.

Example: Given the following parameters and using the nomograph in Figure
4-7;

A = Emission Level -25 dBmW

B = Susceptibility Level -30 dBmW

C =
Length 20 fl, from scale C = 4.5 dB

D = Frequency 1000 Hz, from scale D = 8
dB

Where: X=A-B+C+D Solving for X = -25 - (-30) + 4.5 + 8 = 17.5 dB Plotting
17.5(X) versus 2 inch elevation (E) yields4 inch separation (R).