4.1 Cable Coupling EM Field to
4.1 Cable Coupling EM Field to Cable Coupling. The level of signal coupled
to a cable depends upon several factors. The strength of the signal, its
frequency, the length of the cable, the amount of shielding provided for the
cable, and the distance between the source of the interfering signal and the
cable which must be protected all contribute to coupling. In addition, the
orientation of the cable itself, with respect to the field, is a significant
factor influencing cabling.
To keep it simple for the purpose of this document, two separate cable
coupling methodologies are addressed for estimating the requirements for cable
shielding or separation. The methodology described is useful in evaluating
alternative E3 control techniques.
These coupling methods are shown in Figure 4-1 as "Field to Cable" and
"Coupling Between Cables." Most specific potential problems are not strictly
one or the other, but are a combination of both. Thus, care should be taken in
applying these estimating tools.
The electromagnetic field referred to in this section results from the
electromagnetic environment described in Section 3
. It consists of sources
external to the system; such as ground based equipment, other nearby friendly
aircraft and hostile electromagnetic radiation as well as functional on-board
transmitters such as communications and radar systems. In general, these
problems assume the electromagnetic field results from a source some distance
away from the cable and that the wire is oriented in the field for maximum
coupling. This provides a worst case scenario.
Electromagnetic fields result from:
- Ground and Sea-Based Systems
- Nearby Aircraft
- On-Board Systems
- Navigation Aids
- Static Discharge
The coupling to a wire from a time-varying electromagnetic field such as 60
or 400 Hz power, HF communications (2-30 MHz) or Radars (above 1 GHz)
approximates that of an antenna. The level of voltage or current on the cable
is based upon the cable's orientation with the field (polarization), the
impedance of the cable to ground, and the cable's physical length.
The signal coupled into any antenna is directly proportional to the
effective electrical length of the antenna. The effective length of a wire or
cable will vary. For a cable less than 0.1 wavelengths, the actual length of
the wire will approximate the effective length. A cable with a length equal to
one quarter wavelength will have an effective length equal to 0.78 times its
actual length. The voltage coupled to the cable can be approximated for the
worst case by the following expression.
Where the wire is electrically short,
FI = Field Intensity in Volts/Unit Length and L = Length of Wire
Figure 4-2 graphs the typical electric field coupling. This is true only
for low frequencies when the wire length is less than 0.1 wave length. The
wire length is measured in inches.
1. Assume the worst case for a field parallel to the
2. The field intensity in volts per meter can be converted to open circuit
voltage (V) on the wire by multiplying the effective wire length (L) times the
field intensity (Fl). a. For a cable 20 inches long b. in a field of 100 V/M
c. at a frequency of 2 Mhz, d. the wavelength would be 150 meters
3. Using equation 4-1 :
FI = 100 Volts/Meter
L = 0.504 Meters
V = .504*100 = 50.4 Volts