Quantcast Cable Shields - hdbk419a_vol10292

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MIL-HDBK-419A
8.8 CABLE AND CONNECTOR SHIELDING. Electromagnetic shielding is required not only for equipment
containers but also for many of the cables which connect the equipment units since interference may be
transferred from one circuit or location to another by interconnecting cable. The interference may be radiated
from a cable or transferred into a cable from external fields.  Once interference has been transferred by
radiation or common-impedance circuit elements into a cable circuit of an electronic or electrical complex, it
can be conducted through interconnecting cables to the other elements of the complex.  Because of cable
proximity in cable runs or elsewhere, intra- and/or inter-cable crosstalk may occur as a result of
electromagnetic transference between cables.
8.8.1 Cable Shields.
The effectiveness of a cable shield is a function of two basic interference mechanisms: (1) EM wave shielding
As with other shields, the EM wave shielding
effectiveness and (2) surface transfer impedance,
effectiveness results from attenuation and reflections and is dependent upon such factors as the type and
thickness of the material used and the number and size of openings in the shield. In addition, cable shields
frequently are connected in such a manner as to carry relatively large currents themselves. Although the
interfering currents generally flow on the outer surfaces of the shields (skin depth effects), an electric field and
resulting axial voltage gradient is developed along the inner (shielded) conductor (see Figure 8-33). The ratio of
the induced conductor-to-shield voltage per unit length to the shield current is defined as the surface transfer
impedance,
The effectiveness of a shield is a function of the conductivity of the metal, contact resistance between strands
in the braid, angle and type of weave, strand sizes, percentage of coverage, and size of openings. Analytical
expressions which define
in terms of these parameters are available (8-14). For uniform current distribution
along a cable shield, the resulting
can be used to predict the shield effectiveness of the cable knowing the
terminating impedances of the cable. Typically, the cable is several wavelengths long at the frequency of the
impinging field. Thus, the current distribution on the cable sheath varies with length and is a function of its
orientation to the incident wave and to the surroundings. Since the current distribution will be essentially
unpredictable for other than very specialized conditions, the ability to predict shielding effectiveness of the
cable shield through the use of
is severely limited.
There are several methods for shielding cables. These include: (a) braid, (b) flexible conduit, (c) rigid conduit,
and (d) spirally-wound shields of high permeability materials. The principal types of shielded cables that are
available include shielded single wire, shielded multi-conductor, shielded twisted pair, and coaxial. Cables are
also available in both single and multiple shields in many different forms and with a variety of physical
characteristics. The general properties of five classes of cable shields are given in Table 8-17.
Braid, consisting of woven or perforated material, is used for cable shielding in applications where the shield
cannot be made of solid material. Advantages are ease of handling in cable makeup and lightness in weight.
However, it must be remembered that for radiated fields the shielding effectiveness of woven or braided
materials decreases with increasing frequency and increases with the density of the weave (8-14). The relative
shielding effectiveness of single and double braided cables as a function of frequency is shown in Figure 8-34.
8-59





 


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