Component Video Cables - The Definitive Guide - page 5

3.6 Protective Shields

As indicated in Section 2.1, EMI noise is around and at all times and can enter a component video cable and create interference. In theory, a long component video cable (2-meters or greater), acts as an antenna that can attract external noise. Conscientious manufacturers will implement features such as shields in coaxial cables to minimize EMI from entering. These shields should be made from nonmagnetic materials with thickness much less than skin depth at the frequency of interest. For coaxial cables, there are three basic shielding techniques, including foil shielding, braid shielding and combination shielding.

3.6.1 Foil Shielding

Foil shields are usually made of a thin layer of aluminum foil, bonded to a polyester film (for strength). A good foil shield allows for 100% coverage if applied with an appropriate overlap construction. Foil shields provide an easy termination and low costs, but are somewhat fragile. They are extremely effective in reducing RF noise, but not very effective in EMI (depending on the frequency).

3.6.2 Bi-Foil Shielding

The bi-foil shield is an aluminum-polyester-aluminum tape assembly that offers 100% coverage. There is two times the aluminum in this design as the foil shield discussed in Section 3.6.1, since aluminum is on both sides of the polyester film.

3.6.3 Bonded Foil

This foil is the same as the bi-foil construction, but it is bonded to the dielectric with an adhesive. It partially improves the shielding capabilities, but it is more for ease of cable assembly because the foil doesnât move or pull away from the dielectric when stripped.

3.6.4 Braid Shielding

Braid shielding was already discussed in Section 3.1.2. As mentioned in that section, the braided grounding shield is an effective shield for a wide range of EMI, but not within the RFI range.

3.6.5 Combination Shielding

This technique offers the best of both types of shielding. First a foil shield is placed around the dielectric, and then a braided (or two braided) shield is placed on top of the foil. This combination allows for blocking of a broader range of EMI, including RF noise. There are three basic methods of combination shielding such as triaxial, tri-shielding and quad shielding.

• Triaxial ö Braid-Separator-Braid construction
• Tri-Shielding ö Foil-Braid-Foil combination
• Quad Shield ö Foil-Braid-Foil-Braid combination

3.7 Jackets and Sheaths

Jackets or sheaths over coaxial cables act as a protective covering as well as contain the component elements and shields. Jacket materials are selected typically for cost, flame resistance, durability, flexibility, chemical resistance and to have a good appearance. The most common Jacket materials are as follows:

• PVC : PVC formulations vary widely depending on the desired properties. Because it is relatively inexpensive, easy to extrude and exhibits many excellent mechanical and electrical properties it is used for jackets on many component video cables. The disadvantage of PVC is its overall stiffness and lack of flexibility.
• Polyurethane : Polyurethane in general has excellent low-temp flexibility, high tensile strength, and long flex-life. It exhibits excellent chemical, water, and abrasion resistance, as well as being extremely tough and cut-through resistant. Polyurethane is extremely flammable but can be offered in a flame-retardant version at the expense of tensile strength and surface finish. Polyurethane is restricted to use in jackets only, due to its poor dielectric properties.
• Polyethylene : Polyethylene is the compound most widely used in coaxial and low capacitance cables due to its fine electrical properties. Although polyethylene is flammable, additives can be used to make the polyethylene flame retardant at the expense of the dielectric constant and power loss characteristics. Inherent properties make it ideal for direct burial applications.
• Nylon: Nylon exhibits excellent jacket toughness, especially in thin-wall applications, and has excellent abrasion, cut-through, and chemical resistance. Its main application is to provide these qualities when extruded over softer insulation compounds. Nylon has a very low coefficient of friction, making it a good choice for use in high flex applications. The disadvantage of Nylon is that it absorbs moisture, which, over time, can somewhat degrades the electrical properties of cable.