The present application relates to shielding of wire and cable conductors.

Shielding is used to protect the inner signal of current carrying wire and cable conductors from electro-magnetic interference (EMI), especially radio frequency interference (RFI). Shielding can be accomplished using a conductive material. Typical prior art shields use metallic foil which is wrapped around the insulated conductor or cable, as well as wire or flat wire which can be either braided or served over the insulated conductor or cable. The wire to be protected (shielded) is referred to herein as the“conductor” to distinguish it from“shield(ing) wire” or “shield product.” The shielded cable is then typically covered with a polymeric jacket Shielding wires are typically round, but flat wire has also found wide usage since it can cover a larger surface area while providing weight reductions. However, the use of flat shielding wire has not found broader use especially in woven braid constructions due to the fact that it only allows for a single strand, it is difficult to terminate, it tends to form gaps which is detrimental to shielding effectiveness and higher production cost than round wire due to slower manufacturing rates.

SUMMARY OF THE INVENTION

Electrical wires and cables carrying a signal are generally shielded to protect the signal from EMI and RFI interference. A proper shield must satisfy several requirements including shielding coverage, shield effectiveness, flexibility, flex life, ease of stripping, ease of termination and mechanical strength. These aspects are addressed by shield design and shield material. One important aspect of conductor design is weight. Weight reduction or weight saving is paramount in many products especially in aerospace and automotive applications. An object of this invention is to provide a shield wire and a method for producing this shield wire to save weight while maintaining or improving performance.

Shielding wire -of prior art usage is round in cross section and made to various gauge sizes depending on the wire conductor being shielded. NEMA WC 27500, for example, describes standards for round and flat shield strands. Four sizes of round shield strand from 32 AWG to 38 AWG are identified to be used based on the cable core diameter. Flat shield strand is identified as .0015” (1.5 mils) thick in this standard. Width of flat wire varies but it is typically more than .0100”. Round shield wire can be braided (woven) or spiral wound (served) around the insulated conductor or cable. Other round wire diameters or flat wire thicknesses, however, may also be used.

These and other objects, features and aspects of the invention are further described in the following detailed drawing figures and detailed specification descriptions including preferred embodiments thereof. BRIEF DESCRIPTION OF THE DRAWING

Figure 1. Is a sketch of round, elliptical and oval cross sections of shield wire (note a, b, dimension convention);

Figure 2. Is a graph of percent (%) weight reduction vs. a/b ratio for elliptical shield wire having the same cross-sectional area as round wire;

Figure 3. Is a graph of percent (%) weight reduction vs. a/b ratio for elliptical shield wire having the same width as round wire;

Figure 4. Braid pattern and one diamond of a braid;

Figure 5. Is a photograph of shield coverage using two and three strand carriers of 38 AWG elliptical shield wire having 2:1 aspect ratio;

Figure 6. Is a log-log graph of surface transfer impedance for 24 AWG wire shielded with 38 AWG round shield wire;

Figure 7. Is a log-log graph of surface transfer impedance for 24 AWG wire shielded with 38 AWG elliptical shield wire; and

Figure 8. Is a table of typical drawing steps for converting a round wire starting piece to an elliptical shielding wire form.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Changing the shield wire from round to an elongated cross section shape while maintaining the same cross sectional area will reduce braided shield weight without changing the shield strands’ DC resistance or mechanical properties. The exact weight reduction will depend on the specific shape of the shield strand and its aspect ratio. The cross section of the elongated shape may be an ellipse or oval (race track) or any other cross-section elongated form. Examples of ellipse and oval shapes are shown in Figure 1.

Ellipse is one of the more efficient shapes for weight reduction. The amount of weight reduction for an ellipse depends on the aspect ratio (a/b ratio). The weight reduction for an elliptical shielding wire having axes a and b and the same cross sectional area as a round wire with the radius r is shown in the formula below where W _c is the weight of the round wire and W _e is the weight of the elliptical wire both covering surface of the same cable: The weight reduction for an elliptical shield wire relative to a round wire up to an aspect ratio of 4 is graphically illustrated in Figure 2, the greater the aspect ratio the greater the weight reduction.

A greater weight reduction can be achieved if the elliptical wire maintains the same width as the round wire it is replacing (a = r). In this case the weight reduction will be:

The weight reduction for an elliptical wire having the same width as a round wire diameter relative to the round wire up to an aspect ratio of 4 is graphically illustrated in Figure 3. This greater weight reduction is possible if the standards allow use of a smaller gauge size shielding wire, i.e. if a smaller cross sectional area relative to the round wire is permitted.

Elliptical shield wire combines the best performance aspects of round and flat shield wire while eliminating the negative aspects of each. Elliptical shield wire can be directly used in standard braiding equipment providing ease of application without needing new equipment and added expense. Similar to wire shielded with round braid, elliptical shielded wire can be easily terminated as compared with flat shield wire. In addition, flat shield wire can form gaps when the wire is flexed, resulting in poor EMI shielding due to leakage whereas elliptical shield wire lays flat and does not create gaps in coverage during flexure.

Shield Coverage and Shield Effectiveness:

Shielding of electrical cables using round wire is conducted on braiding machines which use a number of carriers. Each carrier has a number of strands (shield wires) wound onto it. Shield coverage as shown by Vance (Ref. I) ¹ is:

where N is the number of wires per carrier, d is the diameter of a single shield wire and W is the width of coverage provided per carrier including the spacing between carriers as illustrated in Figure 4. In the case of round wire d = 2r while in the case of elliptical shield wire d = 2a or the longer axis of the ellipse. This formula sho ws that in practice shield coverage can be adjusted by adjusting the braiding parameters.

Resistance of the shield wire and transfer impedance which indicates shield effectiveness are important factors in predicting performance of the shield layer. The length of each individual shield wire per unit length of the cable is dependent on the pitch angle of the weave ( a).

¹ E. F, Vance, Shielding Effectiveness of Braided Wire Shields, Stanford Research Institute, IEEE Transactions on

Electromagnetic Compatibility, Vol. EMC- 17 issue 2 (1975) Resistance, which is directly proportional to the length of the shield wire, can then be adjusted by controlling the pitch angle.

Shield hole size is dependent on the percentage of shield coverage, individual shield wire width, number of carriers and number of strands per carrier. Shield effectiveness at high frequencies is related to the hole size. By producing the same or greater coverage using elliptical shield wire, better shielding effectiveness than round shield wire can be achieved.

Shielding coverage of wire and cable using elliptical shield wire may be achieved by other methods such as serve, where the shield wire is helically wound around the cable. This invention is applicable to any method of shielding and not limited to only one specific shielding method such as braiding used as an example here.

Manufacturing Process:

Various techniques maybe utilized to manufacture a wire with elongated cross section. Two of these methods are described below.

In the first method, the wire with elongated cross section may be manufactured using a series of drawing dies. These dies start with a round cross section (typical wire drawing die) and successively change the cross section to the desired elongated shape. Using a standard wire drawing machine, reduction of cross section of wire must be incorporated in the shape change to obtain the required shape change and size at finish. This will allow utilization of standard wire drawing equipment without the need to purchase new equipment, if the shield wire specification requires soft (annealed) temper, an in line annealing machine may also be utilized, although not required. Annealing may be conducted as a separate secondary operation. This method of manufacture allows for production of bare or plated product. Shield wire maybe plated with normal conductor plating such as nickel, silver or tin or any other plating as required.

Alternatively, instead of plating, the product may be clad with a secondary metal or alloy, e.g., copper clad aluminum or copper clad steel.

In a secondary method, rolls may be used to roll the shape. In this case, rolls are ground with the required shape and the round wire is passed through the roll gap to impart the shape. Depending on the rolling equipment full shape change may require more than one pass requiring a tandem rolling mill or more than one rolling step. Similar to wire drawing method to manufacture wire with elongated cross section, rolling method may be used to process plated or clad wire.

A typical sequence of seven progressive die re-shaping steps from a round (circular) starting wire to elliptical shielding wire form 36 AWG with a 2:1 aspect ratio with a ½ Brown & Sharps (B&S) drawing reduction is shown in Figure 8. Example:

Using silver plated copper wire, elliptical wire with a 2:1 aspect ratio and 38 AWG cross section was produced by drawing through specially designed dies. This wire was wound as two and three strands on bobbins for shielding. Eight each of the two strand and three strand bobbins were loaded in a sixteen carrier braider to braid a 24 AWG insulated conductor wire. Braiding was performed successfully on a standard braiding machine with the strands properly covering greater than 85% of wire surface. Figure 5 shows an example of coverage.

An identical procedure was used to shield the same 24 AWG conductor wire with 38 AWG round shield wire with similar coverage. Braid weight per 1000 feet of both shielded wires was measured. Braid weight for the round wire was 2.02 lb/1000ft whereas braid weight for the elliptical wire was only 1.45 lb/1000ft showing a 28.2% weight reduction for the elliptical shield compared with the round shield wire.

Following shielding, the wires were jacketed then tested to measure surface transfer impedance using standard measurement technique. Transfer impedance from 0.1 to 1000 MHz for the shielded wires are shown in Figures 6 and 7.

The lower the surface transfer impedance the more effective the shielding. At lower frequencies, 0.1 to 1 MHz, where resistance of the shield wire dominates, the round shield wire shows a somewhat lower surface transfer impedance. This is not a necessary disadvantage of elliptical shield wire and by controlling shielding factors indicated in Figure 4, equivalency with round wire may be obtained. At higher frequencies, above 2.5 to 3 MHz, which is of greater importance, the elliptical shield has lower transfer impedance showing superiority over the round shield wire. This indicates that in addition to weight reduction, elliptical shield wire

demonstrates superior shield effectiveness at higher frequencies.