Technical Fiflri

This invention relates generally to coaxial cords, and more in particular to a coiled coaxial cord with improved impedance characteristics.

Background

Public safety microphones in conjunction with portable two-way radios typically used by governmental agencies such as police and fire departments use straight cords. The straight cord is ergonomically inferior insofar as the straight cord fails to adjust to different sizes required by different individuals. Use on a short person tends to cause a loop in the cord which has the potential to catch or latch on to objects. Use on a tall person causes restricted movement by the individual or the mounting of microphone in less than desirable locations. As shown in FIG. 1, the typical straight coaxial cord 10 comprises of conductor 18 covered by a dielectric 16, which in turn is covered or surrounded by a braided shield 14. Finally, an outer non-conductive sleeve 12 preferably made of a resilient plastic or rubber covers the braided shield 14. The use of a braided shield in a coiled cord is possible, but this structure results in a poor performing conductor. For instance, the braided shield 14 would bind up in a sharp bend in a coiled cord which would result in degraded impedance characteristics. Furthermore, the flexing of a braided shield leads to binding and fraying of the shield which ultimately results in RF loss and eventual shorting. Thus, a need exists for a shielded coaxial cord that would allow easy flexibility of the cord without the premature degradation and failure of the cord.

Siimmnrv nf the Invention A coiled coaxial cord comprises a conductor surrounded by a layer of dielectric material and a shield surrounding the layer of dielectric material allowing a substantially non- variable impedance characteristic during the relative movement of the coiled coaxial cord.

Brief Description of the Drawings

FIG. 1 is a side view of a partially stripped existing coaxial cord. FIG. 2 is a side view of a partially stripped coiled coaxial cord in accordance with the present invention.

HfttailffH npsfri fion of the Preferred Embodiment

Referring to FIG. 2, there is shown a side view of a partially stripped coiled coaxial cord 20 in accordance with the present invention. The coiled coaxial cord 20 preferably comprises, in order, a conductor 30 surrounded by a layer of dielectric material 28. Then, a shielding means (26 and 28) surrounds the layer of dielectric material 28. The shielding means preferably comprises a foil shield layer 26 surrounding the layer of dielectric material 28 and a spiral wound shield 24 having a plurality of wires surrounding the foil shield layer 26. Finally, the conductor 30, dielectric material 28, foil shield 26, and spiral wound shield 24 can optionally be covered or protected by a plastic or polymeric jacket or sleeve 22.

Preferably, the jacket 22 is made from an elastomer such as polyvinyl chloride. The conductor 30 is preferably a metal wire such as copper, silver plated copper or tin plated copper. The layer of dielectric material 28 is typically made from TEFLON, made by Du Pont. The foil shield layer 26 is preferably made from an aluminum mylar tape. The spiral wound shield 24 is preferably made of tin plated copper wires. Operationally, the coiled coaxial cord 20 should allow someone to either twist or longitudinally compress or extend the cord without degrading the overall impedance characteristic of the coaxial cord. In other words, the relative movement of the coiled cord should not affect the performance of the cord. The spiral wound shield 24 and the foil shield 26 allows the relative movement of the cord. Although the spiral wound shield 24 may slightly bind or fray with movement, the combination of the spiral wound shield with the foil shield prevents the degradation of the impedance characteristic of the coaxial cord. What is claimed is: