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Q: What is Coaxial Cable?

A coaxial cable (often referred to as coax) is a common cable variety designed for providing clean and uninterrupted high speed signals that are isolated from external electromagnetic interference (EMI). This type of cabling is often used to conduct weak (low-amplitude) voltage signals, due to its excellent ability to shield such signals from external interference.

These cables are found in cable television, Broadband Ethernet applications, video equipment, medical applications, closed-circuit television, ham and commercial radio and microwave transmission and even undersea cable systems where long haul, high speed signaling, free from interference, is required.

Coaxial cable is designed to resemble a series of concentric layered rings of varying materials. The centermost point is the metallic conductor. This conductor can either be stranded or solid. The conductor is then coated with a dielectric insulator that isolates the conductor from the shield. The shield, which follows the dielectric layer, consists of a wrapped metallic spiral, a braided mesh of wire, or a metallic tape foil covering the insulation. Over the shield a final jacket is formed that acts as a protective barrier from moisture and other substances based on the material used.

Within a coaxial cable, the electricity flows freely through the conductor at lower speeds, but as the speed increases the electrical cross-section decreases. This means that at higher speeds the electrical signals flow closer to or even along the skin of the conductor. This is called the skin effect and it is a critical characteristic of high speed cables that require very low capacitance to keep the signals intact.

The underlying reason for this is that electrons can act as both particles and waves. This is called the Complementarity Principle. As high enough frequency the particles act as actual waves propagating more than the volts and amps being induced by the electromagnetic wave. In essence, the conductors become wave guides. So a coaxial cable, for instance, that has high capacitance will result in a higher resistance because of the decrease in cross section due to the thinner skin (or reduced area) in which the high speed signals can flow. Also, every conductor has a minute amount of series inductance associated with it. The impedance (resistance) of an inductor increases linearly with frequency. At low frequencies, this inductance has a negligible effect, but it becomes very significant at higher (GigaHz) frequencies and above.

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