or longer. Coaxial cable may be viewed as a type of waveguide. Power is transmitted through the radial electric field and the circumferential magnetic field in the TEM mode. This is the dominant mode from zero frequency (DC) to an upper limit determined by the electrical dimensions of the cable. Connectors Coaxial connectors are designed to maintain a coaxial form across the connection and have the same impedance as the attached cable. Connectors are usually plated with high-conductivity metals such as silver or tarnish-resistant gold. Due to the skin effect, the RF signal is only carried by the plating at higher frequencies and does not penetrate to the connector body. Silver however tarnishes quickly and the silver sulfide that is produced is poorly conductive, degrading connector performance, making silver a poor choice for this application. Important parameters Coaxial cable is a particular kind of transmission line, so the circuit models developed for general transmission lines are appropriate. See Telegrapher's equation. Physical parameters In the following section, these symbols are used: Length of the cable, . Outside diameter of inner conductor, . Inside diameter of the shield, . Dielectric constant of the insulator, . The dielectric constant is often quoted as the relative dielectric constant referred to the dielectric constant of free space : . When the insulator is a mixture of different dielectric materials (e.g., polyethylene foam is a mixture of polyethylene and air), then the term effective dielectric constant is often used. Magnetic permeability of the insulator, . Permeability is often quoted as the relative permeability referred to the permeability of free space : . The relative permeability will almost always be 1. Fundamental electrical parameters Shunt capacitance per unit length, in farads per metre. Series inductance per unit length, in henrys per metre. Series resistance per unit length, in ohms per metre. The resistance per unit length is just the resistance of inner conductor and the shield at low frequencies. At higher frequencies, skin effect increases the effective resistance by confining the conduction to a thin layer of each conductor. Shunt conductance per unit length, in siemens per metre. The shunt conductance is usually very small because insulators with good dielectric properties are used (a very low loss tangent). At high frequencies, a dielectric can have a significant resistive loss. Derived electrical parameters *Characteristic impedance in ohms (Ω). The complex impedance of an infinite length of transmission line