The impedance (Z) is the apparent resistance of a two- or four-terminal device. This is composed of the ohmic resistance (R) and the reactance (X), the reactance. This is the frequency-dependent resistance of the inductances and capacitances.
In general, the impedance "Z" is a complex, frequency-dependent quantity, which consists of a real and an imaginary (j) resistance and is uniquely determined by its magnitude "Z" and its angle Phi (`phi`). The magnitude is calculated according to Pythagoras from the square root of the sum of the squares of the real and the imaginary parts. The angle `phi` via the tangent.
For cables, the impedance is independent of the cable length and is calculated via the square root of the ratio of the inductance to the capacitance. Both quantities are determined by the construction of the cable, by the dimensions of the inner conductor, outer conductor, dielectric and shielding. Inductance is directly dependent on the thickness of the inner conductor; a thicker inner conductor provides a lower inductance, a thinner one a higher one. Capacitance is dependent on the dielectric. A dielectric with a higher dielectric constant or permittivity causes an increase in capacitance, a smaller dielectric constant results in a lower cable capacitance.
Impedance is expressed in ohms and is independent of frequency for cables over wide frequency ranges. With increasing frequencies, the impedance is increasingly determined by the conductor resistance, where the skin effect takes effect. In active quadripoles(amplifiers, repeaters, etc.), the impedance is determined by design measures in the input circuit.
The reciprocal of the impedance is the admittance.