The characteristic impedance or surge impedance of a uniform transmission line, usually written Z0, is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length. The SI unit of characteristic impedance is the ohm.

The characteristic impedance of a lossless transmission line is purely resistive, with no reactive component. Energy supplied by a source at one end of such a line is transmitted through the line without being dissipated in the line itself. A transmission line of finite length (lossless or lossy) that is terminated at one end with a resistor equal to the characteristic impedance appears to the source like an infinitely long transmission line.

In the Polar Si8000 calculating program there are 28 Single ended coplanar structures and 28 Differential ended coplanar structures.

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