Characteristic impedance refers to the impedance of a cable when it is infinitely long. It is a resistance that prevents current from passing through a conductor. It is not a dc resistance in the conventional sense.
The characteristic impedance of a cable is a combination of its conductivity, capacitance and resistance.
Assuming that a uniform cable extends indefinitely, the impedance at the transmitter at a certain frequency is called a 'CharacteristicImpedance'.
These parameters are determined by physical parameters such as conductor size, distance between conductors, and cable insulation properties.
When measuring the characteristic impedance, the equivalent resistance of characteristic impedance can be used at the other end of the cable, and the measurement result will be related to the frequency of the input signal.
The characteristic impedance is measured in ohms.
As the frequency of high frequency increases, the characteristic impedance tends to be fixed.
For example, the coaxial line will be 50 or 75 ohms;
The characteristic impedance of unshielded twisted pair is 100 ohms, while that of shielded twisted pair is 150 ohms.
Normal physical operation depends on the constant characteristic impedance of the cables and connecting devices throughout the system.
The characteristic impedance can usually be slightly altered by cable connections and end junctions.
A hard bend or knot in the cable can also change the cable's characteristic impedance.
In the case of light discontinuities, the reflected signal is weak and attenuates through the cable, so it still works for the network.
Large impedance discontinuities will interfere with data transmission.
Such discontinuity is caused by poor electrical connections, incorrect cable ends, the use of mismatched cables and mismatched connectors, and the wrong way the twisted-cable pairs are twisted in the cable