Distributed Line Constant

The primary line constants are parameters that describe the characteristics of copper (or other conductive material) transmission lines in terms of the physical electrical properties of the line. The primary line constants are only relevant to copper lines and are to be contrasted with the secondary line constants, which can be derived from them, and are more generally applicable. The secondary line constants can be used, for instance, to compare the characteristics of a waveguide to a copper line, whereas the primary constants have no meaning for a waveguide.

Primary Line Constant

Equivalent Circuit

1. Inductance

   When we AC input is applied to the transmission line its inductive reactance (XL) increases with increase in frequency. As the transmission line is made of the millions of small particles, therefore the chain of those particles make the transmission line to behave as inductor.

   The presence of this inductance provides us the opposition to the flow of alternating current. This inductance is also one of the primary line constant and it is measured in H/Km Henry/kilometer.

2. Capacitance

   As the transmission line is made of two conducting wires and in between these wires the dielectric is present, therefore, it is obvious that the capacitance will be present in the transmission lines. The dielectric material used in transmission lines may be air, plastic, rubber, polystyrene or polyethylene etc. the capacitance of the transmission line is measured in f/km.

3. Conductance

   The two wires of the transmission line are separated from each other with the help of insulating material. As each insulating material has more or less conductive effect, therefore, conductance is also one of the primary line constants. It is measured in mho/km.

4. Resistance

   As the transmission line is made of the conducting wire and we know that every wire has some specific resistance according to the type of material used. The transmission line will also have the loop resistance which is known as one of the primary line constants. It is measured in ohm/km.

Surge or Characteristics Impedance Z_o

If the transmission line is made infinity ling and we measure its input impedance from the source, this input impedance of infinity long transmission line is known as its characteristic impedance. It is abbreviated as Z_o. It is also known as surge impedance.

Formula for Surge or Characteristics Impedance Z_o

There are the following formulas used to measure characteristics impedance of transmission line.

1. Here L stands for inductance per unit length of transmission line C stands for capacitance per unit length of transmission line.

2. Here Z stands for impedance of the transmission line and y stands for admittance of the transmission line. (Admittance is opposite to the impedance)

3. Provided that transmission line is infinity long.

4. J stands for angular difference or lage difference
   W stands for 2πf
   G stands for Conductance