A transmission line is fundamentally a connector designed to transmit energy from one point to another. Regardless of their physical form—be it a coaxial cable or a microstrip on a circuit board—all transmission lines are governed by four fundamental distributed electrical parameters. These parameters are intrinsic to the line’s physical construction and collectively determine its behavior and performance at microwave frequencies.

The four primary parameters of a transmission line are:

• Resistance (R): This is the opposition to current flow offered by the conductor material itself. Resistance is directly influenced by the conductor’s resistivity (ρ), length (l), and cross-sectional area (a), as shown by the formula R = ρ * (l/a). At microwave frequencies, resistance is also affected by the Skin Effect, a phenomenon where the current density increases toward the surface of the conductor, effectively reducing the cross-sectional area used for conduction and increasing the line’s resistance.
• Inductance (L): This parameter represents the property of the line that opposes a change in current. It arises from the magnetic field that is induced around the conductors by the flow of alternating current (AC). This varying magnetic field, in turn, induces a voltage that opposes the original current flow.
• Capacitance (C): This effect arises from the electric field that exists between two conductors separated by a dielectric medium (such as air or an insulating material). The conductors act as parallel plates, storing energy in the electric field between them.
• Conductance (G): This parameter represents the leakage current that flows through the dielectric and is the inverse of the leakage resistance of the dielectric material. It accounts for the small current that flows between the two conductors through the insulating medium or to the ground.

For analytical purposes, these primary parameters are often grouped into two secondary parameters: impedance, which combines the effects of Resistance (R) and Inductance (L), and admittance, which combines the effects of Capacitance (C) and Conductance (G).

These four fundamental parameters are not just abstract electrical properties; they collectively define the single most critical operational characteristic of a transmission line: its characteristic impedance.