4
Propagation on Transmission Lines
The transmission line theory makes it possible to understand and control the evolution of voltage and current along the length of controlled impedance interconnects. It also defines the parameters used, including characteristic impedance, VSWSR and insertion losses, and their relationship with circuit materials.
4.1. Transmission line model
The simplest transmission line is a two-conductor system on which a signal is transmitted. Propagation on a transmission line can always be modeled in the form of a complex series impedance and parallel admittance.
Figure 4.1. Complex model of a transmission line
The values of the elements of these impedances and admittances and their evolution with frequency depend on the propagation modes that can be established in the structure. These modes depend on the geometrical configuration of the conductors as well as on the parameters of the insulating materials. Maxwell electromagnetic equations give the relationships between the impedances Zs and Yp with the electrical and magnetic fields [SAG 06].
4.1.1. Modes of propagation on lines
– Pure transverse electromagnetic (TEM) mode without losses
This mode exists mainly in the following hypothesis: σc = ∞ and tan δd = 0. There is no penetration of the electromagnetic field into the conductors. In this case, the parameters Zs and Ys are given by relationship [4.1]. ...
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