Experiment 5 — Coupler Design.

Z 0
Z 0
Through wave
R 2
Z 0
Sampled wave
R 1
Figure 3 - Resistor divider non-directional coupler.
The transmission lines are not electromagnetically coupled and the coupling
equations are derived from the lumped circuit calculation of the resistor
divider.
3.2 Coupled Line Directional Coupler
3.2.1 A Single Section Coupler
There are two modes of current flow in an electromagnetic situation. The first
is one current flowing down one conductor with a contra-flow current back
up the other conductor caused by displacement current coupling between
the two conductors. This is termed the ’odd mode’ current, and it has an
associated odd mode characteristic impedance, styled Z 0o .
Theothermodeisonecurrentflows by displacement current between
each center conductor carrying the same polarity, and the ground that is
common between them. Hence this is called the ’even mode’ current, and it
has an associated even mode characteristic impedance, styled Z 0e .Figure4
shows the polarity of the lines of each mode.
For a single section coupler the even and odd mode characteristic impedances
are defined as:
6