A transmission line with a characteristic impedance Z0 that is terminated by a load impedance ZA shows no reflections at the end of the line if characteristic impedance and load impedance are equal Z0 = ZA (see Section 3.1.8). Furthermore, in order to obtain maximum power from a source with an internal impedance ZI the load impedance ZA should be the conjugate-complex value of the internal impedance (see Section 5.4.2).
If the load impedance is mismatched, special two-port networks (matching circuits) can be designed to transform the actual load impedance to the required input impedance Zin (see Figure 6.11).
Low-loss matching networks can be constructed by using reactive components (capacitors and inductors) or transmission lines. Since these components exhibit frequency dependent behaviour the desired impedance transformation is limited to a certain frequency range.
We will start by looking at resistive load impedances (ZA = RA) and resistive target impedances (Zin = RI). Later on we will expand our view and include universal complex load impedances. Even today the classical Smith chart is a valuable tool when designing matching circuits. ...