RF and Microwave Engineering: Fundamentals of Wireless Communications

6.3 Impedance Matching

A transmission line with a characteristic impedance Z₀ that is terminated by a load impedance Z_A shows no reflections at the end of the line if characteristic impedance and load impedance are equal Z₀ = Z_A (see Section 3.1.8). Furthermore, in order to obtain maximum power from a source with an internal impedance Z_I the load impedance Z_A 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 Z_in (see Figure 6.11).

Figure 6.11 Two-port matching networks for (a) matched (non-reflective) line termination and (b) maximum power transfer.

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 (Z_A = R_A) and resistive target impedances (Z_in = R_I). 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. ...