In this tutorial, we will introduce and explain Smith Charts, and then given an introduction to impedance matching. We will then use the Smith Chart to perform impedance matching with transmission lines and lumped components (capacitors and inductors).

The Smith Chart is a fantastic tool for visualizing the impedance of a transmission line and antenna system as a function of frequency. Smith Charts can be used to increase understanding of transmission lines and how they behave from an impedance viewpoint. Smith Charts are also extremely helpful for impedance matching, as we will see. The Smith Chart is used to display an actual (physical) antenna's impedance when measured on a Vector Network Analyzer (VNA).

Smith Charts were originally developed around 1940 by Phillip Smith as a useful tool for making the equations involved in transmission lines easier to manipulate. See, for instance, the input impedance equation for a load attached to a transmission line of length L and characteristic impedance Z0. With modern computers, the Smith Chart is no longer used to the simplify the calculation of transmission line equatons; however, their value in visualizing the impedance of an antenna or a transmission line has not decreased.

The Smith Chart is shown in Figure 1. A larger version is shown here.

Figure 1. The basic Smith Chart.

Figure 1 should look a little intimidating, as it appears to be lines going everywhere. There is nothing to fear though. We will build up the Smith Chart from scratch, so that you can understand exactly what all of the lines mean. In fact, we are going to learn an even more complicated version of the Smith Chart known as the immitance Smith Chart, which is twice as complicated, but also twice as useful. But for now, just admire the Smith Chart and its curvy elegance.

This section of the antenna theory site will present an introduction to Smith Chart basics.

Smith Chart Tutorial

We'll now begin to explain the fundamentals of the Smith Chart. The Smith Chart displays the complex reflection coefficient [Equation 1, below], in polar form, for an arbitrary impedance (we'll call the impedance ZL or the load impedance). The reflection coefficient is completely determined by the impedance ZL and the "reference" impedance Z0. Note that Z0 can be viewed as the impedance of the transmitter, or what is trying to deliver power to the antenna. Hence, the Smith Chart is a graphical method of displaying the impedance of an antenna, which can be a single point or a range of points to display the impedance as a function of frequency. For a primer on complex math, click here.

Recall that the complex reflection coefficient ( ) for an impedance ZL attached to a transmission line with characteristic impedance Z0 is given by:

[Equation 1]

For this Smith Chart tutorial, we will assume Z0 is 50 Ohms, which is often, but not always the case. Note that the Smith Chart can be used with any value of Z0.

The complex reflection coefficient, or , must have a magnitude between 0 and 1. As such, the set of all possible values for must lie within the unit circle:

Figure 2. The Complex Reflection Coefficient must lie somewhere within the unit circle.

In Figure 2, we are plotting the set of all values for the complex reflection coefficient, along the real and imaginary axis. The center of the Smith Chart is the point where the reflection coefficient is zero. That is, this is the only point on the Smith Chart where no power is reflected by the load impedance.

The outter ring of the Smith Chart is where the magnitude of is equal to 1. This is the black circle in Figure 1. Along this curve, all of the power is reflected by the load impedance.

Let's look at a few examples.

From equation [1], we can solve for ZL to be:

[Equation 2]

From equation [2], with Z0=50 Ohms, a reflection coefficient of 0.5 corresponds to a load impedance ZL=150 Ohms. We can plot gamma_1 on the smith chart:

. Figure 3. plotted on the Smith Chart.

Since is entirely real, the point lies along the real gamma axis (x-axis) in Figure 3, and the imaginary axis value (y-axis) location is 0.

Smith Chart Example 2. Suppose = -0.3 + i0.4

is plotted on the Smith Chart in Figure 4:

Figure 4. plotted on the Smith Chart.

From Equation [2] and using Z0=50, we note that corresponds to a load impedance

ZL = 20.27 + i*21.62 [Ohms].

Smith Chart Example 3. = -i.

is plotted on the Smith Chart in Figure 5:

Figure 5. =-i plotted on the Smith Chart.

From Equation [2] and with Z0=50, corresponds to a load impedance ZL = -i*50 Ohms. That is, the load impedance here is purely imaginary and negative, which indicates a purely capacitive load.

VSWR on the Smith Chart

Since VSWR is only a function of the absolute value of , we can get the VSWR for a load from the Smith Chart as well. That is, a VSWR = 1 would be the center of the Smith Chart, and VSWR=3 would be a circle centered around the center of the Smith Chart, with magnitude =0.5. Circles centered at the origin of the Smith Chart are constant-VSWR circles. Note that the outer boundary of the Smith Chart (where =1) corresponds to a VSWR of infinity.