The Smith Chart

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Intro to Smith Charts

Smith Charts Video

Constant Resistance Circles

Constant Reactance Curves

Impedance Transformations

Impedance Matching: Series L and C

Impedance Matching: Tx Lines

The Admittance Smith Chart

Impedance Matching: Parallel L and C

Impedance Matching: Parallel Tx Lines

Immittance Charts

Impedance Matching on Immittance Charts

Dual-Band Impedance Matching

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).


Introduction to Smith Charts

Up: Smith Chart and Impedance Matching Table of Contents

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.

smith charts

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

basic smith chart

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.

Smith Chart Example 1. Suppose smith chart tutorial=0.5.

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

Smith Chart fundamentals
[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:

example of complex reflection coefficient on 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:

more examples of smith charts

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:

plotting on the smith chart

Figure 5. Smith Charts=-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.


Smith Chart Lecture

Here we present an introduction video to Smith Charts. This video can be skipped if you like; the material is covered in the remaining sections. However, if you like videos, here is another medium of presenting the Smith Chart that may be beneficial. Particularly, the Immittance Smith Chart is discussed in terms of the reflection coefficient. An example antenna impedance is plotted on the Smith Chart and explained. The goal of this movie is to present the information using a different method (video instead of webpage), which will hopefully increase your understanding of Smith Charts.

Smith Chart

The Smith Chart tutorial is shown on this page, in video format. This movie is helpful for explaining the Smith Chart.

To sum up this page so far, the Smith Chart is a plot of the reflection coefficient. Since the reflection coefficient corresponds directly to an impedance, we are actually plotting the impedance of a device on the Smith Chart. In the next section, we'll start filling in some of the crazy lines on the Smith Chart visible in Figure 1.


Next: Constant Resistance Circles

Smith Chart Table of Contents

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