SMITH CHART STUB MATCHING GENERATOR

SMITH CHART STUB MATCHING GENERATOR

If one moves from load to generator on the line, the corresponding chart location moves from the reference point, in clockwise motion, according to an angle (indicated by the light green arc) Notice that in this case the load admittance falls outside the unitary conductance circle. This provides on the admittance chart the physical reference for the load location on the transmission line. The red arrow on the example indicates the load admittance. It is very convenient to analyze the possible solutions on a Smith chart. (inductance if negative, or capacitance if positive)ДЄ stub should be placed at a location where the line admittance has real part equal to Y0Дepending on the length of the transmission line, there may be a number of possible locations where a stub can be inserted for impedance matching. Note that the input admittance of a stub is always imaginary

In order to complete the design, we have to find an appropriate location for the stub. Since the circuit is based on insertion of a parallel stub, it is more convenient to work with admittances, rather than impedances. On the other hand, an open circuited stub may be more practical for certain types of transmission lines, for example microstrips where one would have to drill the insulating substrate to short circuit the two conductors of the line. A short circuited stub is less prone to leakage ofД®lectromagnetic radiation and is somewhat easier to realize. The choice of open or shorted stub may depend in practice on a number of factors. In many cases it is also convenient to select the same characteristic impedance used for the main line, although this is not necessary. The transmission line realizing the stub is normally terminated by a short or by an open circuit. The drawback of this approach is that if the load is changed, the location of insertion may have to be moved. There are two design parameters for single stub matching: The location of the stub with reference to the load dstub The length of the stub line Lstub Any load impedance can be matched to the line by using single stub technique. The examples provided here are solved using graphical tools and a printed Smith chart, rather than the computer program, to emphasize the techniques and approximations involved although some of the numerical results listed were obtained with a computerized Smith chart ( smith-chart.m) available with this text ( see page xi).Single stub impedance matchingImpedance matching can be achieved by inserting another transmission line (stub) as shown in the diagram below A computerized Smith chart can then be used to analyze conditions on lines. Naturally, any chart can also be implemented in a computer program, and the Smith chart has, but we must first understand how it works before we can use it either on paper or on the screen. Some measuring instruments such as network analyzers actually use a Smith chart to display conditions on lines and networks. Although the Smith chart is rather old, it is a common design tool in electromagnetics. As such, it allows calculations of all parameters related to transmission lines as well as impedances in open space, circuits, and the like. The Smith chart is a chart of normalized impedances (or admittances) in the reflection coefficient plane.

This has been accomplished in a rather general tool called the Smith chart. Thus, the following proposition: Build a graphical chart (or an equivalent computer program) capable of representing the reflection coefficient as well as load impedances in some general fashion and you have a simple method of designing transmission line circuits without the need to perform rather tedious calculations. You may also recall, perhaps with some fondness, the complicated calculations which required, in addition to the use of complex variables, the use of trigonometric and hyperbolic functions. The reflection coefficient, in turn, was defined in terms of the load and line impedances (or any equivalent load impedances such as at a discontinuity). Voltage, current, and power were all related to the reflection coefficient. The reflection coefficient was used to find the conditions on the line, to calculate the line impedance, and to calculate the standing wave ratio. A look back at much of what we did with transmission lines reveals that perhaps the dominant feature in all our calculations is the use of the reflection coefficient.

Smith chart stub matching