Notes 18 Spring 2005

Notes 18 Spring 2005 - Notes 18 Spring 2005 Impedance...

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Notes 18 Spring 2005 Impedance matching networks : We’ve determined that a load is said to be matched to its transmission line when the input impedance is equal to the characteristic impedance of the line, in Z 0 in Z Z = , in which case no reflection occurs at the input point. Our job in designing an impedance matching network is, starting with any load impedance, to insert between the end of the transmission line and the load impedance a network that will have an input impedance equal to thereby yielding maximum signal transfer to the load. 0 Z A matched load ensures that the power delivered to the load is a maximum. We place an impedance matching network between the line and the load to achieve a match. Let’s look at what is called a shorted-stub matching network . In doing this it is more convenient to work with admittance than it is to work with impedance because when combining parallel admittances we simply add them, i.e., 1 2 1 p y y y + = . Recall that the input admittance corresponding to the input impedance is the reciprocal of : in Y in Z in Z in in in in jB G Z 1 Y + = = and in in in in jb g z 1 y + = = where the real parts are the input conductances and the imaginary parts are the input susceptances 2 . Also, a normalized load admittance is given by L L o L o L o L L jb g Y B j Y G Y Y y + = + = = . It turns out that to transform an impedance to an admittance on the Smith chart we merely need to move a quarter wavelength ( 4 λ ) along the VSWR circle toward the generator (move to the other side of the vswr circle diameter). In changing over to admittance values we re-name the circles on the Smith chart; the resistance circles become conductance circles and the reactance circles become susceptance circles.
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