TABLE OF CONTENTS At radio frequencies where slotted waveguide or transmission line sections would be excessively long, probe measurements of the relative current (or relative voltage) amplitudes at discrete sampling points along the waveguide provide a convenient and practical technique for measuring the complex impedance and related parameters. The SMITH CHART is useful for interpreting and evaluating data obtained from such measurements [112, 208] as will be described herein. The principle is made use of in the SMITH CHART plotting board shown in Fig. 14.5.
In a waveguide or transmission line propagating electromagnetic wave energy in a single mode, relative amplitude measurements of either current or voltage at three fixed probe positions uniquely determine the standing wave ratio and the wave position, provided that no two of the probes are separated an exact multiple of one- half wavelength. The standing wave pattern, in turn, is related to the impedance and other waveguide parameters, as described in previous chapters. The electrical separations of the probes need not be uniform but must be known. This is generally calculable [10] from the physical construction of the waveguide or transmission line, as described in Chap. 3.
As was shown in Chap. 3, the shape of standing current and voltage waves of a given ratio are identical, and both are different and unique for each different standing wave ratio, varying from a succession of half sine waves as the amplitude ratio approaches infinity to a sinusoidal shape as the amplitude ratio...
