An ideal phase shifter is lossless and matched; it only shifts the phase of the output wave, or in other words, changes the phase difference between the output and input waves. Phase shifters are needed, for example, in phased antenna arrays.
An adjustable waveguide phase shifter can be realized by replacing the resistive card of the attenuator in Figure 6.20(c) with a dielectric card whose depth in the waveguide is adjustable. A structure resembling the attenuator in Figure 6.21 also operates as a phase shifter when the resistive cards are replaced with dielectric cards having proper lengths . Electrically controlled phase shifters are much faster than mechanical phase shifters. They are often based on semiconductor devices such as p-i-n diodes or field effect transistors (FETs).
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