Phase Shifter, Microwave

a device designed to alter the phase of electromagnetic oscillations at the output of a microwave transmission line (such as a hollow or dielectric wave guide, a coaxial long line, or a strip line) with respect to the phase of the oscillations at the input of the line. The phase shift is achieved by changing the electrical length of the line. (The electrical length of a line is equal to 2πl/λ_w, where l is the geometric length of the line and λ_w is the wavelength in the line.) Microwave phase shifters may be adjustable or nonadjustable.

Adjustable phase shifters consist of a feeder section that introduces the phase shift at a certain frequency (or the desired phase shifts over a given frequency range). The magnitude of the shifts can be adjusted as required. The two principal classes of adjustable phase shifters are those with mechanical or electromechanical control of the phase shift and those with electrical control. The first type includes telescoping sections of coaxial lines, adjustable by changing the length l, and dielectric waveguide phase shifters, which consist of waveguide sections containing a movable dielectric plate. In the latter design, the phase velocity of the wave and λ_w (and, consequently, the phase shift) are affected by the position of the dielectric plate in the wave guide. Another type in the first category uses compressible sections of rectangular waveguides, with elastic suspension members mounted on the narrow walls in such a way that the width of the waveguide (and thus λ_w) can be changed. Bridge-type phase shifters are coaxial or wave-guide multibranch microwave devices. They are equipped with two closed stubs with lengths that are matchingly adjustable. The device is fundamentally a directional coupler.

Electrically operated adjustable phase shifters include those with semiconductor components, such as semiconductor diodes with a p-i-n structure or varactors; those that use ferrite devices; those that use a ferroelectric; and plasma devices. The most promising types are those that use p-i-n diodes as switching elements. Such diodes permit the phase shift to be altered in steps, either by changing l directly or by connecting a set of stubs to the line through the diodes. The operation of ferrite phase shifters, which are also widely used, is based on the interaction of the electromagnetic wave with the noncompensated magnetic moments of ferrite sublattices. Ferrite phase shifters may be of the reciprocal type, in which case they produce the same phase shift for both directions of wave propagation, or of the nonreciprocal type; a gyrator is a special example of the latter type.

Nonadjustable phase shifters consist of a phase-calibrated feeder section. The phase shift in the section is provided by selecting the desired length, cross-sectional dimensions (if a wave guide is used), or effective dielectric constant.

Microwave phase shifters are used in various microwave devices, such as systems with a large number of users (to provide the required distribution of the initial phases of the received signals), in the feeders of radio systems (to equalize electrical lengths), and in phased antenna arrays and other coherent radio systems.