The nature of dielectric permittivity electric control at microwaves

The control of dielectric permittivity (ε) by the bias electric field (E_b) at microwaves, and its application in a phase shifter has 40-year history (Gevorgian, S., IEEE Microwave Theor. Tech., vol.49, no.11, p.2117-23, 2001). However, only recently has ferroelectric thin film processing become good enough, and tuned microwave devices based on paraelectric films can now be competitive with the microwave ferrite and PIN diode. Nevertheless, there are still some problems with ferroelectric film microwave applications. Microwave tunable devices (like phase shifters or filters) need low loss dielectric material with permittivity ε(E) which can be controlled by an acceptable voltage. The main mechanisms of electric control of ε_ef are discussed. It is supposed that the most effective low loss is to use a "microwave dielectric-air gap" system which is controlled by a fast electrostrictive actuator.