Tunable Coplanar Waveguide

Abstract

The tuning methods of transmission line based on coplanar waveguide presented. Tuning is achieved by moving signal electrode of the line above the substrate or by moving additional dielectric plate above the electrodes of the line. Signal electrode lifting or presence of additional dielectric plate leads to redistribution of the electromagnetic field and therefore to changing of transmission line effective parameters, such as effective dielectric permittivity and characteristic impedance. Calculation of effective parameters was performed by using finite elements method. Proposed method of calculation is in good agreement with well-known analytical expression for coplanar waveguide based line without tuning. Experimental verification for tunable line based on coplanar waveguide was performed. The values of effective dielectric permittivity were extracted from measured S-parameters by solving an approximation task. The measured and calculated effective dielectric permittivity are in good agreement. Effective dielectric permittivity decreases with increasing of air gap between the signal electrode and dielectric substrate or between the additional dielectric plate and electrodes of the line. Required displacement are up to hundred micrometers, that allows to use MEMS or piezo actuators as control elements. The influence of electro physical and geometrical parameters on the tuning efficiency is shown. There are several ways for improving of the tuning efficiency. First is to decrease distance between the signal electrode and ground electrode, which is applicable for both methods. Another way for method with signal electrode lifting is to increase dielectric permittivity or height of the substrate. In method with moving dielectric plate above the electrodes, dielectric plate is the defining element. Therefore, increasing of dielectric permittivity and height of the dielectric plate or decreasing of dielectric permittivity or height of the substrate leads to the improving of the tuning efficiency and tuning range. The results of the paper allow to effectively design resonant elements and phase shifters based on coplanar waveguide line.Ref. 16, fig. 13, table 4.