The present invention relates to a device including a capacitor system for varying the impedance of a section of a coplanar waveguide. 
German Published Patent Application No.  100 37 385 describes a device including a capacitor for varying the impedance of a section of a coplanar waveguide, in which the capacitance of the capacitor is variable and a metal bridge bridges the interrupted signal line of the waveguide over a predetermined length and is mechanically deformable as a function of an electric voltage which is applied between the metal bridge and a connection which electroconductively connects the ground lines of the waveguide, thus making it possible to initiate a switching operation.  In the off-state (the metal bridge is down), a large part of the power is reflected.  In the on-state (the metal bridge is up), a large part of the power is transmitted. 
The device according to the present invention may provide the advantage that in the on state, the insertion loss is reduced and that, at the same time, the insulation of the switch is increased in the off state.  This may allow advantageous changes in the configuration of the device such as a small clearance between the bridge and the counter-electrode or a small magnetic-force stress on the dielectric.  Moreover, by the additional connection between the ground lines of the waveguide, the attracting area and thereby the force pulling the bridge downward are increased as a result of which the switching voltage is reduced. 
Moreover, it may be an advantage that the first, the second, and the third connections are metallic connections.  In this manner, all material-specific and process-engineering advantages of using metals as electrically conductive connections are used according to the present invention. 
It may also be advantageous that the third connection is mechanically deformable in such a manner that a first clearance between the first connection and the third connection as well as a second clearance between the second connection and the third connection are variable at least in a partial area of the third connection.  In this manner, a capacitor system is produced with a simple arrangement whose total capacitance is variable. 
Another advantage may be that the capacitance of the capacitor system is able to be changed by an electrostatic force between the first connection and the second connection on one side and the third connection on the other side.  Due to this, two switching states of the device according to the present invention may be provided with a simple arrangement, ensuring a reliable and fast switching capability of the device.  Moreover, the switching state of the device is unambiguously defined in this manner at all times.  It is also beneficial that the capacitor system exhibits a first defined total capacitance and a second defined total capacitance as a function of a predetermined electric voltage between the first connection and the second connection on one side and the third connection on the other side.  Due to this, it is possible to determine the operating frequency within wide limits independently of the spacing of the ground lines of the coplanar waveguide by the dimensioning, e.g., of the first, second and third electrically conductive connections and the dielectric layer between the first and third connections and between the second and third connections, respectively.  The insertion loss may also be adjusted in this manner. 
Another advantage may be that, in the case that the capacitor system exhibits the first total capacitance, the first connection forms a first inductance in series with a first partial capacitance of the capacitor system between the signal line and the ground lines, and that, in the same case, the second connection forms a second inductance in series with a second partial capacitance of the capacitor system between the signal line and the ground lines, the common impedance of the first partial capacitance and the first inductance as well as the common impedance of the second partial capacitance and the second inductance corresponding to the ohmic resistance thereof at an operating frequency.  In this manner, it is possible to achieve a high insulation, i.e., a high reflection coefficient while the short-circuit switch is switched off. 
Another advantage may be that the first connection and the second connection have a third clearance along the waveguide, the third clearance approximately corresponding to the equivalent of one quarter of the wavelength at an operating frequency.  In this manner, the reflections at the capacitances formed by the counter-electrodes, i.e., the first and second connections, with the bridge, i.e., the third connection, compensate each other in the switched-on state, that is, in the case that the capacitor system exhibits the second total capacitance.  In this manner, the adaptation of the switch structure is considerably improved, that is, the insertion loss is reduced. 
It is also beneficial that an operating frequency of approximately 77 GHz or approximately 24 GHz is provided.  Due to this, the device according to the present invention is suitable for ACC applications (Adaptive Cruise Control) or for SRR applications (Short Range Radar). 
It may be a further advantage that the predetermined length is provided such that reflections at a transition between the signal line and the second connection compensate each other.  This results in an improvement of the insertion loss of the switch and thereby of the adaptation in the switched-on state. 
Another advantage may be that the ground lines of the waveguide are connected by more than two connections over the predetermined length.  In this manner, it is possible, on one hand, to further reduce the switching voltage; on the other hand, to once more increase the insulation in the switched-off state, and to reduce the insertion loss in the switched-on state. 
It is also expedient that the number of connections connecting the ground lines of the waveguide is odd.  In this manner, the switching voltage may be reduced once more since it is possible to provide a connection connecting the ground lines in the middle of the length where the greatest deflection may be achieved with a given force or the smallest force is required for a predefined deflection. 
An example embodiment of the present invention is depicted in the drawing and will be explained in greater detail in the following description.