Impedance measurements at W-band using an array of bismuth microbolometers

The voltage standing wave on a coplanar waveguide (CPW) is measured at W-band (75-110 GHz) using an array of bismuth microbolometers placed over the CPW. From this information, the impedance of the load at the end of the line can be determined in addition to the propagation and attenuation constant of the CPW.;The CPWs and transitions from the rectangular waveguide to the CPW are fabricated on the same semi-insulating gallium arsenide (S.I. GaAs) substrate to avoid problems in making a good electrical contact between the transition and CPW. The transition consists of a nonradiating slot on the GaAs substrate and a tapered ridge waveguide which makes contact with the center conductor of the CPW.;The microbolometers are spaced above the CPW using a thick layer of polyimide in order not to perturb the voltage standing wave. They sample the electric field along the line at twenty different points by a change of resistance caused by heating by the field. These points are then fitted to the equation for a voltage standing wave using a least squares fitting routine.;Design considerations and results are discussed for measurements on an open, short, offset short and resistive load. Even though the polyimide spacer layer affects the characteristics of the CPW, this measurement technique shows promise as a relatively inexpensive way of obtaining measurements above 100 GHz.