A tuneable probe for noncontacting microwave measurements

This paper describes an electrically tuneable loop coupler and its use for noncontacting measurements in microstrip circuits. Earlier loop coupler probes have the disadvantage of needing mechanical tuning to give sufficient directivity for scalar measurements on substrates having different dielectric constants. This problem is overcome through the use of a PIN diode as an adjustable termination on one of the coupler ports. The probe is evaluated for microstrip substrates with an ε_r ranging from 2:33 to 10 with transmission line characteristic impedances ranging from 30Ω to 80Ω. After tuning, the coupler shows a directivity better than 20 dB up to 3:8 GHz for ε_r = 2:33 and up to 1 GHz for ε_r = 10.