The slotted line technique of impedance measurement

Abstract

Even though it has been largely replaced by other methods, no text on microwave metrology would be complete without at least a brief treatment of the 'slotted line'. The basic form of the measuring instrument includes a section of the appropriate transmission line. In any case, the perturbation due to the slot can be minimised by keeping it narrow and parallel to the current paths. The other source of perturbation is due to the probe itself. In general this is minimised by the use of a sensitive detection system and by keeping the depth of penetration, or coupling, to a minimum. From the viewpoint of careful metrology, its sources of error include impedance discontinuity introduced by the slot, inability to maintain uniform probe coupling to the fields due to imperfections in the (nonideal) transport mechanism, and violation of the 'uniformity requirement', and distortion of field patterns due to probe 'loading'. Unfortunately, however, the success of this scheme is contingent on a known value for the 'shunt admittance', or probe coupling, and this is not easily determined.