A Microwave Impedance Bridge

A six-arm waveguide structure is described which is a true microwave equivalent of a Wheatstone bridge. A theoretical analysis of the equivalent circuit of the device has been made, using the symmetry properties of the structure. The resulting relation among the admittances of the various arms is exactly that of a Wheat-stone bridge with shunting susceptances across each pair of terminals. A device of this sort for use at 10 cm has been built and tested, and was found to behave as predicted. With this bridge it is possible to measure any impedance to about the same accuracy as with a standing-wave detector. A valuable feature of this instrument is that the standard impedances required are three variable reactances (movable shorting plungers) and a Z0termination. The data are obtained in the form of three lengths, the positions of the movable shorts. Since the device is the complete equivalent of a Wheatstone bridge, it can also be used as a four-terminal lattice section in filter design or in any other related application requiring the microwave equivalent of a lattice section. This allows all the greater flexibility which lattice sections have, as compared to tee or pi sections commonly used in microwave work.