1. Field of the Invention
This invention relates to the field of microwave energy transmission. More particularly, the invention relates to microwave circulators for use in planar microwave transmission circuits.
2. Description of the Prior Art
A microwave circulator is a waveguide component which is used to direct the flow of microwave energy at a waveguide junction. More specifically, a microwave circulator is a multiterminal waveguide coupler in which microwave energy is transmitted in a particular direction from one terminal to the next. Such circulators employ a gyromagnetic material which phase shifts microwave energy a predetermined amount depending on its direction of propagation. Microwave circulators typically comprise a ferrite material and external biasing magnets. As is well known, the induced phase shifts for energy passing through the ferrite and, therefore, the direction of energy flow through the circulator is dependent upon the alignment of magnetic moments within the ferrite. These magnetic moments are aligned in a particular direction by the external biasing magnets.
One type of circuit which uses ferrite circulators extensively is a microstrip circuit. A microstrip is a form of waveguide which is essentially an unbalanced transmission line characterized by a planar geometry. A common form of microstrip has a rigid dielectric substrate with two parallel surfaces. A conductive ground plane is bonded to one parallel surface and a narrow flat conductor is bonded to the other parallel surface. Various types of microstrip circulators have been designed for use in coupling several microstrips to each other. A more detailed discussion of a prior art microstrip circulator appears in an article by two of the present inventors, R. A. Stern and R. W. Babbitt, entitled "Millimeter-Wave Microstrip `Drop-In` Circulators," Microwave Journal, Apr. 1989, pp. 137-139.
Currently, microstrip circulators, often called planar circulators, use biasing permanent magnets which are placed outside the microstrip geometry. Although such devices have served the purpose, they have not proved entirely satisfactory because the relatively bulky biasing magnetic structures are not fully compatible with the planar technology of microstrips. As such, in many applications, the packaging densities of microstrip circuits have been severely limited by the relatively large size of the circulator biasing magnets. Those concerned with the development of electronic military systems, such as smart munitions, have long recognized the need for ways to increase the packaging densities of the electronic hardware, including microstrip circuitry, used therein. The present invention fulfills this need.