Three coplanar-stripline circuits have been designed to perform coupling and filtering functions in microwave systems. These circuits could be incorporated into feed networks for microwave antennas, for example. One of the circuits comprises two back-to-back coplanar-strip-to-microstrip transitions. The other two circuits are coplanar-stripline band-stop filters. The principal advantages of these circuits over others designed to perform the same functions are compactness and suitability for integration into monolithic microwave integrated circuits.
"Coplanar stripline" ("CPS") denotes a transmission line that comprises two adjacent conductive strips on one face of a dielectric substrate. "Microstrip" denotes a transmission line that comprises a conductive strip on one face of a dielectric substrate and a patch much broader than the strip (a ground plane) covering all or at least a large portion of the opposite face of the substrate.
The back-to-back CPS-to-microstrip transitions (see Figure 1) serve to couple power from a source to a load; e.g., from a transmitter to an antenna. Proceeding inward from each end of this circuit, one of the conductive strips of the CPS terminates in a radial stub, while the other strip is extended and narrowed to form the microstrip. The radial stub exhibits a resonance, the frequency of which depends on the radius R and angle f; at resonance, there is a radio-frequency short circuit between the strip and the ground plane. The characteristic impedances of the CPS and stripline portions of the circuit are matched by suitable choice of the widths of the conductive strips.
In the CPS portion of the circuit, the electric field is oriented across the gap between the strips and roughly parallel to the faces of the dielectric substrate. In the microstrip portion, the electric field is approximately perpendicular to the faces of the substrate. The angle θ of the edge of the ground plane provides for rotation of the electric field between the CPS and stripline orientations.
The two band-stop filters are of the spur-slot and spur-strip types (see Figure 2). The spur-slot is a resonant structure within one of two strips; it is convenient to use a spur-slot when W is large and S is small. At its resonance, the spur-slot can be modeled as a quarter-wavelength short-circuit stub in series with the transmission line. At resonance, the spur-slot thus prevents the flow of power to the load. The spur-strip is a resonant structure between two strips; it is convenient to use a spur-strip when W is small and S is large. At its resonance, the spur-strip can be modeled as a quarter-wavelength open-circuit stub in parallel with the transmission line. Several spur-slots or spur-strips can be incorporated at intervals of a half wavelength along a transmission line to enhance the band-stop performance.
This work was done by Rainee N. Simons of NYMA, Inc., for Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components and Circuits category, or circle no. 156 on the TSP Order card in this issue to receive a copy by mail ($5 charge).
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