Modeling, design, fabrication, and performance of bi-layer, mode 2, four-arm, cavity-backed, vertically fed, spiral antennas

Spiral antennas are used quite frequently for applications that require broadband operation with a high purity of circular polarization and consistent, azimuthally symmetric radiation patterns. Typically a complex feeding network is required to achieve the proper phase progression between adjacent spiral arms. This network must also be capable of performing well over the same or greater bandwidth as the antenna itself. The design of such modeforming network is challenging, expensive with the complexity depending upon the total number of spiral arms on the structure and the desired mode of operation. In this paper a simple, well performing, vertical coaxial feed able to generate a mode 2, conical pattern from a four-arm spiral antenna is introduced. The modeformer is much simpler than that introduced recently. A requirement for this novel feed configuration is a two-level metallization of the spiral structure. That is, two opposite spiral arms in one plane and the remaining set of opposite spiral arms in an offset plane. Numerical modeling of the proposed design is presented and validated with measured results.