Dual-Polarized Radiating Elements Based on Electromagnetic Dipole Concept

Dual-polarized radiating element is a critical component in the Multi-functional Phased Array Radar (MPAR) system and has direct impact on the dual-polarized array pattern performance. Theoretically, an ideal radiating element consists of co-linear electric and magnetic dipoles, which ensures the orthogonality of co- and cross-pol E-fields in all spatial directions. Initial implementation of such dipole elements has been made by the industry, but much more work remains to be done to achieve a realistic, low-cost and well-performing engineering designs.

The proposed design is a simple circular ring structure that can act as magnetic loop antenna and achieve horizontally polarized omnidirectional pattern in the far field. To achieve this design goal, a uniform current distribution should be supported by the structure of the ring, which makes the 3D radiation pattern to approach the pattern of an ideal dipole. This is generally difficult to achieve for smaller ring antennas because of the high reactance, small radiation resistance, variation of scan impedance and mutual coupling between E and H elements.

Several techniques are used to address the challenge; these include probe and differential feed structures, stacked layers, as well as capacitive loading techniques. For example, one of the candidate designs consists of circular ring fed with a parallel strip feed line, which acts as an impedance transformer. The second example has circular ring fed by coax feed. The third one is a stacked circular ring structure which also uses coax feed. The design process is based on cavity theory model and for initial validations, spectral domain moment method is used. The characteristics of these radiating element designs have been studied with HFSS simulations. The simulations show that these structures result in uni-direction circular current distribution and the ongoing work is focused on improving all aspects of the antenna characteristics including radiation pattern, return loss and cross-polarization isolations at single element level. The latest design optimization results will be discussed and observed tradeoffs will be summarized.

To form a complete EM dipole array, an electric dipole is also being added in the center of the proposed magnetic ring dipole element. By minimizing mutual coupling and dipole structures, the new array radiating element is expected to have close to ideal radiation pattern with orthogonal and “matched” co-polarized and cross-polarized radiation fields. We are working towards fabricating such antenna elements, and testing them in a larger array test bed called Configurable Phased Array Demonstrator (CPAD) at the Radar Innovations Laboratory (RIL), University of Oklahoma.