Mirhamed Mirmozafari from the University of Oklahoma, USA, talks to Electronics Letters about his paper ‘A Highly Isolated Crossed Dipole Antenna with Matched Copolar Beams’, page 470. Mirhamed Mirmozafari My research interests can be broadly described as antenna theory and computational electromagnetics. Presently, I am collaborating with a group of professors and antenna engineers at the Advanced Radar Research Center (ARRC) of the University of Oklahoma. Our research includes developing planar and cylindrical radar structures. A major challenge that we face is incorporating dual-linear polarisation into phased array antennas with high isolation between their corresponding ports and patterns. In addition, we are working on simple and inexpensive solutions that reduce the fabrication costs and complexity of the assembly. Weather observation and air surveillance share the same principle of detecting electromagnetic wave scattering. Therefore, it would be more cost-effective and easier to operate if a single radar network were used in combined missions. This was the basis for multifunction phased array radar (MPAR) that was initiated by the National Oceanic Atmospheric Administration and the Federal Aviation Agency. Research activities for MPAR have begun in industry, academia, and government laboratories to identify technical challenges, mitigate risk and develop technologies. After realising the intrinsic limitations of planar array radar in making accurate polarimetric measurements, my advisor, Dr. Guifu Zhang, and his colleagues and students are developing cylindrical polarimetric phased array radar (CPPAR) at the ARRC. In this Letter, we report a port-to-port isolation that is higher than 56 dB, which to the best of our knowledge is the highest measured isolation reported in the literature to date. It is accompanied by 40 dB isolation between patterns of the antenna. These parameters indicate the ability of the proposed crossed dipole to maintain two independent polarisations. Using two identical geometries for the two polarisations, we achieved a pair of matched co-polar patterns. This, in turn, allows for accurate weather measurements and enables polarimetric phased array radar to distinguish different hydrometeor types, e.g., rain vs melting snow or hail. Crossed dipoles have been widely reported in the literature. However, what makes our proposed antenna significant is the simplicity and the symmetry of the structure. The radiation elements are simple half-wavelength dipoles. High polarisation purity is achieved owing to the symmetry of the radiation elements that are perfectly separated below the principal ground plane. This separation is accomplished by using a stripline structure for each balun. Having considered the stripline structure with two sub-ground planes, one can easily understand that the fabrication process was our major technical challenge. It is worth mentioning that the proposed crossed dipole, though non-planar, is fabricated with inexpensive printed circuit board technology. This antenna was designed for the next generation of cylindrical polarimetric phased array radar for long-range weather measurements and accurate target discrimination. The high polarisation purity achieved by this antenna satisfies the 0.2 dB differential reflectivity required by weather applications. An important objective in designing such a crossed dipole was the elimination of the thick substrate over the large ground plane resulting in a better match between copolar patterns. We plan to develop the linear and the reconfigurable 2D array of the proposed crossed dipole. The linear array will be situated in a large cylindrical array and its features will be characterised. We also plan to examine other types of the balun (e.g. Marchand balun) in order to simplify the geometry. As part of our ongoing research, we are trying to implement similar geometries with 3D printing technology. This opens the door to the next generation of easy-to-fabricate antennas. We, at the Radar Innovations Laboratory of the University of Oklahoma, have discussed the feasibility of cylindrical and planar radar for multi-mission applications and addressed their related challenges. In particular, I have proposed a solution for a phased array antenna. This work is being continued with the development of new technologies with higher performance. Our future objective will be an antenna with high polarisation purity in and out of the principal planes. This would benefit the planar array antennas by providing a high cross-polar isolation when the beam is directed off-axis.
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May 6, 2018
Aliakbar Dastranj 
Open Access