# Compact Design of Annular-Microstrip-Fed mmW Antenna Arrays
## 4 Conclusions  
In this paper, aiming at the wider employment of the mmW frequency band in the future, we propose four novel structures of microstrip antenna array working at 60-GHz waveband in total. Arrays are all composed of outer annular microstrip line and varying quantities and shapes of patch array branches. The quantity of the branches can be designed as 2\(\sim\)6 at designer's option. All the feeding units are contained in the space inside the outer annular microstrip line, in order to obtain a more compact structure. The former two designs are the basic designs which show our design principle. The latter two designs are proposed to obtain the multi-directional radiation feature with the methods of dual annular-microstrip feeding lines and central feeding mode. After simulating and optimizing, we finally achieve the goal of miniaturization of antenna and maintaining symmetrical high-gain radiation in multiple directions under the condition of good impedance matching. Our study can make up for the deficiency of multi-direction characteristics in previous or current research on the mmW microstrip antenna array. In addition, we carried out the physical production and experimental verification of the first basic structure. We obtained the results of the minimum reflection coefficient of \(-\)14.55 dB at 60.65 GHz with the bandwidth of 540 MHz, which achieve a good match with the simulation results, so as to prove the correctness of our design. We anticipate that the design of this novel category of structures can provide potential guidelines for the future mmW coverage antenna design or practice.  
Conceptualization, S.P. and S.-D.L.; methodology, S.P.; software, C.W. and S.-D.L.; validation, S.P.; formal analysis: S.-D.L.; investigation, S.-D.L.; resources, S.-D.L., S.P. and C.W.; data curation: S.-D.L. and H.-Y.R.; writing--original draft preparation, S.-D.L.; writing--review and editing, S.P. and H.-Y.R.; visualization, S.-D.L.; supervision, S.P. and C.W.; project administration, S.P. All authors have read and agreed to the published version of the manuscript.  
This research was supported in part by the Electrostatic Research Foundation of Liu Shanghe Academicians and Experts Workstation, Beijing Orient Institute of Measurement and Test (611704011), Hubei Key Laboratory of Roadway Bridge and Structure Engineering, Wuhan University of Technology (DQJ)201504), the Fundamental Research Funds for the Central Universities  
Figure 20: Radiation pattern of bridged annular-microstrip-fed antenna array (**a**) \(x\)-\(z\) plane, (**b**) \(y\)-\(z\) plane, (**c**) \(x\)-\(y\) plane.  
(191214001), and partly by the National Innovation & Entrepreneurship Training Program for College Students (202010497046).  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors thank the reviewers and editors for their careful works and thoughtful suggestions that help us improve this paper substantially, the assistance in measurement with Open Laboratory of Rohde & Schwarz China, Shanghai Branch from the Fei Xun, and also in memory of the late [PERSON] for her funding support.  
**Conflicts of Interest:** The authors declare no conflict of interest.