# Compact Design of Annular-Microstrip-Fed mmW Antenna Arrays
## 1 Introduction  
Millimeter wave (mmW) frequency band is shown to have much potential usage not only for the near future wireless communication system like 5G NR FR2, but also for the other upcoming signal coverage system based on the protocols of 802.11 ad (Wi-Gig) or 802.11 bd. According to previous research, different types of mmW antenna structures working at mmW frequency bands with novel structures or designing methods have been proposed before. The substrate integrated waveguide (SIW) technology, dual-element arrays, and extension structures have been used in designing the directional horn antenna working at 5G mmW bands [1]. Defected ground structure (DGS) [2], 1 \(\times\) 4 antipodal Vivaldi antenna (AVA) array structure [3], H-shaped and slot-loaded patch with a shorting pin structure [4] have also been proposed in patch antennas working at 5G mmW bands such as 28 GHz. Besides, the rectangular dielectric resonator antenna with circular patch obtains the radiation efficiency at 26 GHz and can be applied in higher frequency of 5G mmW from 24.25 GHz to 27.5 GHz with the optimization of the rectangular slot and cross slot [5]. New structures such as the superposition of identical metallic layers based on metasurface can attain the high gain of 27 dBi at 28 GHz [6]. In the frequency band of 60 GHz, some antennas are combined with IC technology. For example, the low-loss Low Temperature Co-Fired Ceramic (LTCC) has been utilized in wide-band 60 GHz chip antenna and it enables better radiation effectiveness compared with the patch antenna integrated into PCB [7]. A kind of virtual annular antenna has been proposed for use on CMOS IC technology with the achievement of a radiation efficiency of 90% and a gain of 1.43 dBi at about 60 GHz [8]. What is more, different kinds of antennas have been shownsuch as the inverted-F antenna and quasi-Yagi patch antenna which are working at mmW bands [9]. There are also plenty of designs based on microstrip antenna arrays such as the modified 2 \(\times\) 2-element and 2 \(\times\) 2-slot element 60 GHz planar array antenna [10; 11], microstrip grid array antennas [12; 13], microstrip antenna with comb line shape [14], and a microstrip patch antenna array which excites the frequency selective surface (FSS) with two elements [15], etc. Some of the above-mentioned designs are focusing on achieving the high gain. The microstrip grid array antennas can obtain the 14 dBi-highest-gain at 60 GHz and the impedance bandwidth from 57 GHz to 66 GHz [13]. The microstrip antenna with comb line shape can attain the high gain of 20.3 dBi [14]. Some other methods such as loading Artificial Magnetic Conductor (AMC), constructing composite metamaterial, and isotropic meta-surfaces have been proposed to improve gain [16; 17; 18]. However, in this paper, our goal is to achieve more homogeneous coverage with more isotropic radiation instead of high gain, while the articles which are focusing on isotropic or multi-directional radiation property are few. Facing the main issues of obtaining the characteristics of miniaturization, planarity, and integration of antennas and aiming at the potential applications in mmW indoor or outdoor propagation scenarios, a series of four novel microstrip antenna array designs based on different annular-microstrip feeding lines at around 60 GHz are proposed in this paper. In the process of designing the antenna structure to be more compact, the annular microstrip feeding lines which can connect patch units and various compact structures are proposed as the feeding network. On this basis, we improve the surface current distribution of all arrays to be more uniform in order to obtain the multi-directional radiation characteristic under linear polarization mode. Some of our preliminary studies have been already reported in the patents [19; 20; 21] and conference papers [22; 23]. In Section 2, the principle of our design and the two basic designs based on annular microstrip line with different forms of patch arrays will be proposed.