# A Novel High Gain Monopole Antenna Array for 60 GHz Millimeter-Wave Communications
## 1 Introduction  
The rapid changes in wireless communication require a rapid increase in system capacity. This rapid growth and demand for higher data rates were the key factor in the development of the 5 th generation (5G) communication system. The 5G is far superior to previous generations in terms of the extremely high data rate. The main advantage of 5G is providing higher data rates and a more coverage range than those provided by the former generations [1; 2; 3]. Now, researchers' attention is on the 60 GHz band with over 7 GHz of the unlicensed spectrum; this makes the band qualify for high-data-rate, large spectral availability, and more security. In addition, the short-wavelength at 60 GHz allows for the design of highly compact elements with small sizes, which could not be achieved in the past on a chip at other frequencies. 5G wireless systems operate in the mmWave bands of 28, 38, and 60 GHz. The U.S Federal communications commission (FCC) has allocated the spectrum of 57-64 GHz unlicensed band as 60 GHz industrial, scientific and medical (ISM) band. WiGig Institute of Electrical and Electronics Engineers (IEEE 802.11 ad standard), which is the extended version of Wi-Fi, provides access to the 60 GHz band. Due to the advancements in wireless technologies, millimeter-wave (mmWave) microstrip patch antennas and microstrip patch antenna arrays are an essential part of the future. Microstrip antenna arrays possess various advantages, such as ease of fabrication and construction of the array [4]. The array elements can be etched together with the feeding network, forming a unifiedstructure, thus creating a very compact and low-cost design. However, the fabrication of the feeding network in the mmWave frequencies exhibits a lot of challenges during its implementation [5; 6; 7; 8; 9]. These challenges are caused mainly by the increase in microstrip line losses at mmWave frequencies. One cause for these losses is the conductor, dielectric, and radiation losses. At frequencies ranging from 30 to 60 GHz, conductor losses are around 0.15 to 0.2 dB per wavelength, and dielectric losses are around 0.045 dB per wavelength for a 50 \(\Omega\) line on a substrate with \(\varepsilon_{r}=3.55\)[10]. The other major reason for the increased losses is the radiation losses caused by the addition of T-junctions and other discontinuities in the feed network in an attempt to achieve better notching [11]. One way to decrease the radiation losses caused by the added discontinuities is the use of a multiport network model. The minimization of losses in the microstrip feed network will yield a microstrip antenna array with higher efficiency. Recently, there has been an increased interest in the use of a defected ground structure (DGS) to improve the performance of the microstrip antennas, which can be achieved by the etching of a simple shape defect in the ground plane of the microstrip antenna. The DGS may be compared to the equivalent circuit consisting of an inductance and a capacitance in parallel [12; 13; 14; 15; 16]. The monopole antenna has many advantages, such as small size, low profile, easy manufacturing, and integration, as well as low cost. These advantages make it an ideal choice for the 60 GHz band application [17]; however, the monopoles' major disadvantages are low gain and small bandwidth. The choice of feeding technique is one of the factors that can be used to increase the impedance bandwidth. However, the choice of feeding technique depends on its ease of fabrication, cost, hardware implementation, spurious radiation, as well as its impedance matching [18; 19]. Recently, many researches provided several designs for 5G applications. In [20], a compact dual-frequency (38/60 GHz) microstrip patch antenna is proposed. The work of [21] proposes an antenna with DGS on the ground plane to compact the size of the microstrip antenna without the degradation in performance. This antenna is realized using an array consisting of 2 \(\times\) 2 mm-wave microstrip antenna array usable in 5G applications in [22].