Design a Multistub Array Antenna at 28 GHz with Beam Switching Ability

. In this paper, a Millimeter Wave antenna is designed which has a compact, lightweight and planar confguration. Te frequency band is 27–29.5GHz as a candidate band for 5G/millimeter Wave (mmW) systems. Te antenna structure is similar to Quasi Yagi antenna which has driver, director, feeding part, and refector. Substrate Integrated Waveguide used for feeding part of single element antenna and Wilkinson power divider used for antenna array feeding network. Te proposed antenna has been simulated, fabricated, and measured (S11, E , and H pattern). Te simulation and measurement values showed good similarity. Te switched line phase shifter used to consider the beam steering (rotation) ability of the designed antenna which is important in (mmW) systems such as RADARs and mobile handsets. To evaluate this ability, for 0 ° , 30 ° , 45 ° , and 90 ° phase diferences, the beam steering angle ( θ ) simulated and also for − 90 ° and 90 ° implemented. Te results showed that the efciency, S11, and E patterns in the rotated beam is suitable and without degradation in antenna operation. To simulate and evaluate the designed antenna HFSS and CST Software are used.


Introduction
Today the mobile communication service is an essential part of human life. Many researchers all of the world work on electromagnetic felds and their applications. Because of increasing demands for higher data rates (such as wireless broadband connections, massive machine type communications, and highly reliable networks), the research studies on ffth-generation (5G) mobile communication systems have started. Te standardization of this technology is coming in 2020s [1,2]. Compared with previous generations, 5G mobile communication networks will use broader frequency bandwidths. One of the candidate frequencies for this band is 28 GHz . Te other frequency bands can be seen in [3]. In this paper, 28 GHz has been selected because of fabrication considerations. Many types of antenna are used in Millimeter wave band such as Fermi, Vivaldi, and Quasi Yagi [4][5][6].
Quasi-Yagi is an interesting antenna because of its benefts, such as simple structure, lightweight, and easy forming array. So, in this paper, the Quasi-Yagi is selected as base of the proposed antenna [7].
Single element antenna has a small gain to prevail the path loss at mmW band. According to Friis equation, by increasing the frequency, the signal wavelength becomes shorter and consequently the free-space path loss becomes higher [8].
Te array antenna can solve this problem by increasing the gain. Also, it has higher directivity, reduces minor lobes, and shows better performance rather than a single element [9].
Te structure of the proposed antenna is based on Quasi-Yagi, which consists of dipole elements, directors, refectors, and feeding part.
In some 5G devices and systems, it needed to change the main beam direction as the array gain compensates for the small antenna aperture and the large noise bandwidth. Beam steering is a technique for changing the direction of the main beam of a radiation pattern. Also, it is a way to control the feld distribution [10,11]. Tis technique is very important in mmW or 5G systems like cellphones and RADARs to have a good quality in communication.
In the previous author's job [12], three single element antennas have been designed, simulated, and fabricated. Also, in [13] three array antenna have been designed, simulated and one of them fabricated. All of them had good characteristics such as enough gain, end fre pattern, and good S 11 to use in 5G mobile handsets .
In this paper, according to one of the proposed antenna structure in [12] a phased array antenna is designed and the beam steering (rotating) property of the designed array antenna is simulated and implemented in the software. Te array antenna consists of eight single elements. Te feeding part of the array is Wilkinson power divider (WPD) because of its benefts. To evaluate the WPD performance three parameters should be checked: refection coefcients, coupling, and isolation between ports [14] which are considered in [13]. Te switched line phase shifter is used to create the required phase diference.
In the following sections, the design and simulation process is described. According to the obtained results of the proposed antenna (i.e., compact size, beam steering, and other good characteristics) it can be considered as good candidates for mmW/5G devices.

Antenna Design
As mentioned in introduction part in this paper Quasi Yagi structure is used because of its beneft such as its lightweight, planar structure, and low cost which is an ideal structure to apply in handheld devices. In consonance with Quasi Yagi antenna structure, the proposed antenna has a feeding part, director, driver, and refector. To design the feeding part of single element antenna, SIW (Substrate Integrated waveguide) technology is used. SIW is a compact, low-loss, and cost efectiveness structure. Te substrate is Roger RT/duroid 5880 with ε r � 2.2 and h sub � 0.38 mm. Te phase diference between the up and down plane of the substrate.
is 180°and a 50 Ω lumped port is used for the port excitation. Based on the antenna feed system in Figure 1, the TE 10 mode is excited. To calculate the SIW parameters and transition part of the feeding network equations (1)-(4) are used.
where w is the width of waveguide, w t is the width of taper, Lt is the length of taper, Ls is the length of SIW part, s is the distance between two adjacent vias, d is the diameter of via, and L T is the length of taper. So for TE 10 mode, these parameters are equal to the following variables. Ls � 3.5 mm,W T � 3.2 mm, L � 3.5 mm, w � 5.5 mm,L T � 1.6 mm, d � 0.6 mm, s � 1.2 mm. More detail of SIW calculations are in [15,16].
Single element antenna does not have enough gain to overcome the pass loss. Tere are some methods to increase the gain of antenna, for example using of the array antenna. Tus the array antenna is used. For the radiation part the log periodic antenna structure is applied which is endfre, has good gain, and smaller size rather than other structures such as Fermi, Cavity backed, and Vivaldi. For the log periodic antenna, the lengths (L) and widths (w) of adjacent elements are calculated as follows [17]: where λ eff is the efective wavelength, h is the thickness of substrate and w is the width of the longest element. Te number of elements (N) is determined as follows:  International Journal of Antennas and Propagation where σ is the spacing factor, α is the apex angle, τ is the geometry constant, and B AR is the bandwidth of the active region. For 7 dB gain in Figure 2, scale factor (τ) and Relative spacing (σ) are 0.78 and 0.131, respectively. By using of these parameters in equations 5 to 11 the other parameters are calculated. So the number of elements is N � 3. Figure 3 shows the structure of the antenna.
Te antenna array is consisting of eight single elements and an eight way Wilkinson Power Divider (WPD) as a feeding network as shown in Figure 3. In two-way WPD, the isolation resistor is 2Z 0 and the impedance of λ/4 is √2Z 0 . For equal WPD the Z 0 � 50 Ω, the impedance of λ/4 is √2Z 0 � 70.7 Ω and isolation resistor is 2Z 0 � 100 Ω. To design WPD at 28 GHz the TXline calculator is used. Te values for WPD are obtained as: W 50Ω � 1.18 mm, W 70.7Ω � 0.65 mm, and L 70.7Ω � 1.97 mm. Te isolation resistor is 100 Ω (size is 1 × 0.5 mm 2 ) from 0402 SMD family. For eight ways of WPD, three stages of two ways WPD is needed [13]. Te antenna array with feeding network (WPD) is shown in Figure 3. In an antenna array, the distance between elements is about 0.5λ to control mutual coupling.
Te fabricated prototypes of the proposed antennas are shown in Figure 4.
Measurement and simulation results of antenna array (S 11 , Normalized E and H patterns) are shown in Figure 5.
Tere is some diference between measured and simulated results because of SMK losses, fabrication faults, resistors etc. Te overall antenna size of a single element and array are 6.5 mm × 18 mm × 0.38 mm and 52 mm × 29.96 mm × 0.38 mm, respectively, which are suitable for hand held devices.
Te maximum gain of a single element is 8 dBi and array is 14.6 dBi respectively. Te E and H patterns are end fre and S 11 coefcient values are under −10 dB. More gains can be reached by increasing the number of directors, but it increases the size of the antenna. All of these characteristics show that the proposed antenna is suitable for mmW/5G applications.
In [13] the Specifc Absorption Rate of the antenna was evaluated. A three layers human head model situated in a 5 mm distance from the antenna. Te power of the antenna was 15 and 20 dBm according to 5G standard values. Te results showed that all of the SAR values are in standard range (i.e., 1.6 W/Kg according to FCC standard). International Journal of Antennas and Propagation

Introduction.
To better covering the space losses in the communication process the beam steering methods can be used by creating the phase diference. In this part the rules of designing phased array antenna is presented. Figure 6 shows a linear phased array antenna with K elements and equally spaced by a distance d. Te direction of a wave is described by the angle θ between rays and the array normal. According to Figure 6 the array radiation pattern is written by (12).
where S e is the element radiation pattern, S a is array factor, ψ i is the phase diference, K is the number of elements, and K 0 is the free-space wave number. All the coefcients a i form an amplitude taper. Let us assume a  International Journal of Antennas and Propagation uniform, normalized amplitude distribution, so for K elements: So only S a is considered. If a linear phase taper is chosen: Since −90 < θ 0 < 90, the array factor may be written as follows: For the linear phased array antenna situation, maximum array factor is θ � θ 0 from (16).
So by choosing θ 0 in ψ i � k 0 (K − i)d sin(θ 0 ) the array factor will have its maximum at θ 0 .
A phased array antenna is a complicated structure in designing and fabrication thus, reduction of complexity, cost, and size is very important to use in more devices specially handhelds. Create several subarrays within the antenna which each group is controlled by a phase-shifter reduces the number of phase-shifters and the cost. Each group is seen as a single element to other groups (Figure 7).
In this paper to reach enough gain at mmW frequency bands (to overcome the path losses), eight elements are used which are arranged linearly with about 0.5 λ distance between them. In Figure 8-show the antenna array and subarrays. In this case the spacing between two elements is around 1.1λ because two elements are combined and assumed as one element according to Figures 7, 8(a), and 8(b).

Phase Diference.
Tere are several phase shifters like Switched Line Phase Shifters, Loaded Line phase shifters, High-Pass/Low-Pass phase shifter, and Quadrature phase shifter. In this paper, Switched Line Phase Shifters (Figure 9) is used because of the simplest confguration and it provides a good insight into the concept behind phase shifters. Also, this method has certain advantages over other phase shifters as φ is a linear function of frequency and independent of time and temperature changes since it solely depends on the length of the transmission lines [19].
By creating a time delay diference between two paths (changing the length of L with respect to L 0 ) the desired phase shift can reach in the transmission line. All of the diodes are assumed as an ideal diode.
Reference line is L 0 and the delay line length is L 0 +L. Diferent path lengths which result in a phase delay (φ) [18,[20][21][22]. As mentioned above, to design the beam switching network of the array antenna, the switched line phase shifter is used. For a desired beam direction (θ) in Figure 8(b) it is necessary to create the related phase shift by using equations (17)- (20).

Feeding Network Feeding Network Feeding Network
Feeding Network Figure 7: Grouping antennas into a sub-array [18].
International Journal of Antennas and Propagation

Simulation
By the calculated parameters from Table 1 antenna array is simulated in Figure 10. Tis fgure shows the results of the simulation for 30°, 45°, and 90°phase diferences and corresponding beam direction (θ). Figure 10(a) shows the phase diference 30°(θ � 4°), and Figures 10(d), 10(g), and 10(j) are the results of normalized E-pattern with respect to 0°phase diference, S 11 and efciency of this state respectively. Figure 9(b) shows the phase diference 45°(θ � 7°) and Figures 10(e), 10(h), and 10(k) are the results of normalized E-pattern with respect to 0°phase diference, S 11 and efciency of this state respectively. Figure 10(c) shows phase diference 90°(θ � 11°), and Figures 10(f ), 10(i), and 10(l) are the results of normalized E-pattern with respect to 0°p hase diference, S 11 and Efciency of this state respectively. As it can be seen the results of simulation are in agreement with the values from calculations. As the results show, in all cases the efciency, S 11 and pattern are suitable and without any degradation in antenna performance. Figure 11 presents the beam steering at 0°, 30°, 45°, and 90°phase diferences in one chart.  Figure 9: Basic schematic of phase shifter operation using diodes [19]. International Journal of Antennas and Propagation To compare the simulated and calculated "θ", Table 2 is provided. From Table 2, the calculated and measured θ are very similar for various phase shifts.

Implementation of the Beam Switching
Because of similarity, in this part, only beam switching is designed for θ � 11°and 11°. As Figure 11 shows switched line phase shifter technique with diodes is implemented. Phase diference is producing by changing in the length L 0 to L 3 which is calculated in Table 3 and using "on" and "of" states of diodes. From equations (13)-(17) for θ � 11°and θ � −11°the required phase shift is 90°and −90°, respectively.
In Figure 12(a) the overall structure of the proposed antenna is shown. Te diodes used to switch and select the     International Journal of Antennas and Propagation paths. In Figure 12 when the green diodes are on "on" state and blue diodes are "of," θ � −11°and When the green diodes are on "of" state and blue diodes are "on," θ � 11°. From Figure 12 the rotation of the patterns are observed clearly. By the changing the phase diference angles and associated lengths, favorite beam steering angle (θ) can reach. But increase in the number of switches leads to increase the complexity of the design. Tus it is better to use from other beam steering methods [23][24][25]. Table 4 presents a comparison between some beam steering methods in various frequencies. As it can be seen from the table, the size of this work is the smallest. Te structure of the proposed antenna. is planar, easy to fabrication, and integrates with other devices. In some methods [26][27][28][29] the. ferrite used which can increase the losses and size of the antenna. Te small size of the antenna makes it suitable to apply in handheld devices such as cellphones, tablets, and laptops. However, the methods to create beam steering in [26][27][28][29] do not have any mechanical switches (for example microelectro-mechanical switches (MEMS), or pin diode, and varactor) which can add some losses to the system. In [30][31][32] the antennas have good efciency and almost enough gain but the size of the proposed antenna in this work is smaller. Also, the proposed antenna has the required gain for 5G application (At least is 12 dB).

Conclusions
An end fre antenna which is planar, with compact size, lightweight, simple structure, and high gain is proposed, simulated, and fabricated. Te feeding part is the Wilkinson power divider which provides an ability to beam switching by using switched line phase shifter technique. Te switching action is done by "on" and "of" states of diodes which allow selecting the path of current in the transmission lines. Te simulation results for the phased array antenna show that when beam switching and beam rotation is done the S 11 coefcients, efciency, and pattern of the antenna are acceptable. Also, the operation of the antenna is not degraded. By choosing the phase shift degree, beam direction can be changed. Tus, this type of antenna is a good candidate to mmW/5G (27-29.5 GHz) application and systems especially for handheld devices.

Data Availability
Te data used to support the study are included in the paper.