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

Authors: A. Lak
Journal: International Journal of Antennas and Propagation
 (2022)

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## Abstract

In this paper, a Millimeter Wave antenna is designed which has a compact, lightweight and planar configuration. The frequency band is 27–29.5 GHz as a candidate band for 5G/millimeter Wave (mmW) systems. The antenna structure is similar to Quasi Yagi antenna which has driver, director, feeding part, and reflector. Substrate Integrated Waveguide used for feeding part of single element antenna and Wilkinson power divider used for antenna array feeding network. The proposed antenna has been simulated, fabricated, and measured (S11,E, and H pattern). The simulation and measurement values showed good similarity. The 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 differences, the beam steering angle (θ) simulated and also for −90° and 90° implemented. The results showed that the efficiency, 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.


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## Body

## 1. Introduction

Today the mobile communication service is an essential part of human life. Many researchers all of the world work on electromagnetic fields 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 fifth-generation (5G) mobile communication systems have started. The 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 (27–29.5 GHz). The 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 [4–6].Quasi-Yagi is an interesting antenna because of its benefits, 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].The 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].The structure of the proposed antenna is based on Quasi-Yagi, which consists of dipole elements, directors, reflectors, 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 field distribution [10, 11]. This 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 fire pattern, and good S11 to use in 5G mobile handsets (27–29.5 GHz).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. The array antenna consists of eight single elements. The feeding part of the array is Wilkinson power divider (WPD) because of its benefits. To evaluate the WPD performance three parameters should be checked: refection coefficients, coupling, and isolation between ports [14] which are considered in [13]. The switched line phase shifter is used to create the required phase difference.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.


## 2. Antenna Design

As mentioned in introduction part in this paper Quasi Yagi structure is used because of its benefit 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 reflector. To design the feeding part of single element antenna, SIW (Substrate Integrated waveguide) technology is used. SIW is a compact, low-loss, and cost effectiveness structure. The substrate is Roger RT/duroid 5880 withεr=2.2 and hsub = 0.38 mm. The phase difference 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 Figure1, the TE10 mode is excited. To calculate the SIW parameters and transition part of the feeding network equations (1)–(4) are used.(1)d<s<2d,(2)w=c2fcεr,(3)WT≈0.4w,(4)λg/2<LT<λg,where w is the width of waveguide, wt 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 LT is the length of taper. So for TE10 mode, these parameters are equal to the following variables.Figure 1
The structure of the proposed antenna.Ls = 3.5 mm,WT = 3.2 mm, L = 3.5 mm, w = 5.5 mm,LT = 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. There are some methods to increase the gain of antenna, for example using of the array antenna. Thus the array antenna is used. For the radiation part the log periodic antenna structure is applied which is endfire, 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]:(5)τ=Lm+1Lm=Wm+1Wm,(6)εeff=εr+12+εr−121+12hw−0.5,(7)L1=λeffmax/4=c/4εefffmin,where λeff is the effective wavelength, h is the thickness of substrate and w is the width of the longest element. The number of elements (N) is determined as follows:(8)σ=1−τ4tanα,(9)N=1+lnBAR×Bln1/τ,(10)B=fmaxfmin,(11)BAR=1.1+7.71−τ2cotα,where σ is the spacing factor, α is the apex angle, τ is the geometry constant, and BAR is the bandwidth of the active region.For 7 dB gain in Figure2, 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.Figure 2
Computed contours of constant directivity versusσ and τ for log-periodic dipole arrays.Figure 3
Antenna array structure.According to Figure1, the calculated dimension are L1 = 2.8 mm, L2 = 4.22 mm L3 = 1.068 mm, L4 = 1.425 mm, L5 = 1.9 mm, W1 = 0.75 mm, W2 = 1.25 mm, W3 = 0.14 mm, W4 = 0.1875 mm, and W5 = 0.25 mm.The antenna array is consisting of eight single elements and an eight way Wilkinson Power Divider (WPD) as a feeding network as shown in Figure3. In two-way WPD, the isolation resistor is 2Z0 and the impedance of λ/4 is √2Z0. For equal WPD the Z0 = 50 Ω, the impedance of λ/4 is √2Z0 = 70.7 Ω and isolation resistor is 2Z0 = 100 Ω. To design WPD at 28 GHz the TXline calculator is used. The values for WPD are obtained as: W50Ω = 1.18 mm, W70.7Ω = 0.65 mm, and L70.7Ω = 1.97 mm. The isolation resistor is 100 Ω (size is 1 × 0.5 mm2) from 0402 SMD family. For eight ways of WPD, three stages of two ways WPD is needed [13]. The 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.The fabricated prototypes of the proposed antennas are shown in Figure4.Figure 4
(a) Single element. (b) Antenna array [13].
(a)(b)Measurement and simulation results of antenna array (S11, Normalized E and H patterns) are shown in Figure 5.Figure 5
(a) NormalizedH pattern, (b) Normalized E-pattern, and (c) S11 coefficient of the proposed antennas at 28 GHz; [13].
(a)(b)(c)There is some difference between measured and simulated results because of SMK losses, fabrication faults, resistors etc. The 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.The maximum gain of a single element is 8 dBi and array is 14.6 dBi respectively. TheE and H patterns are end fire and S11 coefficient 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 Specific Absorption Rate of the antenna was evaluated. A three layers human head model situated in a 5 mm distance from the antenna. The power of the antenna was 15 and 20 dBm according to 5G standard values. The results showed that all of the SAR values are in standard range (i.e., 1.6 W/Kg according to FCC standard).


## 3. Linear Phased Array Antenna

### 3.1. Introduction

To better covering the space losses in the communication process the beam steering methods can be used by creating the phase difference. In this part the rules of designing phased array antenna is presented.Figure6 shows a linear phased array antenna with K elements and equally spaced by a distance d. The 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).(12)Sθ=∑i=1KSiθ=Seθ.Saθ=Seθ∑i=1KaiejK0K−idsinθ+ψi,Figure 6
A linear phased array of K radiators[11].whereSe is the element radiation pattern, Sa is array factor, ψi is the phase difference, K is the number of elements, and K0 is the free-space wave number. All the coefficients ai form an amplitude taper. Let us assume a uniform, normalized amplitude distribution, so for K elements:(13)ai=1,i=1,2,…,K.So onlySa is considered. If a linear phase taper is chosen:(14)ψi=k0K−idsinθ0,−90<θ0<90.Since−90<θ0<90, the array factor may be written as follows:(15)Saθ=∑i=1Kejk0K−idsinθ−sinθ0.For the linear phased array antenna situation, maximum array factor isθ=θ0 from (16).(16)sinθ−sinθ0=0.So by choosingθ0 in ψi=k0K−idsinθ0 the array factor will have its maximum at θ0.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 (Figure7).Figure 7
Grouping antennas into a sub-array [18].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 0.5λ 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).Figure 8
(a) Eight elements array with 0 degree phase difference, (b). Four sub arrays.
(a)(b)


### 3.2. Phase Difference

There 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 (Figure9) is used because of the simplest configuration 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].Figure 9
Basic schematic of phase shifter operation using diodes[19].By creating a time delay difference between two paths (changing the length ofL with respect to L0) the desired phase shift can reach in the transmission line. All of the diodes are assumed as an ideal diode.Reference line isL0 and the delay line length is L0+L. Different path lengths which result in a phase delay (φ) [18, 20–22]. 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).(17)L0=λεeff1−02π,(18)L1=λεeff1−φ2π,(19)L2=λεeff1−2φ2π,(20)L3=λεeff1−3φ2π.And, for φ calculate from (21).(21)φ=2πλdsinθ.


### 3.3. Calculations

In this paper, the calculations for the favorite (selective) phase shiftφ = 0°, 30°, 45°, 90°, and −90° and associated beam direction (θ) are done. For example for φ=30°, since at 28 GHz, λ=10.7mm and taking into account to substrate characteristic εeff=1.87, from (13) to (17) θ can be found as follows:(22)L0=10.71.871−0360°=7.86mm,L1=10.71.871−30°360°=7.21mm,L2=10.71.871−60°360°=6.55mm,L3=10.71.871−30°360°=5.89mm,θ=sin−1φ∗λ/2πd=sin−1π/6λ2π∗1.21λ=3.94°.Table1shows the calculations for 30, 45, 90, and -90 degrees.Table 1
Calculations for30°, 45°, and 90°
<!−−ColCount:4−−>φ (degree)L0 (mm)L1 (mm)L2 (mm)L3 (mm)θ (degree)307.867.216.555.893.94457.866.335.424.516907.865.893.931.9611.9


## 3.1. Introduction

To better covering the space losses in the communication process the beam steering methods can be used by creating the phase difference. In this part the rules of designing phased array antenna is presented.Figure6 shows a linear phased array antenna with K elements and equally spaced by a distance d. The 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).(12)Sθ=∑i=1KSiθ=Seθ.Saθ=Seθ∑i=1KaiejK0K−idsinθ+ψi,Figure 6
A linear phased array of K radiators[11].whereSe is the element radiation pattern, Sa is array factor, ψi is the phase difference, K is the number of elements, and K0 is the free-space wave number. All the coefficients ai form an amplitude taper. Let us assume a uniform, normalized amplitude distribution, so for K elements:(13)ai=1,i=1,2,…,K.So onlySa is considered. If a linear phase taper is chosen:(14)ψi=k0K−idsinθ0,−90<θ0<90.Since−90<θ0<90, the array factor may be written as follows:(15)Saθ=∑i=1Kejk0K−idsinθ−sinθ0.For the linear phased array antenna situation, maximum array factor isθ=θ0 from (16).(16)sinθ−sinθ0=0.So by choosingθ0 in ψi=k0K−idsinθ0 the array factor will have its maximum at θ0.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 (Figure7).Figure 7
Grouping antennas into a sub-array [18].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 0.5λ 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).Figure 8
(a) Eight elements array with 0 degree phase difference, (b). Four sub arrays.
(a)(b)


## 3.2. Phase Difference

There 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 (Figure9) is used because of the simplest configuration 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].Figure 9
Basic schematic of phase shifter operation using diodes[19].By creating a time delay difference between two paths (changing the length ofL with respect to L0) the desired phase shift can reach in the transmission line. All of the diodes are assumed as an ideal diode.Reference line isL0 and the delay line length is L0+L. Different path lengths which result in a phase delay (φ) [18, 20–22]. 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).(17)L0=λεeff1−02π,(18)L1=λεeff1−φ2π,(19)L2=λεeff1−2φ2π,(20)L3=λεeff1−3φ2π.And, for φ calculate from (21).(21)φ=2πλdsinθ.


## 3.3. Calculations

In this paper, the calculations for the favorite (selective) phase shiftφ = 0°, 30°, 45°, 90°, and −90° and associated beam direction (θ) are done. For example for φ=30°, since at 28 GHz, λ=10.7mm and taking into account to substrate characteristic εeff=1.87, from (13) to (17) θ can be found as follows:(22)L0=10.71.871−0360°=7.86mm,L1=10.71.871−30°360°=7.21mm,L2=10.71.871−60°360°=6.55mm,L3=10.71.871−30°360°=5.89mm,θ=sin−1φ∗λ/2πd=sin−1π/6λ2π∗1.21λ=3.94°.Table1shows the calculations for 30, 45, 90, and -90 degrees.Table 1
Calculations for30°, 45°, and 90°
<!−−ColCount:4−−>φ (degree)L0 (mm)L1 (mm)L2 (mm)L3 (mm)θ (degree)307.867.216.555.893.94457.866.335.424.516907.865.893.931.9611.9


## 4. Simulation

By the calculated parameters from Table1 antenna array is simulated in Figure 10. This figure shows the results of the simulation for 30°, 45°, and 90° phase differences and corresponding beam direction (θ). Figure 10(a) shows the phase difference 30° (θ=4°), and Figures 10(d), 10(g), and 10(j) are the results of normalized E-pattern with respect to 0° phase difference, S11 and efficiency of this state respectively. Figure 9(b) shows the phase difference 45° (θ=7°) and Figures 10(e), 10(h), and 10(k) are the results of normalized E-pattern with respect to 0° phase difference, S11 and efficiency of this state respectively. Figure 10(c) shows phase difference 90° (θ=11°), and Figures 10(f), 10(i), and 10(l) are the results of normalized E-pattern with respect to 0° phase difference, S11 and Efficiency 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 efficiency, S11 and pattern are suitable and without any degradation in antenna performance. Figure 11 presents the beam steering at 0°, 30°, 45°, and 90° phase differences in one chart.Figure 10
a). Phase difference 30 degree(θ=4°), d, g, j are results of normalized E – pattern with respect to 0° phase difference, S11 and Efficiency of this state, respectively. (b). Phase difference 45 degree(θ=7°), e, h, k are results of normalized E-pattern with respect to 0° phase difference, S11 and Efficiency of this state respectively. (c). Phase difference 90 degree(θ=11°), f, i, l are results of normalized E-pattern with respect to 0° phase difference, S11 and Efficiency of this state respectively.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)Figure 11
Normalized E patterns for various phase shifts and beam rotating.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.Table 2
Simulated and calculated “θ”
φ (degree)30°45°90°θ simulated3.94611.9θ calculated4711


## 5. Implementation of the Beam Switching

Because of similarity, in this part, only beam switching is designed forθ=11° and 11°. As Figure 11 shows switched line phase shifter technique with diodes is implemented. Phase difference is producing by changing in the length L0 to L3 which is calculated in Table 3 and using “on” and “off” states of diodes. From equations (13)–(17) for θ=11° and θ=−11° the required phase shift is 90° and −90°, respectively.Table 3
L0 to L3 values for θ=11° and θ=−11°.
φ (degree)L0 (mm)L1 (mmL2 (mm)L3 (mm)θ (degree)907.865.893.931.9611.9−907.869.82511.7913.775−11.88In Figure12(a) the overall structure of the proposed antenna is shown. The diodes used to switch and select the paths. In Figure 12 when the green diodes are on “on” state and blue diodes are “off,” θ=−11° and When the green diodes are on “off” state and blue diodes are “on,” θ=11°.Figure 12
(a). The linear array antenna with beam rotation ability, (b). Green diodes “on”, and Blue diodes “off,” (c). Green diodes “off,” and Blue diodes “on,” (d). the result of beam rotation forφ=90θ=11° and φ=−90θ=−11° degrees with respect to 0 degree.
(a)(b)(c)(d)From Figure12 the rotation of the patterns are observed clearly. By the changing the phase difference 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. Thus it is better to use from other beam steering methods [23–25].Table4 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. The structure of the proposed antenna. is planar, easy to fabrication, and integrates with other devices. In some methods [26–29] the. ferrite used which can increase the losses and size of the antenna. The 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 [26–29] 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 [30–32] the antennas have good efficiency 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).Table 4
Comparison of some beam steering.
Ref. Num.Freq.(GHz)Gain (dB)Radiation atθ = 0°5G applicationSize (λ03)Size reductionEfficiency[26]6.59–6.699YesNo1.5 × 11.58 × 0.20586.2%N.A[27]6.1–7.214–16YesNo37 × 3.17 × 0.57999.2%65%[28]6.59–6.72≈ 5YesNo1.5 × 11.58 × 0.20586.2%N.A[29]6.2–6.87.1YesNo0.21 × 9.34 × 0.74866.66%78.5%[30]27.5–28.58.01YesYes5.6 × 12.6 × 0.07490.6%90%[31]25–2710.5–12YesYes8.41 × 3.73 × 0.3595.5%95.6%[32]2811.62YesYes7.55 × 7.55 × 0.2296.1%96.21%This work27–29.514.6YesYes2.8 × 4.86 × 0.036—> 95%


## 6. Conclusions

An end fire antenna which is planar, with compact size, lightweight, simple structure, and high gain is proposed, simulated, and fabricated. The feeding part is the Wilkinson power divider which provides an ability to beam switching by using switched line phase shifter technique. The switching action is done by “on” and “off” states of diodes which allow selecting the path of current in the transmission lines. The simulation results for the phased array antenna show that when beam switching and beam rotation is done the S11 coefficients, efficiency, 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. Thus, this type of antenna is a good candidate to mmW/5G (27–29.5 GHz) application and systems especially for handheld devices.



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*Source: 8679798-2022-11-01.xml*