Triple-Band Dual-Polarized Dipole Antenna for 5G Sub-6 GHz Communications

A triple-band ± 45° dual-polarized dipole antenna is presented in this paper. The proposed antenna covers two n77 bands and one n79 band in 5G NR frequency spectrums with S11, S22 <− 15 dB return loss. The profile antenna exhibits the measured impedance bandwidths of 250 MHz, 150 MHz and 350 MHz from the operating bands 3.6–3.85 GHz, 4.05–4.2 GHz and 4.8–5.15 GHz respectively. Antenna is fabricated with four substrates; one radiator, one reflector and two feeding baluns. Antenna is designed and optimized with HFSS simulator and fabricated for experimental verification. Antenna gives a stable radiation pattern of 8.55 dBi high gain with 70° half power beam width (HPBW) that makes it a good candidate for wireless 5G sub-6 GHz and multiband base station applications. Finally, antenna is tested in a realistic application environment to show the utility of the proposed antenna for wireless sub-6 GHz IoT applications.


Introduction
Internet of things (IoT) is expected to be adopted for most of the electronic communication applications in near future. The polarization diversities and frequencies will work at several bands in future. IoT is bringing an advanced digital revolution to our daily life. As a result, there is a huge production of sensors and integrated antennas associated with IoT age. The IoT technology associated with integrated antennas is expected to make our future communications a very sophisticated and simple. The concept of internet of things (IoT) is known as the extension of internet now. It is a worldwide inter-connected emerging network of different objects associated with sensors, input/output devices, actuators and communication systems. The efficiency of internet of things (IoT) and wireless internet connection are improved with the wireless sensor technology [1].
In fact, antennas are playing a vital role in wireless sensor technology that leads to an emerging future with the development of IoT techniques. The compact and low profile integrated antennas have drawn a large attention in last few years due to multi-band and multifunction features in communication systems [2]. A loop antenna is presented in [3] for an IoT application with the antenna placement at different places. A multi-standard MIMO antenna is proposed in [4] for IoT applications. A loop antenna is proposed in [5] for a biotelemetry application to operate in GSM900, GSM1800 and Bluetooth Low Energy. In the recent years, different antenna modules and antennas has been designed for upcoming multimedia high speed and large data volume challenges. The technological development in different cellular and wireless applications has generated a high speed data rate requirement that leads to a fifth generation (5G) communication system. In present communication systems, dual-polarized antenna has attained a large attraction to reduce multipath fading. For dual-polarized antennas, a significant research is present [6][7][8][9][10][11][12][13][14]. Many countries are using (IMT-2020) standard 5G networks and services for research and services. The sub-6 GHz frequency spectrum i.e. 3.3-3.6 GHz and 4.8-5.0 GHz in China and 3.4 GH to 3.8 GHz in Europe has been licensed. The N78 (3.3-3.8 GHz) 5G band has been assigned in many countries. The broadband MIMO antennas with high impedance bandwidths have become a good solution for 5G demands. A dual-polarized printed dipole antenna with 52% impedance bandwidth is presented in [15]. The base station broadband antennas for 2G, 3G and 4G networks are proposed in [16][17][18][19][20][21][22]. The comparison of the proposed work with the related ones [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] is presented in Table 1.
A low cost, low profile triple band ± 45° dual-polarized dipole antenna is proposed for wireless 5G applications. In the first step, an un-slotted printed dipole with ± 45° cross pair arrangement antenna is designed. In the next steps, slots are added in the center of each quadrant patch and then at corners. Radiation patterns and reflection coefficient are noted at each stage of electromagnetic analysis to get optimum antenna structure.
The paper is organized as follows: design of the profile antenna with EM analysis lies in Sect. 2. Simulated and experimental results are presented in Sect. 3. The final summary lies in Sect. 4.

Antenna Design
The antenna geometry is shown in Fig. 1. The profile antenna consists of one reflector, one radiator and two baluns. The radiator has a 0.8 mm thick RT5880 substrate printed on the top. The RT5880 substrate has dielectric constant and loss tangent 2.2 and 0.0004 respectively. The reflector has RT5880 substrate with 0.8 mm thickness printed at bottom. Baluns are erected perpendicularly between radiator and reflector.
The main radiator of the proposed antenna has two dipoles with ± 45° cross pair arrangement as shown in Fig. 1a. Radiator is a 32 × 36 mm 2 printed substrate mounted on both baluns at 23.8 mm height. The patch is divided into four equal quadrants on the substrate. Each quadrant has 15.5 × 17.5 mm 2 dimension and is connected with the other in a ± 45° cross pair dipole arrangement. Balun shorts each dipole with ground separately. The two center lines slots divide the main radiator patch into four equal quadrants. The quarter circle slot is placed at each corner of the main radiator. The quarter circle has 2.8 mm radius. Each quadrant has an E-shaped slot at the center. The whole radiator arrangement demonstrates ± 45° dual-polarized cross pair dipoles that are shorted with the ground via baluns.
Geometric configuration of baluns is shown in Fig. 1c and d. Each balun has a micro strip patch at one side and L-probe feed line on the other side on a FR4 dielectric substrate. Both baluns are erected perpendicularly in a ± 45° cross arrangement. Balun1 is mounted on balun2. The open slots on both baluns make adjustment of balun1 on balun2. Both baluns have bumps at upper and lower edges. Radiator is mounted on the upper bumps of the baluns and the lower bumps are inserted in the reflector bores. Micro strip patches on both baluns short radiator with the ground through balun bumps. The feed line on the other side of each balun is shorted with the inner conductor of port/coaxial cable.
The outer conductor of coaxial cable/port is connected to the ground. The feed lines on both baluns have different dimensions but the micro strip patches and relevant bumps have same dimensions as shown in Fig. 1c and d. The reflector is a 120 × 120 mm 2 RT5880 dielectric substrate with ground patch at bottom. This ground is connected with the outer conductors of both ports port1 and port2 as shown in Fig. 1b. Hence a ± 45° dual-polarized cross pair dipole antenna structure is developed by connecting each feed line to a separate port and each dipole to the ground.

EM Analysis
The numerical simulations and optimization of the profile antenna are performed on HFSS. The slotted radiator develops a dual band s-parameter while the L-probe feed line achieves a better 50 Ω matching with a wide band feature. Based on the cited work and literature review, an un-slotted radiator with cross pair dipoles mounted on feeding baluns is designed for a dual-polarized radiation pattern in the first step. It exhibits 8.55 dBi gain and impedance bandwidth 3.6-4.1 GHz with S11, S22 <− 15 dB as shown in Figs. 2 and 3.
In the 2nd step E-shaped and corner circle slots are added to get a multi-band feature. The antenna exhibits 8.55dBi gain with impedance bandwidth 3.6-3.85 GHz, 4.05-4.20 GHz and 4.8-5.1 GHz with S11, S22 <− 15 dB as shown in Figs. 2 and 3. So, a triple band dualpolarized feature is achieved with a ± 45° dual-polarized cross pair dipole arrangement along with the feeding baluns and reflector. A uniform stable radiation pattern is observed with no evident drop when slots are added to get multi-band feature as shown in Fig. 2. Figure 3 shows the reflection coefficient of the profile antenna with slotted and un-slotted radiator. It is observed that the un-slotted radiator structure gives a single band while the slotted radiator configuration exhibits triple band with the same uniform radiation pattern.

Simulation and Experimental Results
Based on optimization and above EM analysis with both dipole configurations, a triple band dual-polarized dipole antenna is proposed for 5G wireless applications. The profile antenna is fabricated and measured as shown in Fig. 4. Experimental results were taken in anechoic chamber using vector network analyzer. The reflection coefficient of the proposed antenna is shown in Fig. 5. It is observed that the antenna gives impedance bandwidth 3.6-3.85 GHz, 4.05-4.20 GHz and 4.8-5.15 GHz. The simulated and measured radiation patterns of the proposed antenna at 3.6 GHz center frequency in E and H planes are shown in Figs. 6 and 7. The − 45° polarized radiation pattern is omitted due to the similarity in results. The simulated and measured radiation patterns are very consistent and uniform in E and H planes. The radiation patterns at various frequencies within the proposed bands are shown in Fig. 8. It is observed that the profile antenna gives a uniform and stable radiation pattern over the entire band. The profile antenna gives 8.55 ± 0.6dBi gain over the entire band. Both simulated and measured results are in a good agreement and a small discrepancy is due to the fabrication and insertion loss. The half power beam width (HPBW) of the proposed antenna is 70°. Furthermore, the comparison of the proposed antenna with reference papers of related works is presented in Table 1. For IoT applications, the profile antenna was tested in a realistic environment. The port1 was connected to a 5 GHz supported core I5 DELL laptop through a USB port connector as shown in Fig. 9. It was observed that the profile antenna got connected with Wi-Fi WLAN (Wi-Fi 4(802.11n)) at 5 GHz and support browsing as shown in Fig. 9.

Conclusion
In IoT applications, there is an advantage of applying an antenna as a wireless sensor technology. In this paper, a triple band ± 45° dual-polarized dipole antenna is proposed for 5G wireless applications. Antenna covers 3.6-3.85 GHz, 4.05-4.20 GHz and 4.8-5.15 GHz measured bands and exhibits 8.55 dBi high gain with 70° half power beam width (HPBW). Antenna supports WLAN (Wi-Fi 4(802.11n)) when tested for an IoT application in a realistic environment at 5 GHz. The measured results confirm the validity of the proposed antenna design that makes it a good candidate for 5G sub-6 GHz wireless applications. The antenna performance and comparison with related works makes it eligible for massive MIMO antenna array for multi-band 5G base station applications.