A Dual Orthogonal Fed Monopole Antenna for Circular Polarization Diversity

Article information

J. Electromagn. Eng. Sci. 2022;22(3):283-290
Publication date (electronic) : 2022 May 31
doi : https://doi.org/10.26866/jees.2022.3.r.88
Department of Electronic Engineering, Kyonggi University, Suwon, Korea
*Corresponding Author: Youngje Sung (e-mail: yjsung@kgu.ac.kr)
Received 2021 May 19; Revised 2021 August 13; Accepted 2021 October 20.

Abstract

A monopole antenna with circular polarization diversity and high isolation is proposed in this paper. The proposed antenna consists of two bent monopole antennas and a partial ground plane. Two monopole antennas are implemented on opposite planes with respect to the ground plane. The ground plane is a right-angled isosceles triangle, and the antenna is located at one end of the ground plane. Circular polarization (CP) characteristics are obtained using this structural asymmetry. In the proposed structure, the resonant frequency at S11 is controlled by the antenna length, and the null frequency at S21 (isolation) is controlled using the bending angle of the monopole and the distance between the two antennas. From the measured results, the CP operating bandwidth of the proposed antenna is 13.8% (1.76–2.02 GHz). The measured isolation is −50 dB at the resonant frequency and −20 dB within the operating bandwidth.

I. Introduction

Circular polarization (CP) diversity has received much attention in the communication field because it can implement right-handed circular polarization (RHCP) and left-handed circular polarization (LHCP) in the same frequency band [1]. The CP polarization diversity antenna has two advantages. First, it has CP characteristics, which make it less susceptible to polarization mismatch than linear polarization (LP) and have the advantage of reducing fading loss [2, 3]. These characteristics are appropriate for the propagation environment in urban areas, where tall buildings are closely located. Second, the isolation between two polarizations (LHCP and RHCP) is very high, so different communication systems can be used at the same frequency [4]. In recent years, many communication services have been supported owing to the development of wireless communication technology, and resources in the low-frequency band below 10 GHz are rapidly becoming exhausted. Polarization diversity is one solution to this problem.

In general, the CP polarization diversity antenna uses a single-feed method, which is structurally less complex than an antenna with a dual-feed. However, in the case of a single-feed antenna, at least two PIN diodes are generally used, and DC power and a bias circuit are required to implement control of the diodes. This may distort the radiation pattern of the antenna or degrade the antenna gain [57].

When the CP polarization diversity antenna is implemented with a dual-feed structure, it is common to use a wide-slot or monopole structure to obtain broadband characteristics. A wide slot with a curved edge is used instead of a rectangular or square slot to maximize the bandwidth of the antenna in a given space [8]. Both structures exhibit a wide CP bandwidth of 72.5% and 54%, respectively, but they are difficult to redesign because the numerical value or shape of the curve is unclear. In [9], a monopole structure is used, the 3 dB axial ratio (AR) bandwidth is 80%, and the isolation is less than −15 dB. This structure has the disadvantage of using 13 design parameters.

In [10], LP and CP antennas operating in the same frequency band are implemented on different layers, but the isolation characteristic is not as high as approximately −15 dB, and the polarization mismatch between LP and CP is theoretically 50%. Therefore, the antennas are limited to polarization diversity use. A dual-feed polarization diversity antenna has been proposed using two different slots. In [11], a CP bandwidth of 7.8% is obtained using slots of different shapes, and a CP bandwidth of 2% is achieved using different size H-shaped slots in [12].

In addition, in [13], a dual-feed polarization diversity antenna using a dielectric resonator (DR) structure is proposed, and it has a CP bandwidth of 13.23%. When power is supplied to port 1, the beam is inclined 30° to the left, and when it is fed to port 2, it is inclined 30° to the right. Therefore, a problem arises where the radiation pattern is not constant according to polarization. In [14], CP bandwidths of 3.2%/7.7% are obtained at the downlink/uplink using a phased power divider with embedded filtering functions and phase control. Here, the isolation is greater than 20 dB.

The antenna proposed in [15] has a very wide CP bandwidth of 72.5% and 56%, respectively, in the dual band. On the other hand, design parameters for curved slots are not presented, and there are many disadvantages in the design parameters. The antenna proposed in [16] has a small size, but it also has a narrow CP bandwidth of 4.16%.

In this work, optimal CP characteristics are obtained by properly placing a monopole antenna at the end of a partial ground plane, and bidirectional radiation pattern and isolation characteristics are realized by placing the same monopole antenna back to back. The proposed antenna shows LHCP characteristics in the +z direction and RHCP characteristics in the −z direction when port 1 is excited. If port 2 is excited, the polarization reverses.

II. Single-Layer Dual-Feed Dual-CP Antenna

Extensive research has been conducted on two printed monopole antennas with a rectangular partial ground plane. Two monopole antennas are located within λg/2, and the isolation between the two antennas is higher than −10 dB. The closer the distance between the two antennas, the worse the isolation characteristics tend to be. Various decoupling networks, such as a partially extended ground plane [17], slit [18], and neutralization line [19], are used between the two antennas to improve isolation.

As shown in Fig. 1(a), when the ground plane is oblique, it becomes structurally asymmetric. Therefore, unlike in previous research, the monopole antenna can easily obtain CP characteristics. Fig. 1(b) shows the simulation result of the antenna shown in Fig. 1(a). As shown in Fig. 1(b), the resonant frequency of the antenna is 1.68 GHz, and the isolation is better than −30 dB even though there is no decoupling network. However, the frequency with the minimum AR is 1.21 GHz, which is a considerable distance from the resonant frequency of the antenna. As a result, the CP bandwidth satisfying 10 dB impedance bandwidth and 3 dB AR bandwidth is 7.8% (1.48–1.6 GHz). S21 is less than −15 dB.

Fig. 1

Dual-feed dual-circular polarized monopole antenna: (a) geometry and (b) simulated antenna performance.

To broaden the CP bandwidth of the antenna shown in Fig. 1(a), the resonant frequency of the antenna and the frequency with the minimum AR should be matched. As is well known, the resonant frequency of the antenna depends on the length of the radiating element located on the non-ground plane. However, when the length of the antenna is changed, the frequency response of the AR also changes, so that the resonant frequency of the antenna and the frequency with the minimum AR do not coincide.

Fig. 2(a) shows four monopole antennas with various feeding shapes. At this time, the length, position of the monopole antenna, and GND shape do not change, but only the feeding structure changes. Fig. 2(b) shows the simulated AR characteristics when the length of the horizontal line increases in the feeding structure. At this time, the length of the vertical line of the feeding structure is fixed to 15.8 mm. In Model A, the feeding structure consists only of vertical lines, and the feeding point is 57 mm away from the left corner. The simulated AR of Model A shows good CP characteristics near 1.4 GHz. In Model B, the feed point is shifted to the left by 22 mm, and the horizontal line of the feed structure is increased to keep the other parameters. Accordingly, a new AR dip is formed at 2.4 GHz. At this time, there is little change in the position of the previously formed AR dip. In Model C, the feeding point is moved further to the left, and the length of the horizontal line is further increased. Accordingly, the first AR dip frequency is hardly affected, and the second AR dip frequency decreases. The feeding structure of Model D consists of only horizontal lines without vertical lines. Accordingly, the first AR dip frequency increases, and the second AR dip frequency decreases.

Fig. 2

Monopole antennas with different feeding points: (a) geometry and (b) simulated AR.

Fig. 3(a) shows three models with different antenna positions. The antenna represented by the dotted line in Fig. 3 indicates Model A in Fig. 2(a). Model E moves Model A to the right by 10 mm, and Models F and G move Model A to the left by 10 mm and 20 mm, respectively. To move the antenna without changing the length and shape, the length of the vertical line in the feeding structure is changed. It can be seen that the frequency with the minimum AR decreases as the length of the vertical line increases. Based on these facts, it can be seen that the first AR dip is formed by the vertical line of the feeding structure, and the second AR dip is formed by the horizontal line of the feeding structure.

Fig. 3

Monopole antennas with different positions: (a) geometry and (b) simulated AR.

Fig. 4 shows the surface current distribution at the two frequencies with the minimum AR. At the two frequencies with the minimum point, a surface current is formed in the right diagonal direction on the monopole antenna, a surface current is formed in the left diagonal direction along the hypotenuse of the GND, and the two surface currents are perpendicular to each other. Accordingly, the proposed structure exhibits CP characteristics. At the first minimum AR frequency, the surface current formed in the entire GND rotates clockwise, and at the second minimum AR frequency, the surface current formed in the GND located below the feed structure rotates clockwise. Based on this fact, it is confirmed that LHCP characteristics appear in both frequency bands.

Fig. 4

Surface current distribution according to phase change: (a) first minimum AR point and (b) second minimum AR point.

With feeding point D, the feed line is implemented in a straight line. As the frequency at the second minimum point decreases slightly, the values of the two minimum points are low, and good CP characteristics are obtained over a wide operating band. However, when feeding point D is used, the feeding structure is overlapped geometrically, so that two monopoles cannot be implemented on a single layer. To solve these problems, two monopoles are implemented on a dual layer.

III. Antenna Geometry

Fig. 5 shows the proposed dual-feed dual-circularly polarized antenna structure. The proposed antenna is implemented using two substrates. Both substrates are FR4 (loss tangent = 0.02) with a thickness of 1.6 mm. Monopole #1, shown in blue in Fig. 5, is located in the top layer and lies horizontally. Monopole #1 uses a line width w corresponding to 50 ohms for the substrate used for the simple structure and is located a distance s away from the lower edge of the substrate. The GND, shown in green in Fig. 5, is implemented in the middle layer and is located between the two substrates. It acts as the GND of the two antennas (Monopole #1 and Monopole #2).

Fig. 5

Proposed dual-feed dual-circularly polarized antenna.

Monopole #1 is mainly divided into two parts. In the area with the ground plane, the shape of the monopole is implemented horizontally, and its length is L1. In the region where there is no ground plane, the stripline is tilted counterclockwise by α° with respect to the horizontal axis, and its length is L2. On the other hand, Monopole #2, shown in red in Fig. 5, is placed vertically and is located a distance away from the left edge of the substrate. In the non-GND region, the stripline is tilted clockwise by α° with respect to the vertical axis. Monopole #1 and Monopole #2 are symmetrical with respect to the right diagonal when viewed from above. The distance between the two antennas is set to d, and the overall size of the substrate is 100 mm × 100 mm (0.63λ0 × 0.63λ0). The antenna parameters are L1 = 64 mm, L2 = 38.5 mm, s = 15.8 mm, w = 3 mm, d = 66 mm, α = 25°, and G = 80 mm.

IV. Parameter Study

Since L1 is the area with the ground plane, its length can change with no change in the resonant frequency of the antenna. Therefore, as shown in Fig. 6(a), the resonant frequency of the antenna versus the length of L2 is examined, while the other parameters remain unchanged. As L2 increases, the resonant frequency of the antenna decreases, but the matching characteristic deteriorates and the operating bandwidth decreases. The frequency at which the null in isolation is formed hardly changes with respect to the change in length of L2. As shown in Fig. 6(b), the AR frequency response is less affected by the change in the length of L2 than the resonant frequency. In this case, although the AR characteristics somewhat deteriorate as L2 increases, the AR is lower than 3 dB from 1.7 to 2.1 GHz and still shows good characteristics. As L2 increases from 36.5 mm to 40.5 mm, the CP bandwidths satisfying both S11 < −10 dB and AR < 3 dB are 16.6% (1.659–1.96 GHz), 18.4% (1.624–1.953 GHz), and 17.3% (1.599–1.901 GHz), respectively. Accordingly, L2 is selected to be 38.5 mm.

Fig. 6

Simulated results according to the change in length L2: (a) reflection coefficient and isolation performance and (b) axial ratio.

Fig. 7 shows the simulation result versus distance d between the two antennas. In this case, L1 = 64 mm, L2 = 38.5 mm, s = 15.8 mm, w = 3 mm, α = 25°, and G = 80 mm. As the distance between the two antennas increases from 62 mm to 70 mm, the resonant frequency of the antenna increases from 1.79 GHz to 1.86 GHz, and the matching characteristics of the antenna worsen. In addition, the frequency of the isolation null increases from 1.71 GHz to 1.85 GHz. As d increases, the frequency of the isolation null gradually approaches the resonant frequency of the antenna. In the case of a dual-feed antenna, it is desirable to match the frequency of the isolation null and the resonant frequency of the antenna. In the case of the proposed structure, the resonant frequency of the antenna is equal to the frequency of the isolation null when d is 70 mm. However, the antenna matching and isolation null characteristics are the worst among the five cases considered above.

Fig. 7

Simulated results according to the change of distance d: (a) reflection coefficient and isolation performance and (b) axial ratio.

The lower edge of the AR frequency response increases rapidly as d increases, but the upper edge hardly changes. The CP bandwidth gradually decreases to 13.8% when d = 66 mm, 16.1% when d = 64 mm, and 18.3% when d = 62 mm. In terms of CP bandwidth, the smaller d, the better the characteristic. However, when d is 62 mm and 64 mm, the isolation characteristics are poor. Accordingly, d is selected to be 66 mm.

Fig. 8 shows the simulation results of the antenna characteristics versus the size G of the ground plane. Except for G, there is no change in the other parameters as follows: L2 = 38.5 mm, w = 3 mm, d = 66 mm, and α = 25°. As G increases, both the resonant frequency of the antenna and the null frequency of the isolation decrease but the change in null frequency is faster than the change in resonant frequency. The AR frequency response also decreases as G increases. As G increases from 75 mm to 85 mm, the bandwidth satisfying both S11 < −10 dB and AR < 3 dB is 16.2% (1.649–1.94 GHz), 18.4% (1.624–1.953 GHz), and 17.4% (1.6–1.905 GHz), respectively. Accordingly, G is selected to be 80 mm.

Fig. 8

Simulated results according to the change of ground plane G: (a) reflection coefficient and isolation performance and (b) axial ratio.

Fig. 9 shows the simulation results of the reflection coefficient and AR versus the rotation angle α. In this case, L1 = 64 mm, L2 = 38.5 mm, s = 15.8 mm, w = 3 mm, d = 66 mm, and G = 80 mm. As the rotation angle α increases, the distance between the two monopole antennas becomes greater. This is similar to the increase in distance d mentioned above. Thus, the effect of the rotation angle α on the reflection coefficient, isolation, and AR is similar to that shown in Fig. 7. One notable difference is the change in the isolation null value. As d changes, the isolation null value changes greatly, but it is hardly affected by the change in the rotation angle. When the rotation angle α is 30° or 35°, the AR value is greater than 3 dB, and good CP characteristics cannot be obtained. When the rotation angle is 15° or 20°, there is a big difference between the resonant frequency of the antenna and the isolation null frequency, and the isolation characteristics are poor at the resonant frequency. Accordingly, the rotation angle α is selected to be 25°. Fig. 10 shows the fabricated antenna.

Fig. 9

Simulated results according to the change in rotation angle α: (a) reflection coefficient and isolation performance and (b) axial ratio.

Fig. 10

Fabricated antenna.

V. Simulation and Measured Results

Fig. 11 presents the simulated and measured frequency response of the fabricated antenna. The measured 10 dB impedance bandwidth is 20.8% (1.64–2.02 GHz) and is indicated by the shaded yellow area in Fig. 8(a). In addition, the measured 3-dB AR bandwidth of the proposed antenna is 21.3% (1.76–2.18 GHz), denoted by the shaded yellow area in Fig. 8(b). Here, AR is a value in the boresight direction (θ = 0°) of the proposed antenna. The overlapping CP bandwidth (S11 < −10 dB and AR < 3 dB) is 13.8% (1.76–2.02 GHz).

Fig. 11

Simulated and measured results: (a) reflection coefficient and (b) axial ratio.

Fig. 12 presents the simulated and measured radiation patterns of the proposed antenna. Note that the proposed antenna is a bidirectional radiator, meaning that the radiation patterns on both sides of the antenna are similar. The measured peak gain at ϕ = 0°, θ = 0° is 1.39 dBic and the measured peak gain at ϕ = 0°, θ = 180° is 1.26 dBic. The radiation efficiency is 78%.

Fig. 12

Simulated and measured radiation patterns when port 1 is excited: (a) 1.76 GHz, (b) 1.89 GHz, and (c) 2.02 GHz. The left and right images are xy-plane and xz-plane, respectively.

VI. Conclusion

In this study, a dual-CP antenna using two curved monopoles and an isosceles triangle-shaped partial ground plane is implemented. Owing to the asymmetry of the antenna relative to the ground plane, CP performance is easily obtained. By analyzing the AR frequency response according to the position of the feed structure, the broadband CP characteristic is obtained. A good isolation characteristic of −45 dB is achieved without the additional structure. In addition, the proposed structure has the advantage of being able to independently control the resonant frequency and isolation null frequency.

Acknowledgments

This work was supported by a Kyonggi University Research Grant, 2020.

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Biography

Youngje Sung was born in Incheon, Korea, in 1975. He received B.S., M.S., and Ph.D. degrees from Korea University, Seoul, Korea, in 2000, 2002, and 2005, respectively. From 2005 to 2008, he was a Senior Engineer with the Antenna R&D Laboratory, Mobile Phone Division, Samsung Electronics, Korea. In 2008, he joined the Department of Electronic Engineering, Kyonggi University, Suwon, Korea, where he is currently a professor. His research interests include reconfigurable antennas, cellphone antennas, wideband slot antennas, multifunction devices, compact circular polarized antennas, and compact dual-mode filters. Prof. Sung is serving as a reviewer for the IEEE Transactions on Microwave Theory and Techniques, IEEE Transactions on Antennas and Propagation, IEEE Microwave and Wireless Components Letters, IEEE Antennas and Wireless Propagation Letters, Progress in Electromagnetic Research, IET Electronics Letters, IET Microwaves, Antennas and Propagation.

Article information Continued

Fig. 1

Dual-feed dual-circular polarized monopole antenna: (a) geometry and (b) simulated antenna performance.

Fig. 2

Monopole antennas with different feeding points: (a) geometry and (b) simulated AR.

Fig. 3

Monopole antennas with different positions: (a) geometry and (b) simulated AR.

Fig. 4

Surface current distribution according to phase change: (a) first minimum AR point and (b) second minimum AR point.

Fig. 5

Proposed dual-feed dual-circularly polarized antenna.

Fig. 6

Simulated results according to the change in length L2: (a) reflection coefficient and isolation performance and (b) axial ratio.

Fig. 7

Simulated results according to the change of distance d: (a) reflection coefficient and isolation performance and (b) axial ratio.

Fig. 8

Simulated results according to the change of ground plane G: (a) reflection coefficient and isolation performance and (b) axial ratio.

Fig. 9

Simulated results according to the change in rotation angle α: (a) reflection coefficient and isolation performance and (b) axial ratio.

Fig. 10

Fabricated antenna.

Fig. 11

Simulated and measured results: (a) reflection coefficient and (b) axial ratio.

Fig. 12

Simulated and measured radiation patterns when port 1 is excited: (a) 1.76 GHz, (b) 1.89 GHz, and (c) 2.02 GHz. The left and right images are xy-plane and xz-plane, respectively.