A Millimeter-Wave Wideband Circularly Polarized Planar Spiral Antenna Array with Unequal Amplitude Feeding Network

A 2× 2 wideband circularly polarized (CP) antenna array operating at millimeter wave (mmWave) band is presented. .e array element is a wideband CP Archimedean spiral radiator with special-shaped ring slot. .e elements are fed by an unequal amplitude (UA) feeding network based on a microstrip line (MSL) power divider. .e side lobe level is improved by this UA feeding network. In addition, a cross slot is employed to isolate the elements for decoupling. A prototype is fabricated, and the measured results show that the proposed array achieves an impedance bandwidth (IBW) of 6.31GHz (22.5% referring to 28GHz) and an axial ratio bandwidth (ARBW) of 7.32GHz (26.1% referring to 28GHz). .e peak gain of the proposed array is 11.3 dBic, and the gain is greater than 9.3 dBic within the whole desired band (from 25GHz to 31GHz). .e proposed array consists of only one substrate layer and can be built by the conventional printed circuit board technology. Attributed to the characteristics of wide bandwidth, simple structure, low profile, and low cost, the proposed antenna array has a great potential in mmWave wireless communications.


Introduction
With the rapid popularization of the fifth-generation (5G) wireless communication technology and the growth of wireless data traffic, millimeter wave (mmWave) communication has become one of the most attractive technologies in the 5G mobile communication system [1]. Antennas are usually arranged into arrays for high gain to compensate the large path loss in mmWave band. Moreover, since the circularly polarized (CP) antenna can suppress multipath and prevent rain and fog interference, the mmWave CP arrays with wide bandwidth has been widely concerned by academia and industry.
In recent decades, many researchers have studied the mmWave wideband CP array. One of the most widely used techniques to realize CP array is the sequentially rotated phase (SRP) feeding. In a typical SRP feeding array, four single-feeding radiation elements are rotated clockwise or anticlockwise, and the elements are fed with equal amplitude and 90 degrees phase difference. e SRP feeding technique is beneficial to improve the bandwidth and polarization purity of CP array. e reported structures of SRP feeding network in the mmWave band can be divided into three categories: (1) microstrip line (MSL) SRP feeding network [2][3][4][5], (2) substrate-integrated waveguide (SIW) SRP feeding network [6][7][8][9], and (3) grounded coplanar waveguide (GCPW) SRP feeding network [10,11]. A 61 GHz beam steering antenna with left-handed linear polarized polarization (LHCP) fed by MSL clockwise SRP network was presented in [2]. A 4 × 4 wideband CP cross-aperturecoupled stacked array operating at 60 GHz and a 4 × 4 CP crossed slot with parasitic patch array operating at 28 GHz were presented in [3,4], respectively. A 4 × 4 mmWave wideband CP microstrip array with right-handed circular polarization (RHCP) was proposed in [5], which consisted of an edge-coupled parasitic patch structure and a novel series anticlockwise SRP feeding network. A low-profile cavitybacked slot CP 2 × 2 array and a 2 × 2 wideband RHCP diamond ring slot antenna array at the X-band were realized by employing the SIW SRP feeding network structure in [6,7], respectively. Jianfeng Zhu presented a dual CP planar aperture array operating at 60 GHz fed through two input ports connecting with the SRP feeding network [8]. A 45 GHz wideband CP array using inclined slot in SIW was developed by using SRP feeding network in [9]. Moreover, an mmWave CP differentially fed dipole array [10] and an mmWave CP array with the truncated-corner square patch as radiation element [11] were realized by employing GCPW SRP feeding network and substrate-integrated gap waveguide (SIGW) SRP feeding network, respectively. e SRP feeding can enhance the axial ratio bandwidth (ARBW) of CP array, even if the element has a narrow ARBW. However, the above designs have shown that the structure of SRP feeding network and the placement of antenna elements are complex.
Another feeding method to realize wideband CP array is the full-corporate (FC) feeding technique based on the power divider. In FC feeding network, the elements are excited with equal amplitude and equal phase. Ming Du presented wideband CP patch array are fed by MSL FC feeding network with low-temperature cofired ceramic (LTCC) technology in [12]. A wideband CP 4 × 4 mmWave array operating at 45 GHz was realized by utilizing MSL FC feeding network to excite the microstrip elements in [13].
e SIW technique has also been applied in FC feeding network. In [14], a 60 GHz 2 × 2 planar array with RHCP was fed by SIW FC feeding network. Long Zhang presented an mmWave wideband CP 4 × 4 antenna array which consisted of SIW FC feeding network and S-dipole elements in [15], and this design was improved in [16], in which a 8 × 8 array with S-dipole radiation elements was presented. A 16 × 16 and a 8 × 8 high-gain wideband CP array operating at 60 GHz were developed by using magneto-electric dipoles in [17,18], respectively, in which the radiation elements were excited by SIW FC feeding network. Yue Zhao proposed a high-gain CP scalable array operating at 60 GHz [19], and this array consisted of cross-slot-coupled patch elements fed by a SIW FC feeding network. FC feeding network may suffer from high side-lobe level, and unequal feeding network can alleviate this problem. An mmWave low side-lobe linearly polarized SIW array was designed by using wideband unequal feeding network [20]. However, wideband CP arrays fed by unequal feeding network are seldom reported. Moreover, most of the aforementioned designs contain multiple substrate layers which increase complexity in fabrication.
In this paper, a 2 × 2 mmWave wideband CP antenna array is proposed. is array has a single layer structure, and it is fed by a parallel feeding network with equal phase unequal amplitude (UA) which can improve the side-lobe level.
e array operates at 28 GHz with the impedance bandwidth (IBW) of 22.5% and ARBW of 26.1%. e peak gain of this array is 11.3 dBic. A prototype is fabricated with simple printed circuit board (PCB) technique. e simulated and measured results agree very well. e proposed mmWave wideband CP array has the advantages of simple structure, low profile, and low cost, which make it have great application prospects in the field of mmWave communications in the future. e contents of this paper are arranged as follows. e design procedure of the proposed array is presented in Section 2. e verification and measured results are discussed in Section 3. Section 4 is a conclusion of the whole paper.

Antenna Design
e geometry of the proposed mmWave wideband CP array is shown in Figure 1.
is antenna array is simply constructed on single substrate with two copper layers. e substrate is Rogers RT/Duroid 5880 with a thickness of 0.254 mm, dielectric constant of 2.2, and loss tangent of 0.0009. e wideband CP radiating element of the array consists of a planar Archimedean spiral on the top layer of the substrate and a special-shaped ring slot etched on the bottom layer. Four of these radiating elements are arranged as a 2 × 2 array. Besides, a cross slot with a width of W s is introduced into the bottom copper layer of the array to isolate each radiating element for decoupling. A MSL feeding network based on the unequal power divider is employed to feed the array, and this feeding network feeds each element with the equal phase but UA. A GCPW structure connects to the MSL feeding network to mount the coaxial connector for measurement. e diameters of the connector mounting holes on the two sides of GCPW are 2 mm. All the simulations on the proposed array are performed by Computer Simulation Technology (CST) Studio Suite 2019. Figure 2 shows the top view and bottom view of the proposed radiating element. is radiating element is based on our previous work [21], in which a planar Archimedean spiral on the top layer and a specialshaped ring slot on the bottom layer are used to generate annular currents for CP radiation. CP radiation can be generated by both archimedean spiral and special-shaped ring slot, and these two radiators couple together to significantly improve the ARBW and radiation pattern. e ARBW of the element achieves 8.35 GHz from 23.85 GHz to 32.2 GHz (29.82% referring to 28 GHz), which is beneficial to build wideband CP array. e gain of the element is 6.49 dBic at 28 GHz and is larger than 5.2 dBic within the desired frequency band. In this letter, four of such radiating elements are employed to construct a 2 × 2 antenna array. e dimensions of the radiating element are shown in Table 1.

Array Design.
Four of the radiating elements are arranged into a 2 × 2 array, and the distance between adjacent radiating elements is denoted as D e in Figure 1. D e is a critical parameter which impacts the radiating performance of the array greatly. Large D e can lead to grating lobe, while strong mutual coupling may occur between elements if D e is 2 International Journal of Antennas and Propagation too small, which will deteriorate the overall performance of the array. In this work, D e is set to be 0.6λ 0 to 0.8λ 0 , where λ 0 is the free space wavelength at the central frequency of 28 GHz which is about 10.7 mm. Figure 3 shows the reflection coefficient S 11 and axial ratio (AR) varying against D e . It can be seen that wider IBW and ARBW can be x z y θ Figure 1: Geometry of the proposed mmWave wideband CP antenna array. obtained when D e is set to be 8 mm. Meanwhile, a cross slot with width of W s is introduced into the bottom layer of the radiating elements to reduce mutual coupling [22], whose lengths are denoted as L 8 , L 9 , and L 10 in Figure 1. e proposed antenna array is fed by a parallel network consists of a four-way MSL power divider and a GCPW transmission line, as shown in Figure 1. e connector is mounted on the edge of GCPW for measurement. For characteristic impedance of 50 Ω, the feed line width W f of GCPW and MSL is set to be 0.74 mm, and the gap width g in GCPW structure is set to be 0.2 mm. e four-way power divider is composed of a two-way MSL power divider in the first stage and two parallel two-way MSL power dividers in the second stage. A quarter wavelength impedance transformer with a length of L t is applied to connect the two stages of the power divider. e wavelength λ g in the MSL is figured out to be 7.22 mm at 28 GHz, so the value of L t is set as 1.8 mm. e width of the impedance transformer is tapered to enhance the bandwidth of impedance. e lengths of remaining MSLs are denoted as L 1 , L 2 , L 3 , L 4 , L 5 , L 6 , and L 7 . e array elements are fed by the FC power divider with equal amplitude and phase at first [23]. In order to carry out comparative analysis, the results of the proposed array are simulated with and without the FC power divider. Figure 4 shows the simulated gain and AR of the proposed array with and without the FC power divider. It is obvious that this array without a power divider has wider ARBW and higher gains than those of the one with the FC power divider. e power divider not only affects the bandwidth of the proposed array but also affects its radiation patterns. To suppress the side lobe, an unequal feeding network is designed to feed each element with equal phase but UA. e characteristic impedances assigned to each microstrip line of FC feeding network and UA feeding network are shown in  Figure 1 can be determined by the following equation: where Z 0 is the characteristic impedance, ε r is the relative dielectric constant of the substrate, h is the thickness of the substrate, W is the width of the microstrip line, and t is the thickness of the copper layer which is 35 μm here. e simulated magnitude and phase of the S-parameters of the proposed four-way FC power divider and UA power divider are shown in Figures 7(a) and 7(b), respectively. In Figure 7(a), the magnitude of transmission coefficient at each of the four output ports is around −7 dB, which is slightly lower than the theoretical value of −6 dB for a fourways FC power divider due to the loss of MSL. e phases are exactly the same at each output port for the FC power divider. In Figure 7(b), the magnitudes of transmission coefficients at the four output ports are different, ranging from −5.9 dB to −8.9 dB within the desired band from 25 GHz to 31 GHz. Since the UA feeding network is not symmetry, the phase deviates a little from each other at the output ports.
is deviation is less than 20 degree within the desired band, for simplicity no extra phase compensation is applied.    International Journal of Antennas and Propagation e simulated radiation patterns of the proposed array at 31 GHz with FC feeding network and UA feeding network are shown in Figure 8. ere is a large side lobe at 60 degree in the radiation pattern, and the side lobe level is about −5 dB when using the FC feeding network (black dot dash line in Figure 8). In comparison, the side lobe level is improved to be -9 dB for UA feeding network (red solid line in Figure 8). So, the UA feeding network is employed in this letter, and its dimensions are listed in Table 2.

Experimental Results
A prototype is fabricated to verify the proposed 2 × 2 mmWave wideband CP array based on the parameters shown in Tables 1  and 2. e prototype is fabricated by using the printed circuit board (PCB) technology which has a low cost. e array is connected with a 50 Ω southwest microwave 2.92 mm end launch connector for measurement. Figure 9 shows the top view and back view of the fabricated prototype, as well as its measurement scene in an anechoic chamber. e antenna under test (AUT) transmits CP waves to the auxiliary antenna (linearly polarized horn antenna). A controlled polarizer is connected with the auxiliary antenna so that the auxiliary antenna can be rotated from 0°to 360°. e maximum received power by the auxiliary antenna is recorded as P max , and the minimum received power is recorded as P min (specify: P max and P min in dBm). With these results, AR of the proposed array can be obtained simply by the following equation (specify: AR in dB): (2) Figure 10 shows the simulated and measured reflection coefficients (S 11 ) and AR of the proposed array. e red dotted line shows that the bandwidth for S 11 lower than -10 dB is from 25.01 GHz to 31.32 GHz, which means the IBW of the proposed array is about 6.31 GHz (22.5% referring to 28 GHz). As shown, the measured ARBW, within which AR is lower than 3 dB, is from 24.47 GHz to 31.79 GHz. So, the measured ARBW is about 7.32 GHz (26.1% referring to 28 GHz). e simulated results generally agree with the measured results. e simulated and measured gains of the proposed array are illustrated in Figure 11. It shows that the measured peak gain is 11.3 dBic at 26.5 GHz, and the variations of gains are smaller than 2 dB within the operating frequency band from 25 GHz to 31 GHz. e measured gain is approximately 1 dB lower than the simulated gain, which may be caused by connector loss and inaccuracies of substrate parameters in simulation. e simulated radiation efficiency of the proposed array is 94% at 28 GHz and greater than 88% within the desired band.
Based on the coordinate system in Figure 1, the simulated and measured normalized radiation patterns at the 25 GHz, 28 GHz, and 31 GHz in the xoz plane and the yoz plane are depicted in Figures 12(a)-12(c) and 13(a)-13(c), respectively. e measured results agree well with the simulated ones. It can be observed that the primary polarization of the proposed array is LHCP at the top and RHCP at the bottom. e radiation patterns are similar within the operating band except the direction of maximum radiation in the xoz plane tiled from z-axis as the frequency increases. For example, the maximum radiation at 31 GHz points to the angle of 14°from z-axis, as shown in Figure 12(c) which is due to the asymmetry of the planar spiral. Table 3 shows the comparisons between the proposed array and some similar mmWave wideband CP arrays. e proposed array achieves the bandwidth and gain comparable to other similar designs by applying an UA feeding network, and it can be fabricated with low cost PCB technology. Since the array in this work has only one substrate layer and one feeding port, its structure is much simpler than many other designs, and it is easy to be integrated into a circuit board.   Figure 9: Photos of the top view, the back view, and the measurement system for the fabricated array.
International Journal of Antennas and Propagation

Conclusions
A low-profile mmWave wideband CP 2 × 2 antenna array is proposed. A four-way UA feeding network based on the MSL power divider is applied to feed each planar Archimedean spiral radiator. e UA feeding network leads to about 4 dB reduction in the side lobe level compared with the FC feeding network. A cross slot is introduced into the bottom layer to isolate radiating elements for decoupling. A prototype is fabricated with PCB technology, and its performances are measured and evaluated. e measured results show good agreement with the simulated results. e proposed array obtains an IBW of 22.5% and ARBW of 26.1% referring to 28 GHz. e measured peak gain of the proposed array is about 11.3 dBic. e characteristics of wideband, simple structure, low profile, and low cost make the proposed array as a potential candidate in mmWave communication applications.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.  International Journal of Antennas and Propagation 9