8 × 2-Element 60-GHz-Band Circularly Polarized Post-Wall Waveguide Slot Array Antenna Loaded With Dipoles

In this paper, an <inline-formula> <tex-math notation="LaTeX">$8\times 2$ </tex-math></inline-formula>-element 60-GHz band circularly polarized post-wall waveguide slot array antenna loaded with dipoles is proposed. The antenna is composed of a feeding circuit, radiating slots on radiating waveguides, and dipoles for polarization conversion. By integrating the feeding circuit and the radiating waveguides into a single dielectric layer, the total number of dielectric layers is reduced to two which greatly increases ease of mass production. The feeding circuit divides power into two and feeds the two radiating waveguides from their center. <inline-formula> <tex-math notation="LaTeX">$8\times 2$ </tex-math></inline-formula> radiating elements are composed of a slot and a dipole placed on the radiating waveguides. The elements are designed by standing-wave excitation. The antenna is fabricated by printed circuit board technology and its dimensions are <inline-formula> <tex-math notation="LaTeX">$44.5\times 20.0\times2.16$ </tex-math></inline-formula> mm<sup>3</sup>. The measured results show a 2.1-dB axial ratio, 15.6-dBi gain, and 57.2% efficiency at 61.5 GHz. The proposed antenna realizes a circular polarized array antenna with the minimum number of dielectric layers and electric performance comparable to conventional antennas.


I. INTRODUCTION
The millimeter-wave band enables high data rate wireless communication because wider frequency bands are available than in the microwave bands [1]. The wireless communication is limited to line of sight because of low diffraction and high propagation losses. Many applications have been proposed for wireless communication between a base station and a user terminal or between user terminals [2], [3]. Antennas for base stations are required to be simple structures in terms of mechanical stability and productivity.
High gain antennas are used for base stations in the millimeter-wave band to compensate for the high propagation losses. Microstrip line patch array antennas are widely used for high gain planar antennas in the microwave band. However, they suffer from high transmission losses in the millimeter-wave band [4]. Post wall waveguides [5], also known as substrate integrated waveguides (SIW) [6], feeding circuits are widely used in the millimeter-wave band because of their low transmission losses [7].
The associate editor coordinating the review of this manuscript and approving it for publication was Davide Comite . Circular polarization (CP) is adopted in antennas of base stations in order to reduce polarization mismatch between a base station and the user equipment [3]. A hollow waveguide fed circularly polarized slot array with printed dipoles [8], [9] or hexagonal apertures [10] offer low transmission losses but is not suitable for mass production. Low temperature cofired ceramics (LTCC) based SIW CP array antennas realize wide bandwidths using 20 dielectric and 8 metal layers [11]. The measurement results are degraded because of fabrication tolerances, and the number of layers should be decreased as far as possible to reduce the complexity and fabrication costs. An ellipse-loaded circular slot array for the 60-GHz band realizes measured wideband and high-efficiency characteristics [12]. The slot array antenna [12] is fabricated by a printed circuit board (PCB) process and suitable for mass production. However, this antenna requires a reflector which makes the structure complex. A circularly polarized slotted SIW cavity antenna has a simple feeding circuit [13], [14] and 4 × 4 elements are placed on the cavity and fed by a single coaxial probe. However, it requires an air spacer between the slots and dipoles which weaken the physical strength and complicates production. Other shapes of polarization conversion elements have been investigated such as stacked curl [15], ME-dipole [16], [17], double spiral [18], helical [19], and dielectric resonator antennas [20]. However, they require more than three substrate layers.
In this paper, we propose a 60-GHz-band circular polarized array antenna with a much simpler structure than conventional ones [8]- [20] and which can be fabricated by conventional PCB technology. The antenna is composed of two dielectric layers and four metal layers. It does not require spacers or reflectors. The target bandwidth is channel two of IEEE 802.11ad [1], which is 59.5 -61.5 GHz. The antenna is suitable for base stations and enables more practical mass production because of the simple structure and the fabrication by PCB technology. The design method and characterization of the fabricated antenna are detailed.

II. ANTENNA STRUCTURE
The antenna is composed of two copper-clad laminated substrates as shown in Fig. 1 (a). The substrate in the lower layer has a feeding circuit and radiating waveguides. The other substrate has dipoles. The substrates are made of poly tetra fluoro ethylene (PTFE) and have a 2.16 dielectric constant and 0.0005 loss tangent with 29-µm thick copper layer. The two substrates are bonded by a bonding film with dielectric constant, loss tangent, and thickness of 2.28, 0.003, and 67 µm, respectively. The antenna is fed by a rectangular waveguide WR-15 from the back. The feeding circuit divides the power into two and feed the center of the radiating waveguides. Slots are placed on the broad wall of the radiating waveguide with constant spacing in the x-axis direction and excited with the same amplitude and phase. Dipoles are placed on each slot as shown in Fig. 1 (b) to change polarization from linear to circular. Around the dipoles there are post walls to prevent unwanted surface waves.

III. DESIGN
The radiation and the feeding part of the antenna are designed separately and then the designed parts are combined. The design frequency is 60.5 GHz and the target bandwidth is channel two of IEEE 802.11ad, 59.5 -61.5 GHz [1].

A. RADIATION PART
First a desired admittance value of a single radiating element is determined and then the actual antenna dimensions are designed. Because a longitudinal slot on a waveguide is expressed by a shunt admittance element [21], the single radiating element is expressed by a single shunt admittance element as shown in Fig. 2 (a). The entire 8 × 2-element array is expressed by eight admittance elements as shown in Fig. 2 (b). The admittance value is determined for uniform excitation and matching. Because of the symmetricity of the structure, a quarter of the array, a 4 × 1-element subarray, is designed. The equivalent circuit of the four-element array is expressed by four cascaded shunt elements with normalized admittanceŶ and short termination as shown in Fig 2 (c). The array is designed by standing wave excitation [22], each element is placed with a constant spacing of half guided wavelength. To excite the elements uniformly, all of the admittance and the dimensions of the four elements are identical. Because the elements are placed with the half guided wavelength separation, the equivalent circuit can be simplified to a single shunt element of 4Ŷ as shown in Fig. 2 (d). The resulting desired admittance for the uniform excitation and matching isŶ = 1/4. A 1×2-element subarray is used for designing the radiating element dimensions to include mutual coupling between the elements as shown in Fig. 3. The dipole and the slot are shifted from the center of the waveguide to control the radiation power. The dipole is rotated by θ d from the waveguide axis to change the polarization from linear to circular. The width of the rectangular waveguide is determined so that the propagation constants of both the rectangular waveguide and post-wall waveguide are the same using the approximated equations [23], and the guided wavelength is 5.48 mm.
The design of the radiation part is as follows: Step 1: Change the length and the angle of the dipole so that a low axial ratio is obtained. The frequency characteristics of the axial ratio can be controlled by the dimensions of the dipole as shown in Fig. 4 Step 2: Change the slot length to control the imaginary part of the normalized admittance as shown in Fig. 5 (a). The slot length affects both the real and imaginary part of the normalized admittance.
Step 3: Change the shift amount of the slot to change the real part of the normalized admittance as shown in Fig. 5 (b). The shift amount has the smaller effect on the imaginary part of the admittance.   Step 4: Repeat steps 1 to 3 until a low axial ratio and desired normalized admittance are realized. The designed dimensions are shown in Table 1.

B. FEEDING PART
The feeding part consists of four components: a waveguide to the post-wall waveguide transition [24], a T-junction [25], an H-bend [26], and a τ -junction [27], [28] as shown in Fig. 6 (a). Each component is separately designed as shown in Fig. 6 (b) -(d) except for the waveguide to post-wall transition and the T-junction. The designed parameters and the reflections are shown in Table 2 and Fig 7, respectively. The reflection is below −10 dB over the target bandwidth.

C. FULL STRUCTURE
The designed radiation and feeding part are combined and the complete structure shown in Fig. 8 is analyzed. The designed reflection, gain, and axial ratio are shown in Figs. 9 -11, respectively. The reflection is below −15 dB at the target frequencies. The gain is 16.9 dBi and the antenna efficiency is  75.1% at 60.5 GHz with a 0.54-dB conductor loss and a 0.20-dB dielectric loss. The axial ratio is below 1.3 dB over the target bandwidth. The 1-dB gain down bandwidth is 5.6 GHz, which is wider than the two-channel bandwidth of the IEEE 802.11ad.

IV. MEASUREMENTS
The designed antenna was fabricated by PCB processes as shown in Fig. 12. The reflection and radiation characteristics of the fabricated antenna were characterized by a vector network analyzer and a near-field antenna measurement system in an anechoic chamber.     The frequency characteristics of the measured reflection are similar to the simulated ones and below −14.5 dB over the  target bandwidth as shown in Fig. 9. The measured frequency characteristics of the realized gain are better than 15.9 dBi in the target bandwidth as shown in Fig. 10. The difference between the realized gain and gain corresponds to the return loss [29] and the measured realized gain at 62.7 GHz is degraded because of the return loss. However, the measured frequency characteristics of the directivity is 1.0 dB lower than the simulation at the design frequency and the 1-dB down directivity bandwidth becomes narrower, from 9.2% to 7.7%. It is considered that the degradation of the directivity is a result of misalignment of the substrates in the y-direction. The simulated directivity and realized gain with misalignment of the dielectric for both the x-and y-directions is shown in Fig. 13 and Fig. 14, respectively. The x-direction misalignment has less of an effect on the directivity. However, the ydirection misalignment degrades the directivity more than that in the x-direction and a 150-µm misalignment decreases directivity by 1 dB below the designed value. The tendencies of the realized gain are similar to those of the directivity. The y-direction misalignment has a larger influence to the realized gain. The realized gain with a 100-µm misalignment shows similar characteristics to that of the measured realized gain.
The frequency characteristics of the axial ratio were degraded by 1 dB at the design frequency and below 2.1 dB at the target bandwidth as shown in Fig. 11. The measured polarization mismatch is 0.3-dB higher than the designed polarization at the design frequency. The spinning linear patterns at 59.5, 60.5, and 61.5 GHz were measured as shown in Fig. 15 and higher sidelobe levels are observed at ϕ = 0 •  because of the fabrication tolerance. The oscillation of the measured radiation patterns results from the spinning linear measurement method [30] and its amplitude corresponds to the axial ratio. Radiation patterns with the y-direction misalignment are simulated as shown in Fig. 16. The y-direction   allows the conclusion that the measured higher side lobe levels result from the y-direction misalignment. Table 3 details the designed antenna with previously published CP array antennas at 20-GHz and higher bands as shown in Table 3. The antennas [9], [14], and [12] require additional mechanical components decreasing ease of mass production. The other antennas in Table 3 are fabricated by LTCC or PCB, which are suitable for mass production. For mass production, the number of dielectric and metal layers should be the smallest possible. The proposed antenna realized the minimum number of dielectric layers with a comparable axial ratio, gain, and efficiency.

V. CONCLUSIONS
This paper presents an 8 × 2-element 60-GHz band circularly polarized post-wall waveguide slot array antenna loaded with dipoles. The antenna is composed of a feeding part, a radiating part, and a polarization conversion part. The integration of the feeding part and the radiating part into a single dielectric enables reduction of the total number of dielectric layers. The measured results show a 2.1-dB axial ratio, 15.6-dBi gain, and 57.2% efficiency at 61.5 GHz. The antenna consists of two dielectric and four metal layers and is fabricated by PCB. The number of dielectric layers is the minimum among the published investigations and suitable for mass production.