1. Introduction

The demand for high-speed wireless data transmission is growing in modern times for applications such as 5G, IoT, radar, and satellite communications ⁽¹⁾. These demands have necessitated the need to move to higher frequency bands such as W-band, which offer huge bandwidth to increase channel capacity ⁽²⁾. These higher frequency bands however have disadvantages such as high path loss, limited coverage, and suffer from atmospheric attenuations ⁽³⁾. High gain antennas such as parabolic reflectors, lens antennas, and microstrip phased arrays are usually employed in this band to solve the aforementioned problems. Although highly efficient radiators, the parabolic reflectors are generally bulky and difficult to manufacture at higher frequencies due to their curved surface. Microstrip arrays on the other hand are of low profile and lightweight, their feeding networks based on microstrip lines suffer from serious losses making them less efficient.

A W-band reflectarray antenna using a metal mesh grid unit cell is therefore proposed in this paper. Just like all reflectarrays, this antenna combines the advantages of the parabolic and phased array. It offers the advantages of low profile, low cost, high gain, surface mount ability, and high efficiency ^(4,5).

그림. 1. 제안한 반사배열 안테나 구조

Fig. 1. Configuration of the proposed reflectarray antenna

2. Reflectarray Antenna Design and Analysis

The electromagnetic waves emanating from the feed arrive at each element on the aperture at different times as shown in Fig 1and this is referred to as the spatial phase delay ⁽⁶⁾. The elements on the aperture surface re-radiate the incident waves from the feed antenna. To compensate for these phase delays on the aperture, several unit cell structures have been proposed, some of which are, patch with variable size, parallel dipoles, rings with phase delay delays ⁽⁷⁾.

그림. 2. Floquet port 기반 (a) 모의실험환경과 (b) 다양한 금속 그물 망 단위 구조

Fig. 2. (a) Floquet port-based simulation environment and (b) test geometry of metal grid mesh unit-cell

The proposed variable size metal grid mesh patch as the radiating unit cell element is shown in Fig 2. The design process of the proposed antenna is divided into 3 parts, the feed antenna design, the unit cell design, and the aperture design. A 15 dBi pyramidal horn antenna called FMWAN1018 produced by Fairview Microwave Co. Ltd. is selected as the feed antenna. The unit cell structure is set up using floquet boundary conditions as shown in Fig 2a. For fabrication convenience, the metal grid mesh patch is printed above a 1.57 mm-thick Rogers RT/Duroid 5880 with a dielectric constant of 2.2. The unit cell is of size , 1.875 mm which is about half wavelength at 80 GHz. The width of the metal grid, is selected as 0.01 mm for the unit-cell design. The length of the square metal mesh grid, is varied from 0.2 mm to 1.6 mm. The slot size is represented by s. Different mesh grid cells are tested as shown in Fig 2b. The total slot number, N is chosen as 0, 1, 4, and 16. At N = 0, the unit cell is a full metal patch. The characteristics of the proposed unit cell structure are tested and shown in Fig 3. The reflection coefficient of the various unit cell configurations is shown in Fig 3a.

그림. 3. 금속 그물 망 단위 소자의 구조변화에 대한 (a) 반사크기와 (b) 반사위상 분석결과

Fig. 3. simulated results of the (a) reflection magnitude and (b) phase for geometry variations of metal grid mesh unit-cell

A maximum reflection loss of around 0.3 dB occurs when N is set as 1. However, the overall reflection loss due to the unit cells is very small and will not have a significant effect on the overall antenna performance.

그림. 4. 금속 그물 망 단위구조 변화에 대한 반사배열 안테나 설계

Fig. 4. Geometry of the reflectarray antenna with metal grid mesh unit-cell variations

In Fig 3b, the reflection phase for the various configurations of the proposed unit cell is presented. A phase ranges of 342.2°, 324.9°, and 305.1° are obtained for N = 1, 4, and 16, respectively. A phase range of over 300° is obtained for the various metal mesh configurations, which is sufficient for a reflectarray antenna design.

For the aperture design, that is, = 30 mm, which is 8 wavelengths at 80 GHz is used. The feed antenna is placed 53 mm away from the center of the square aperture. The phase delay distribution on the aperture surface is compensated for by the unit cells reflection phase. The table below shows the phase of the unit cells used for the phase compensation. For a full metal reflectarray antenna, that is N = 0, shown in Fig 4a, 5 unit cells of size, from 0.9 mm to 1.3 mm were selected and their reflection phase are shown in Table 1. For the proposed reflectarray, 2, 3, and 4 unit cell sizes were selected for N = 1, 4 and 16, respectively. The configuration and the results of the various reflectarray with their respective unit cells is shown in Fig 4 and Fig 5, respectively. A maximum gain of 23.19 dBi is achieved for a full metal unit cell reflectarray antenna at the operating frequency, while gains of 20.01, 22.82, 23.14 dBi are obtained for reflectarrays with N = 1, 4, and 16, respectively, as seen from Fig 5a. The cross-polarization levels for the various configurations shown in Fig 5b are over 20 dB with the lowest recorded for N = 1, while the highest is recorded for N = 0. The cross-polarization levels are fairly good across all configurations. The sidelobe levels (SLL) are seen in Fig 5c with N = 1 recording the lowest, that is about 7.0 dB. The half -power beamwidth (HPBW) for the reflectarray configurations is almost constant with values around 5° ± 0.2° at the operating frequency as displayed in Fig 5d. At N = 0 and 16, the antenna achieves very good performance, however at the cost of many phase steps (i.e., unit cells) as compared to the other configurations.

The reflectarray configuration with N = 1, uses the least number of unit cells, however, this configuration records the lowest gain, cross-polarization level, SLL, and HPBW at the operating frequency. This configuration, even though ideal for the least number of unit cell sizes, shows poor antenna performance among the other configurations. The reflectarray configuration with N = 4, achieves a fairly good antenna performance as the configurations with N = 0 and 16 while using a small number of unit cells. This antenna configuration, achieves a gain of 22.82 dBi as discussed earlier, a 39.1 dB crosspolarization level, a 9.5 dB SLL, and a 5° HPBW. To design reflectarray with a smaller number of unit cells (2-state), the configuration with N = 4 is selected and tested with various unit cell size combinations.

3. Results and Discussions

As discussed in the previous section, the reflectarray configuration with N = 4 is selected and a reflectarray with 2-state unit cell size is designed. The optimizing results are shown in Fig 6. The design of the 2-state reflectarray is achieved by the combination of two unit cell sizes.

그림. 5. 금속 그물 망 구조 변화별 반사배열 모의실험 결과. (a) 안테나이득, (b) 교차편파비, (c) 부엽크기비, (d) 반치각

Fig. 5. Simulated results of the reflectarray antenna with variations of metal grid mesh geometry. (a) Antenna gain, (b) cross-pol. ratio, (c) side-lobe level, and (d) half-power beamwidth

그림. 6. 2상태 및 N=4 조건을 만족하는 금속 그물 망 단위구조를 이용한 반사배열 안테나 특성 분석결과. (a) 안테나이득, (b) 부엽크기비, (c) 교차편파비

Fig. 6. Parametric analysis results of the reflectarray antenna based on 2-states and N=4 metal grid mesh unit-cell. (a) Antenna gain, (b) side-lobe level, and (c) cross-pol. ratio

The following unit cell sizes, of from 0.7 mm to 1.3 mm are selected and tested. The 2-state reflectarray antennas gain for the various cell size combinations is shown in Fig 6a, while the sidelobe levels and the cross-polarization levels are shown in Fig 6b and Fig 6c, respectively.

그림. 7. 제안한 반사배열 안테나의 반사계수

Fig. 7. Reflection coefficient of the proposed reflectarray antenna

그림. 8. 제안한 반사배열 안테나의 80 GHz에서의 원역장 방사패턴. (a) E-평면, (b) H-평면

Fig. 8. Far-field radiation patterns of the proposed reflectarray antenna at 80 GHz of test frequency. (a) E-plane and (b) H-plane

A maximum gain of 23.17 dBi, a sidelobe level of 5.9 dB, and a crosspolarization level of 33.11 dB are achieved for = 0.8 mm and 0.9 mm unit cell combination. A gain of 22.87 dBi, a sidelobe level of 9.6 dB, and a cross-polarization level of 36.61 dB are obtained with the unit cell combination, p = 0.9 mm and 1.0 mm. Also, a gain of 22.23 dBi, a sidelobe level of 10.2 dB, and a crosspolarization level of 31.8 dB are achieved for p = 0.9 mm and 1.10 mm unit cell combination. The 2-state unit cell combination of p = 0.9 mm and 1.1 mm, is selected and used for the final design because an appreciable gain is obtained at a good sidelobe level and a fairly good cross-polarization level as can be seen from Fig 6.

Using the aforementioned 2-state unit cell combination, the reflection coefficient of the proposed reflectarray is shown in Fig 7. The reflection coefficient is well below –10 dB across wide bandwidths from 70 GHz to 90 GHz, this indicates that the proposed antenna operates across a wide band. Also, the radiation pattern of the proposed antenna is shown in Fig 8.

A half-power beamwidth of 4.9°, a sidelobe level of 10.2 dB as well as a cross-polarization level over 50 dB are obtained in the E-plane as shown in Fig 8a. Likewise, a half-power beamwidth of 4.9°, a sidelobe level of 9.5 dB, as well as a cross-polarization level of around 31.8 dB is achieved in the H-plane as shown in Fig 8b.

4. Conclusion

The The reflectarray antenna for W-band communication system operating at 80 GHz has been proposed. This antenna structure is based on a square aperture illuminated by a pyramidal feed horn antenna. The aperture surface is made up of 2-state metal grid mesh unit cells with sizes of 0.9 mm and 1.1 mm forming a total number of 193 elements that re-radiate the waves from the feed antenna. A gain of 22.23 dBi is realized at the operating frequency while a half-power beamwidth of 4.9° is obtained in both the E-plane and H-plane. A sidelobe level greater than 9.5 dB in both planes as well as a crosspolarization level over 31.8 dB is achieved. The antenna operates over a wide bandwidth from 70 to 90 GHz with a return loss well below –10 dB. The proposed reflectarray antenna using a 2-state metal grid mesh unit cells achieves a fairly good antenna performance and can therefore be used for W-band communication system.