Ka band reflectarray unit cell with 1 bit digital phase resolution

Ka Band Reflectarray Unit Cell with 1 Bit Digital Phase Resolution Ka band Reflectarray Unit cell with 1 bit Digital Phase Resolution Minh Thien Nguyen School of Electrical Engineering International U[.]

2021 8th NAFOSTED Conference on Information and Computer Science (NICS) Ka-band Reflectarray Unit-cell with 1-bit Digital Phase Resolution Minh Thien Nguyen School of Electrical Engineering International University Vietnam National University Ho Chi Minh City, Vietnam nmthien@hcmiu.edu.vn Van Su Tran School of Electrical Engineering International University Vietnam National University Ho Chi Minh City, Vietnam tvsu@hcmiu.edu.vn Binh Duong Nguyen School of Electrical Engineering International University Vietnam National University Ho Chi Minh City, Vietnam nbduong@hcmiu.edu.vn Abstract— A simple and low-profile reconfigurable unit-cell design for Ka band reconfigurable reflectarray antennas is presented in this paper The unit-cell is based on a single substrate and a ground plane that allows a simple fabrication process One p-i-n diode is used to control the reflection phase shift with a step of 180° The optimization of the unit-cell structure is carried out with full wave simulation software Radiation characteristics of a 10x10-element reflectarray is also validated in Ka- frequency band Simulation results show that the unit cell exhibits a good 1-bit phase control within a wide bandwidth and the array achieves an excellent beam-steering capability with low loss and wide scan angle Keywords—Reflectarray reconfigurable reflectarray element, reflectarray antenna, Fig A typical reflectarray antenna I INTRODUCTION In the recent decades, reflectarray antennas have been highlighted as high gain and low-profile antennas for many applications such as radars, remote sensing, satellite and point-to-point communications Reflectarray antennas represent a very attractive options to replace conventional phased arrays and parabolic reflectors Reflectarray antennas have several benefits such as lighter weight, lower profile and less expensive than a parabolic reflector which is typically bulky and lossy due to high refraction index materials A reflectarray antenna, as depicted in Fig 1, consists of array of unit-cells and a feed source The unit-cells receive the spherical waves from the feed source and reflect it back into free space The reflected beam is collimated by adjusting the reflection phase of each reflectarray unit-cell Various approaches have been introduced to vary the reflection phase of the unit-cells such as: varying the size of radiating patch elements [1-2], rotating the patch elements [3], varying the delay lines coupled to the radiating elements [4-5] Recently, electronically reconfigurable reflectarray antennas are attractive solutions for wireless applications that demand wideband and beam-steering features The key point to reconfigure the antenna pattern is the capability of controlling the phase shift of each unit-cell Several works have provided different solutions by integrating the tunable components into the unit-cell for the electronic reconfigurability In some designs, the phase shift of the unit-cells is controlled by using varactors [6], RF-MEMS [7-8] or p-i-n diodes [9-12] The p-i-n diodes are more widely adopted components in reconfigurable reflectarray designs thanks to their moderatecost and low loss at high frequency band Wideband unit-cells in [9-10] are designed by using and p-i-n diodes that are inserted on the radiating patch elements to modify the surface current, hence provide 1-bit phase resolution Patch elements 978-1-6654-1001-4/21/$31.00 ©2021 IEEE coupled with delay lines could be inserted with p-i-n diodes [11-12] to adjust the electrical length of the delay lines to vary the reflection phase shift Those designs could effectively control the phase shift through controlling the p-i-n diodes, yet multiple stack structures make the fabrication more difficult and costly In the paper, we propose a design of a wide band, 1-bit unit-cell for beam-steering reflectarray antennas, working at Ka-band The proposed unit-cell can provide two phase states with a step of 180° in a wide frequency range by employing a single p-i-n diode Full wave simulations have been conducted to show the reflection coefficients of the unit-cells and validate the radiation characteristics of a fully populated beam-steering reflectarray antenna II UNIT-CELL DESIGN A Unit-cell Geometry and Principles of Operation This section describes in detail the design and operation of the 1-bit reconfigurable reflectarray unit-cell It is designed to operate with linear polarization where the electric field is oriented along the x-axis as shown in Fig The unit-cell structure consists of a radiating element printed on the top of the substrate, a phase shifter element, and a ground plane The radiating element is made of two half-ring patches The substrate is Roger substrate RO5870 ( = 2.33, = 0.0012, ℎ = 0.78 ) On the bottom layer is the phase shifter element which has a shape of an annual slot which is loaded with a rectangular gap A metallic plate acting as ground plane is placed beneath the substrate with a distance of 0.5 mm The size of the unit-cell is 5.2 mm, corresponding to 0.48 and the total thickness of the unit-cell is 1.28mm, corresponding to 0.12 , where is the wavelength in free space at 28 GHz Detail dimensions are shown in Table I 555 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) (a) Fig Geometry of the reconfigurable unit-cell TABLE I Element Radiating layer Phase shifter layer Substrate Air gap Cell size DIMENSIONS OF THE PROPOSED UNIT-CELL Dimensions (unit: mm) = 2.2; = 0.8; = 1.08; = 0.2 = 2.4; = 2.2; = 0.6; = 0.3 ℎ = 0.78 (b) Fig Reflection phase (a) and magnitude (b) of the unit-cell in two different phase states = 0.5 a = 5.2 In the unit-cell, the phase shifter layer has a shape of an annual slot loaded by a rectangular gap to control the reflection phase shift through controlling the p-i-n diode that is inserted into to the rectangular gap The geometry of the phase shifter layer has been investigated for X-band transmitarray unit-cell in [13] In this work, the unit-cell is optimized for operating at Ka-band The annual slot is designed so that it acts as a reflecting layer when the p-i-n diode is ON When the diode is OFF, the annual slot allows to pass the waves received from radiating element to the ground plane which is also another reflecting layer The combination between the radiating element, the annual slot and the ground plane allows a difference of phase compared to the case when diode is ON The phase difference can be adjusted by distance from the substrate to the ground plane Therefore, two different phase states can be obtained B DC Biasing Topology for the P-i-n Diode In order to bias the diode, a DC biasing network must be designed As shown in Fig 2, the bottom layer is loaded with an additional gap to separate the ring slot into two fractions The larger part is connected to GND pole of the power supply by a small strip-line bypassing the annual slot The smaller fragment is connected to +Vcc through a metallized via hole linked to the top layer The additional separation gap is placed orthogonally with respect to the rectangular slot loaded with the diode Three 0402-standard-size capacitors of pF are also mounted upon the gap to ensure the RF current can flow through The positions of the separation gap and the capacitors are optimized so that they have a minimal impact on the operation of the annual slot C Frequency Response of the Unit-Cell A full wave simulation software HFSS is used to optimize and simulate the proposed unit-cell In general, a reflectarray antenna is a planar-periodic structure In the simulation, lumped components are chosen to model the p-i-n diode For ON state, the diode is simulated as a 4Ω resistor For OFF state, a capacitor of 30 fF is used The frequency response of the unit-cell presented in Fig is simulated considering a plane wave with normal incidence As can be seen, the unit-cell responses in two reflection phase states as the diode is switched ON and OFF The phase difference is about 180° and the phase curves are maintained almost parallel within a wide bandwidth of 10 GHz, from 24 GHz to 34 GHz Regarding to the reflection magnitude, the loss within operative bandwidth is low in both phase states It is better than 1.5 dB in the state when the diode is ON and is better than 0.1 dB when the diode is OFF III UNIT-CELL VALIDATION FOR A 10X10 BEAM-STEERING REFLECTARRAY ANTENNA To validate the operation of the reconfigurable unit-cell, a fully populated 100-element reflectarray antenna is simulated The radiation characteristics of the reflectarray antenna in different beam scenarios could show us the performance of the unit-cells regarding to the beam-steering capability of the array The simulated reflectarray antenna system is shown in Fig The size of the array antenna is 52x52 mm2, corresponding to 4.8 x4.8 at 28 GHz The feed source which is a small pyramidal horn antenna with the aperture of 20x14 mm2, is placed at a focal length of 60 mm, corresponding to a ratio F/D of 0.84 The feed source is tilted at 14° with respect to the z-axis to prevent blocking the main beam of = 0°, φ = 0° 556 2021 8th NAFOSTED Conference on Information and Computer Science (NICS) two phase states by using a single p-i-n diode Simulated results have shown that the unit-cell has a linear phase response in both phase states and a low reflection loss for a large bandwidth of 10 GHz The reflectarray antenna constructed from the unit-cells has also simulated and shown good radiation characteristics in different beam-steering angles REFERENCES [1] [2] [3] Fig A 100-element reflectarray antenna model in simulation The phase shift that required at each unit-cell to focus the main beam in a given direction ( , φ) is determined by (1) = ( − ( cosφ + y ) ) Where is propagation constant in free space, distance between the feed source to the unit-cell (1) is the For an ideal reflectarray design, the reflection phase must be adjusted in each unit-cell to match these phases obtained by (1) However, 1-bit reconfigurable reflectarray antenna is based on the unit-cells that merely provide two phase states Therefore, the real phase ψ of each unit-cell must be quantized using (2) ψ = 0° − 90 < i < 90 180° 90 ≤ i ≤ 270 [4] [5] [6] [7] (2) [8] The radiation patterns at 28 GHz for different beam scenarios are provided in Fig For the configuration of broadside main beam, the maximum directivity reaches 18 dB while the side lobe level is lower than 10 dB compared to main lobe level The reflectarray antenna could steer the main beam up to ±40 with very low scan loss of circa 0.5 dB comparing to the level of broadside beam For the configurations when the main beam is titled at ±40°, the scan loss increases slightly to dB [9] [10] [11] [12] [13] Fig Radiation patterns at 28 GHz for different main beam angles IV CONCLUSION The design of a unit-cell for 1-bit reconfigurable reflectarray antenna has been simulated The unit-cell structure, which is simple and easy to fabricate, could provide 557 S D Targonski and D M Pozar, "Analysis and design of a microstrip reflectarray using patches of variable size," Electr Lett Vol.29, No pp 657-658, April 1993 D M Pozar and S D Targonski, "A microstrip reflectarray using crossed dipoles," IEEE Antennas and Propagation Society International Symposium, pp 1008-1011, June 1998 J Huang and R J Pogorzelski, "A Ka-band microstrip reflectarray with elements having variable rotation angles," in IEEE Trans Antennas and Propagat., vol 46, no 5, pp 650-656, May 1998 D.-C Chang and M-C Huang, "Multiple-polarization microstrip reflectarray antenna with high efficiency and low cross-polarization," in IEEE Trans Antennas and Propagat, vol 43, no 8, pp 829-834, Aug 1995 E Carrasco, M Barba and J A Encinar, "Reflectarray Element Based on Aperture-Coupled Patches With Slots and Lines of Variable Length," in IEEE Trans Antennas and Propagat., vol 55, no 3, pp 820-825, March 2007 S V Hum, M Okoniewski, and R J Davies, “Modeling and design of electronically tunable reflectarrays," IEEE Trans Antennas Propag., vol.55, no 8, pp 2200-2210, Aug 2007 E Carrasco, M Barba, B Reig, C Dieppedale, and J A Encinar, “Characterization of a reflectarray gathered element with electronic control using ohmic RF-MEMS and patches aperture-coupled to a delayline,” IEEE Trans Antennas Propag., vol 60, no 9, pp 4190– 4201,Sep 2012 O Bayraktar, O A Civi, and T Akin, "Beam switching reflectarray monolithically integrated with RF MEMS switches," IEEE Trans Antennas Propag., vol 60, No 2, pp 854-862, Feb.2012 S Montori, F Cacciamani, R.V Gatti, R Sorrentino, G Arista, C Tienda, J A Encinar, G Toso., “A transportable reflectarray antennafor satellite Kuband emergency communications," IEEE Trans Antennas Propag., vol 63, no 4, pp 1393-1407, April 2015 M T Zhang et al., “Design of novel reconfigurable reflectarrays with single-bit phase resolution for Ku-band satellite antenna applications," IEEE Trans Antennas Propag., vol 64, no 5, pp 1634-1641, May 2016 B D Nguyen, K T Pham, V.-S Tran, L Mai, N Yonemoto, A Kohmura, and S Futatsumori, “Electronically tunable reflectarray element based on C-patch coupled to delay line,” Electronics Letters, vol 50, no 16, pp 1114–1116, Jul 2014 Hongjing Xu, Shenheng Xu, Fan Yang and Maokun Li, “Design and Experiment of a Dual-band 1-bit Reconfigurable Reflectarray Antenna with Independent Large-angle Beam Scanning Capability,” IEEE Antennas Wirel Propag Lett, vol.19, pp 1896-1900, Nov 2020 B.D Nguyen, C Phichot: “Unit-Cell loaded with PIN diodes for 1-bit Linearly Polarized Reconfigurable Transmitarrays”, IEEE Antennas Wirel Propag Lett, vol.18, pp 98-102, Jan 2019  ... to the unit- cell (1) is the For an ideal reflectarray design, the reflection phase must be adjusted in each unit- cell to match these phases obtained by (1) However, 1- bit reconfigurable reflectarray. .. increases slightly to dB [9] [10 ] [11 ] [12 ] [13 ] Fig Radiation patterns at 28 GHz for different main beam angles IV CONCLUSION The design of a unit- cell for 1- bit reconfigurable reflectarray antenna has... gap The geometry of the phase shifter layer has been investigated for X -band transmitarray unit- cell in [13 ] In this work, the unit- cell is optimized for operating at Ka- band The annual slot is