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[.]

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 element, reflectarray antenna,

reconfigurable reflectarray

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

moderate-cost and low loss at high frequency band Wideband unit-cells

in [9-10] are designed by using 2 and 4 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

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 2 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

Fig 1 A typical reflectarray antenna

Fig 2 Geometry of the reconfigurable unit-cell

TABLE I DIMENSIONS OF THE PROPOSED UNIT-CELL

Radiating

layer = 2.2; = 0.8; = 1.08; = 0.2

Phase shifter

layer = 2.4; = 2.2; = 0.6; = 0.3

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 2 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

(a)

(b) Fig 3 Reflection phase (a) and magnitude (b) of the unit-cell in two different phase states

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 3

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 4 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°

Fig 4 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)

Where is propagation constant in free space, is the

distance between the feed source to the unit-cell

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

(2)

The radiation patterns at 28 GHz for different beam

scenarios are provided in Fig 5 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 2 dB

Fig 5 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

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

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