DESIGNING OF A SMALL WEARABLE CONFORMAL PHASED ARRAY ANTENNA FOR WIRELESS COMMUNICATIONS By Sayan Roy Major Advisor: Dr. Benjamin D. Braaten Dept. of ECE, NDSU, Fargo, ND, USA

Contents Introduction Defining the Problem Phased Array Antenna Realization of Conformal Phased Array Antenna Designing of Phased Array Antenna Test Platform Scanning Properties of Phased Array Antenna Test Platform Four Element SELFLEX Array Design Scanning Properties of SELFLEX Array Conclusion

Introduction to Array Antenna Conformal Antenna Phased Array Antenna

Antenna For any communication device, an antenna system serves the purpose for external communication wirelessly.

Today’s Antenna Systems

Array Antenna Array means a collection of similar entities. Set of individual antenna elements connected together to behave as a single unit Advantages Higher Gain Beam Steering Capability Reliable Higher SNR

Beam steered 45° from Broadside direction Beam Steering In any Antenna system, the transmitting or receiving signal has two attributes: Amplitude (A) and Phase (φ). Beam Steering can be achieved in an array antenna by changing the progressive phase differences between antenna elements. Beam steered 45° from Broadside direction

Beam Steering of a Patch Array Antenna

Conformality Conformality can be described as a map projection which has the property of preserving relative angles over small scales. In Mathematics, a conformal map is a function which preserves angles.

Conformal Antennas Often mechanical design of a communication system requires that the associated antenna should be mounted on a curved surface. Applications Aerospace Designs Wearable Antenna Spacesuit Mobile Devices For last couple of years, designers have been showing interest in simulating conformal antenna performance to optimize antenna parameters in presence of conformal surface.

Defining the Problem

Relation between Conformality and Beam Steering A conformal surface changes its curvature with time and may be planar or non-planar. When an antenna system lies on a planar conformal surface, the field pattern of the antenna behaves normally.

Relation between Conformality and Beam Steering (cont.) However, when the surface of the antenna becomes non-planar, the performance of the antenna starts to degrade.

Relation between Conformality and Beam Steering (cont.) Beam Steering concept can be implemented to recover the field pattern of the antenna system by proper correction in relative phases between elements of the array. This type of antenna is known as Phased Array Antenna.

Defining the Problem: Can we recover the radiation pattern of a conformal array ?

Phased Array Antenna. Defining Co-ordinate. Theory of Array Factor Phased Array Antenna Defining Co-ordinate Theory of Array Factor Concept of Phase Scanning Phase Compensation Technique of a Conformal Array Antenna

Defining Co-ordinate (θ,φ) is the direction in space

Array Factor (AF) The array factor due to isotropic point sources is the weighted sum of the signals received by the elements. Mathematically, where N = number of elements is the complex weight for element n k=2π/λ is the wave number (xn, yn, zn) is the location of element n

Array Factor (AF) (cont.) Unique for each array Depends on number of elements, relative magnitude and phase of current on each element, relative inter-element spacing and geometrical orientation of the elements. Use Pattern Multiplication Rule If the response of a single element of a linear array is 𝑬 𝒔 then the total response of the array 𝑬 total can be written as, 𝑬 total = 𝑬 𝒔 𝑨𝑭

Concept of Phase Scanning Phase Scanning Circuitry Why? Electronic Beam Steering Technique Time Delay Scanning Frequency Scanning Phase Scanning Why Phase Scanning? Ease of Implementation Cheaper Digital Control Circuitry Fast Response Time High Sensitivity

Concept of Phase Scanning (cont.) How? By controlling the progressive phase difference between each individual elements of an array. Implementation Diode Phase Shifter Ferrite Phase Shifter Industrial Solution Digitally controlled fixed step phase shifter Analog controlled continuous phase shifter

Phase Scanning Technique Implementation Series Phasers Advantage: Sharing Equal Power Disadvantages: Unequal Inter-element Phase Shift, so complex control circuitry. Summed up Attenuation Parallel Phasers Advantages: Phase Shifters act independently Simpler Control Circuit Disadvantage: Each phase shifter does not share equal power Example Switched Line Phase Shifter Ferrite Phase Shifter

Conformal Antenna- Challenges and Solution For a conformal antenna, the surface of the substrate changes with time during operation. When the surface remains planar, the antenna behaves normally. However for non-planar orientation, the radiation pattern gets distorted. Solution By applying the concept of phase steering, correct radiation pattern can be recovered.

Realization of Conformal Phased Array Antenna Realization of Conformal Phased Array Antenna Equation for Phase Correction Proposed System Block

Determining possible conformal surfaces in terms of application Conformal Antennas are used basically as wearable antennas which may be shaped as wedge or cylindrical in non-planar orientation.

A linear conformal array antenna placed on a Wedge shaped surface

A linear conformal array antenna placed on a Cylindrical surface

Equation for Phase Correction An x- and z- translation is incurred from the original flat position for each array element. Fields arriving at the reference plane associated with A±2 lagged from fields arriving at the reference plane associated with A±1. So, the phases of current at A±2 should be positive enough to compensate the phase delay by that free space propagation to maintain equivalent planar orientation. As the phase has being corrected towards the source, the phase correction will be additive in nature. For wedge shaped surface, this correction can be achieved by introducing phase 𝚫 𝜱 𝒏 𝒘 to each element where 𝜱 𝒏 𝒘 = +𝒌𝑳|𝒏|𝐬𝐢𝐧 𝜽 𝒃 For cylindrical surface, this correction can be achieved by introducing phase 𝚫 𝜱 𝒏 𝒄 to each element where 𝜱 𝒏 𝒄 =+𝒌𝒓 𝒔𝒊𝒏 𝜱 𝒏 − 𝒔𝒊𝒏 𝜱 𝒏−𝟏

Designing of Phased Array Antenna Test Platform

Phased Array Antenna Test Platform

4-element antenna array with connectors g=2.0 mm, h=35.6 mm, t=1.3 mm w=43.6 mm. Rogers 6002(εr=2.94) 60 mil substrate. Resonant Frequency: 2.46 GHz

Four port Receiver RF Circuit Board Consists of Voltage controlled Analog Phase Shifters Voltage Controlled Attenuators Amplifier and Power Combiner Industry Available Each component was tested and verified prior to application with single prototype

Control Voltage vs. Normalized Phase of the Phase Shifter

Four port Receiver RF Circuit Board (cont.) Multiple Input Single Output System RT/duroid 6002 60 mil (εr=2.94) Controlled by DAC Circuit through LabVIEW GUI

DAC Circuit 12 bit, octal, 64 pin, low power DAC Output ranges from 0V to 33 V for unipolar operation Allows programmable gain of x4 or x6 w.r.t the applied reference voltage Features Serial Peripheral Interface that can be operated at 50 MHz and is logic compatible with 1.8V, 3V or 5V The register consists of a R/W bit, 5 address bits and 12 data bits Operated in both synchronous and asynchronous mode TQFP(Thin Quad Flat Package)-64 (10 x 10mm) used

LabVIEW GUI National Instrument LabVIEW USB 6008 peripheral device was used to communicate with the GUI 4 phase shifters and 4 attenuators can be controlled by 8 separate output channels from DAC with precision up to 300 mV

Connection Setup of the system

Scanning Properties of Phased Array Antenna Test Platform

Phase Compensation Calculation The expression for Array Factor can be redefined in Spherical coordinate as: where 𝒖= 𝐬𝐢𝐧 𝜽 𝐜𝐨𝐬 𝜱 𝒖 𝒔 = 𝐬𝐢𝐧 𝜽 𝒔 𝐜𝐨𝐬 𝜱 𝒔 𝒗 𝒔 = 𝐬𝐢𝐧 𝜽 𝒔 𝐬𝐢𝐧 𝜱 𝒔 𝒗= 𝐬𝐢𝐧 𝜽 𝐬𝐢𝐧 𝜱 θs is the elevation steering angle Φs is the elevation steering angle A is the amplitude to each element Element factor 𝒆 𝜽 =𝑨 𝐜𝐨𝐬 𝜽 and 𝒘 𝒏 =𝒆(𝜽) 𝒆 𝒋𝜶 Then the compensated Array Factor ( 𝑨𝑭 𝒄 ) will be 𝑨𝑭 𝒄 =𝑨𝑭 𝒆 𝒋 𝜱 𝒏

Return Loss Measurement

Properties on a flat surface ( 𝜽 𝒃 =0°)

Properties on a wedge ( 𝜽 𝒃 =30°)

Properties on a wedge ( 𝜽 𝒃 =45°)

Properties on a cylinder (r=10cm)

Gain Calculation The primary objective through this correction is to recover the gain. If the reference gain of the system for a particular orientation is Gr(θ,Φ) and the compensated gain after the correction is Gc(θ,Φ), then for ideal condition Gr (θ,Φ) = Gc (θ,Φ) However, the projected spacing between the elements deviates from λ/2 value for any non-planar orientation. Due to this geometrical limitation, compensated gain can never be achieved to be equal to the reference gain. This gain shift (Gs) has been measured for all conformal cases and compared with analytical result.

Gain Calculation (cont.) Gs (θ,Φ) = Gc (θ,Φ) - Gr (θ,Φ) Surface ( 𝜽 𝒃 =30°) ( 𝜽 𝒃 =45°) Cylinder Gs, analy. -0.6 dBi -1.3 dBi -0.8 dBi Gs, meas. -1.0 dBi -1.8 dBi -1.6 dBi Projected Spacing 0.43λ 0.35λ non-uniform

Test Platform Results Advantages Disadvantages Practically validates the theory of beam steering Ability of recovering the radiation pattern has been demonstrated for a general array Gain Calculation has been presented showing low loss of gain Disadvantages Manual control required for any changes of conformal surface The array was formed by individual element with separate feeding points. But an array should be acting as an individual element. Gain shift

Four Element SELFLEX Array Design

SELFLEX Array Design Challenges Solution Can we design a conformal array on a single substrate with phase correction capability? Can we achieve radiation pattern recovery for a conformal array in an autonomous manner? Can we reduce the gain shift? Solution By designing a SELFLEX (SELF-adapting FLEXible) array antenna.

Proposed System Block Diagram

Corporate Feed Network Why? Matching. Technique Corporate Feed Structure by using quarter-wave transformer Example Bifurcated T waveguide or coaxial T-junctions.

SELFLEX Array Design Features: Single feed point Insertion of phase shifters into corporate feed network Introduce the sensor circuit as the feedback network with autonomous controller circuitry for radiation pattern recovery

Sensor Circuit Setup

How it Works A flexible resistor senses the amount of curvature of the surface each time and feed that value to the controller circuit. The controller circuit consists of an instrumentation Op-Amp AMP04 that offers the phase shifter with necessary voltage correction for any conformal orientation. The phase shifters placed on the corporate feed network then process the signals from each array element resulting correction of radiation pattern of the array autonomously.

Scanning Properties of SELFLEX Array

Return Loss Measurement

Properties on a flat surface ( 𝜽 𝒃 =30°)

Properties on a flat surface ( 𝜽 𝒃 =45°)

Properties on a cylinder (r=10cm)

Gs (θ,Φ) = Gc (θ,Φ) - Gr (θ,Φ) Gain Calculation Gs (θ,Φ) = Gc (θ,Φ) - Gr (θ,Φ) Surface ( 𝜽 𝒃 =30°) ( 𝜽 𝒃 =45°) Cylinder Gs, analy. -0.6 dBi -1.3 dBi -0.8 dBi (Test Platform) Gs, meas. -1.0 dBi -1.8 dBi -1.6 dBi (SELFLEX) Projected Spacing 0.43λ 0.35λ non-uniform -0.9 dBi -1.4 dBi -1.2 dBi

Conclusion Conformal Phased Array Antenna Theory of Beam Steering Implementation of RF block Designing, printing and testing of a primitive conformal array that has the ability to compensate phase on each element with external manual control by the user Designing, printing and testing of a 1x4 self-adapting antenna that can autonomously preserve its radiation field during conformal application

Questions ? Thank You