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In this thesis, a unique design of a self-adapting conformal phased-array antenna system for wireless communications is presented. The antenna system is comprised of a sensor circuit and one 1x4 printed microstrip patch antenna array on a flexible substrate with a resonant frequency of 2.47 GHz. When the performance of the antenna starts to degrade under non-planar orientation, the sensor circuitry compensates the phase of each array element of the antenn
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A Thesis Submitted to the Graduate Faculty of the North Dakota State University of Agriculture and Applied Science
By Sayan Roy
In Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE
Major Department: Electrical and Computer Engineering
August 2012
Fargo, North Dakota
Title DESIGNING OF A SMALL WEARABLE CONFORMAL PHASED ARRAY ANTENNA
FOR WIRELESS COMMUNICATIONS
By
Sayan Roy
The Supervisory Committee certifies that this disquisition complies with North Dakota State University’s regulations and meets the accepted standards for the degree of
MASTER OF SCIENCE
Dr. Benjamin D. Braaten Chair Dr. David A. Rogers
Dr. Mark Schroeder
Dr. Alan Denton
Approved:
5 th^ July, 2012 Dr. Rajendra Katti Date (^) Department Chair
First, I would like to thank my advisor, Dr. Benjamin D. Braaten, for his continuous support, patience, and guidance in completing this research. He always supported me by providing intriguing fundamental thoughts for this research. Without his guidance, I would have never been able to complete this work. I would also like to thank my committee members, Dr. David A. Rogers, Dr. Mark Schroeder and Dr. Alan Denton, for their continuous encouragement and support. They have taught me many things and helped me in overcoming any difficulties I had along the way in this research. I acknowledge DMEA, NASA NDEPSCoR and DERPA/MTO for their financial support for this project. I would especially like to thank Dr. Neil F. Chamberlain with the NASA Jet Propulsion Laboratory (JPL), California and Dr. Michael Reich with the Center for Nanoscale Science and Engineering (CNSE), Fargo for their collaboration and input on various aspects of this reseach. Finally, I would like to thank my family for their support and understanding that I had to leave my hometown in India to pursue this work.
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To Ma and Baba.
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24 Voltage Variable Attenuator under test............................................. 27
25 |S 11 | of the Attenuator at 2.45 GHz................................................ 28
26 Magnitude of S 21 of the Attenuator at 2.45 GHz...................................... 28
27 Phase of S 21 of the Attenuator at 2.45 GHz......................................... 29
28 Low Noise Amplifier under test................................................... 29
29 |S 11 | in dB of the Amplifier from 2.4 to 2.6 GHz...................................... 30
30 Phase of S 21 of the Amplifier from 2.4 to 2.6 GHz.................................... 30
31 Power Combiner under test...................................................... 32
32 |S 11 | of the Combiner at Combiner Side from 2.4 to 2.6 GHz............................ 32
33 |S 11 | of the Combiner at one of the branch from 2.4 to 2.6 GHz.......................... 33
34 DAC circuitry in details......................................................... 34
35 a) Picture of the 1 x 4 antenna test platform attached to a non-conducting wedge; b) picture of the 1 x 4 antenna test platform attached to a non-conducting cylinder................... 36
36 Measured S 11 of the 1 x 4 antenna test platform...................................... 37
37 Measured and analytical scanned patterns in the x − z plane for the 1 x 4 antenna test platform on a flat surface (θb = 0 ◦)....................................................... 38
38 Measured and analytical patterns at 2.45 GHz in the x-z plane for the 1 x 4 antenna test platform on a wedge with θb = 30 ◦....................................................... 39
39 Measured and analytical patterns at 2.45 GHz in the x-z plane for the 1 x 4 antenna test platform on a wedge with θb = 45 ◦....................................................... 40
40 Measured and analytical patterns at 2.45 GHz in the x-z plane for the 1 x 4 antenna test platform on a cylinder with a radius of curvature of 10cm...................................... 41
41 Schematic of the 1 x 4 Self-adapting flexible (SELFLEX) array with embedded sensor circuitry... 45
42 Picture of the manufactured 1 x 4 SELFLEX array prototype (g = 2.0 mm, h = 35.6 mm, m = 19.8 mm, s = 11.0 mm, t = 1.3 mm, u = 33.4 mm and w = 43.6 mm).................... 45
43 a) Schematic of the sensor circuit used to measure the resistance and control the phase shifters (Vcc = 15V, Rgain=4.7kΩ and Vref = −Vcc = -0.4V) and b) a picture of the flexible resistive sensor used for measuring surface deformation........................................ 46
44 Measured output of the phase shifter controlled by the sensor circuit where θb is the bend angle and ∆φwn is the phase compensation for the nth^ antenna element in the array............... 46
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1.1. History In the year of 1873, famous physicist James Clerk Maxwell mathematically described the nature of electromagnetic waves. In his book, “A Treaties on Electricity and Magnetism”, he proposed four sets of partial differential equations that explain the quantitative and qualitative analysis on electromagnetic waves. These equations are known as Maxwells Equations and have been considered to be one of the greatest discoveries of 19th century in the world of mathematical physics. A few years later, another famous physicist Heinrich Hertz experimentally demonstrated the existence of electromagnetic waves in free space. However, it took another twenty-two years to apply the concept of Maxwells equations in practice when two scientists from two different countries, Sir Jagadish Chandra Bose from India and Guglielmo Marconi from Italy separately demonstrated the engineering behind radio waves and the usefulness of wireless communications through radio waves in the year of 1895. Afterwards, the wireless communications soon gained popularity due to two World Wars. Today wireless communications is being exploited from clinical practice to extraterrestrial communications. Space science is one of the domains that experienced a dramatic advancement after the invention of powerful antennas. Antenna is one of the basic building blocks in the world of wireless technology. The fields of antenna and wave propagation has drawn the attention of researchers for last thirty years resulting in many inventions of new types of antennas with superior performance and versatile features. Conformal antennas is one of these new types of antennas. A conformal antenna can be described as an antenna that conforms to a prescribed shape. The shape can be some part of an airplane, a spacesuit, a high-speed train or other types of physical entities. The purpose will be to build antenna systems in such a way that the antenna integration makes the antennas less disturbing yet maintaining the optimum performance [1]. Usually, a conformal antenna is cylindrical, spherical, or some other shape, with the radiating elements mounted on or integrated into the smoothly curved surface. The IEEE Standard Definition of Terms for Antennas (IEEE Std 145-1993) gives the following definition: 2.74 conformal antenna [conformal array]. An antenna [an array] that conforms to a surface whose shape is determined by considerations other than electromagnetic; for example, aerodynamic or hydrodynamic.
1.2. Background Antennas are used to transmit and receive electromagnetic signals in wireless communication systems. From the view of a receiving antenna, the quality of the antenna depends upon how well it can receive the faintest electromagnetic waves. In general, antennas with very large aperture can detect faint signals much better than antennas with a comparatively smaller aperture. However, a larger aperture demands bulky systems and complex construction engineering which sometime exceeds the feasibility for physical implementation of the antenna. One way to overcome this challenge is to implement antenna array concepts where a number of identical antennas with very small apertures can be cascaded in different manners based on their functionality. The output of each small antenna is then combined to enhance the total received signal that is equivalent to the signal received with a single antenna with a large aperture. Mathematically, an antenna array can offer an aperture that exceeds the aperture of a single antenna and thus it can be capable of detecting extremely faint signals from far away sources [2]. The compromised factor here is the complexity. Since the electromagnetic signals received by each antenna array element differs from the signals received by other array elements in terms of amplitude and phase, they must be combined coherently to achieve the desired output. Though it is more complex to set up an antenna array compared to a single antenna, weighting the signals before combining them enables enhanced performance features such as interference rejection and beam steering without physically moving the aperture. The trade-off for these attractive features is increased complexity and cost. 1.3. Motivation for Work Now-a-days wireless systems are exploited in the domain of space communication, harvesting energy, tracking inventory and streaming entertainment to billions of people around the globe. The microstrip antenna shown in Fig. 1 is one of the most popular antennas used currently in wireless communications because of its simple geometry, ease of design, compactness, durability and low manufacturing cost. A more detail geometry related to the designing of a printed microstrip antenna is illustrated in Fig. 2. This type of antenna consists of a single conducting plane, usually made with copper, printed on the top layer of a dielectric material. A ground plane, also made with copper, is then printed on the bottom layer of the dielectric substrate. The radiation of the antenna can be achieved by generating an electric field between the two conductor layers of the antenna by applying a voltage between the two conductors on the top and the bottom of the substrate. Widespread use of printed microstrip antennas has drawn a lot of attention in the area of research that includes but is not limited to, ultra wide-band antennas, reconfigurable antennas, metamaterials-based antennas and millimeter-wave integrated phased arrays [3]. However, most of the geometries of the aforesaid systems are limited to
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1.4. Proposed Work
Figure 3. High level block diagram of the proposed antenna system.
One of the drawbacks of an antenna array is the lack of ability to recover the original radiation pattern when it undergoes some sort of change in its physical structure. As illustrated in Figures 4(a) and 4(b), an antenna array has the capability of changing the direction of radiation by controlling the individual phases of the voltages being supplied to each microstrip antenna array, known as beam steering. To do that, implementation of flexible sensor circuitry in a planar conformal antenna array is being proposed in this thesis. In particular, the flexible sensor circuit will be used with suitable phase compensation circuitry to dynamically determine the changes in the curvature of the antenna surface and the circuit then modifies necessary input signals to each array element through a feedback path. The block diagram of the proposed setup has been shown in Fig. 3.
(a) A printed microstrip antenna array.
(b) A printed microstrip array on a curved surface illustrating a change in radiation direction as a result of curvature and radiation correction using phase compensation circuitry.
Figure 4. A printed microstrip array on planar and curved surfaces with direction of maximum radiation
2.4.1. Feed Network A feed network distributes energy to the elements of the array by means of phase shifters according to a desired amplitude function. Corporate binary feed, as shown in Fig. 5 is common in arrays of dipoles, open-end guides and patches. Such feed circuits are commonly binary but can be modified to design 3-way or 5-way dividers, depending upon the number of array elements. The critical component in the corporate feed is the power divider that can be realized by bifurcated T waveguide or coaxial T junctions [6]. One challenge in design of this type of feed network is that each of the elements is required to be impedance matched and isolated or the reflected signal from each element results in a parasitic radiation pattern that will be superimposed on the required pattern. This condition plays an important role in the design of feed networks, where it is often necessary to use a directional couplar or a matched transmission line.
Figure 5. Corporate feed structure for an array system.
2.4.2. Phase Scanning Circuitry One primary goal of developing phased-array antennas is to achieve beam steering electronically and thus to eliminate the mechanical movement of an antenna system. Electronic beam steering in an array antenna can be realized by time delay scanning, frequency scanning or phase scanning techniques. However, ease of implementation, cheaper digital control circuits, fast response time and high sensitivity make the phase scanning method the most popular. For proper functionality, a clever choice for a phase shifter is a switched line or ferrite phase shifter with analog or digital control. A good choice for the placement of phase shifters along the feed line is also a very important factor. The orientation may be in series or in parallel, as shown in Fig. 6. Although the series phasers have the advantage of sharing equal power, the disadvantage is the phase compensation circuit because the basic interelement phase shift must be multiplied by the number of elements and the attenuations of the phasers add up along
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Figure 6. Parallel and Series Feeds.
the feed line. On the contrary, for parallel combination, although each phase shifter does not share the same power, the major advantage is all phasers are independent of each other and thus modeling of the control circuit becomes simpler. The mathematical approach to the phase compensation calculation will be discussed in the next section. 2.5. Defining Coordinate System For notation purposes, henceforth it will be assumed that any linear planar array will be lying on the x-y plane with the z-axis pointing broadside to the array unless otherwise mentioned, as shown in Fig. 7(b). One objective of this project is to build an array system for a receiver, therefore any antenna system will be considered as a receiver module with respect to a point source acting as a trasmitter, as shown in Fig. 7(a). The rectangular coordinate system has been used throughout the discussion with proper notation. To define the angular separation of the array element from an axis, two parameters have been defined, denoted as the elevation angle or scan angle (θ) and the azimuthal angle (φ). The
