This book presents the proposal for the novel designs of the wideband Magneto-Electric (ME) dipole antenna for various applications in wireless communication. The standard ME dipole consists of a planar dipole, which acts like an electric dipole, and a vertically oriented quarter wave shorted patch, equivalent to a magnetic dipole. The research demonstrates the good electrical characteristics for the proposed designs of the wideband ME dipole antenna and shows low cross-polarization radiations, stable antenna gain across the operating frequency band and almost identical radiation patterns in the E- plane and H-plane. Literature confirms the design of the ME dipole antenna as linearly polarized, circularly polarized or differentially fed antenna and this thesis presents five novel structures of ME dipole antenna for applications like airborne radar, Ultra-Wide Band (UWB), mobile communication and satellite communication. After examining a number of feeding techniques designed to enhance the impedance bandwidth of ME dipole antenna, which includes co-axial-to-patch probe feeding, transition of microstrip-to-parallel strip line feeding, differential feeding, the book presents all the novel designs using co-axial probe feeding.
Table of Content
Chapter 1 Introductory Chapter
1.1 Introduction
1.2 Review of Previous Research
1.2.1 Wideband unidirectional patch antenna
1.2.2 Bandwidth Enhancement Techniques for Conventional unidirectional Microstrip Patch Antenna
1.2.2.1 U-slot Technique
1.2.2.2 L-shaped Probe Technique
1.2.3 The Dipole Antenna
1.2.3.1 Bandwidth Enhancement Techniques for Dipole Antenna
1.2.4 Complementary Antenna
1.2.4.1 Complementary Antenna Composed of Slot Antenna and Parasitic Wires
1.2.4.2 Complementary Antenna Composed of Slot Antenna and a Monopole
1.2.5 Magneto-Electric Dipole Antenna
1.2.5.1 Magneto-Electric Dipole Antenna with Modified Ground Plane
1.2.5.2 Magneto-Electric Dipole Antenna with Differential Feed
1.2.5.3 Circularly Polarized Magneto-Electric Dipole Antenna
1.2.5.4 Planar Printed Magneto-Electric Dipole Antenna
1.2.5.5 Reconfigurable Magneto-Electric Dipole Antenna
1.2.5.6 Millimeter Waves Magneto-Electric Dipole Antenna
1.3 Objectives
1.4 Structure of the Book
Chapter 2 Design of an End-Fire Magneto-Electric Dipole Antenna
2.1 Introduction
2.2 Antenna Description and Design Geometry
2.3 Current Distribution in the Magneto-Electric Dipole Antenna
2.4 Simulation and Measured Results
2.5 Parametric Study
2.5.1 Effect of Variation in length of Capacitive Arm
2.6 Conclusion
Chapter 3 Design of a Differentially-fed Magneto-Electric Dipole Antenna
3.1 Introduction
3.2 Electrical Parameters of Differentially fed Antenna
3.3 Antenna Description and Design Geometry
3.4 Simulation and Measured Results
3.5 Conclusion
Chapter 4 Design of Magneto-Electric Dipole Antenna with Modified Ground Plane
4.1 Introduction
4.2 Principle of Operation
4.3 Design of E-Shaped Antenna without Cavity
4.4 Performance of E-Shaped Antenna without Cavity
4.5 E-Shaped Antenna with Rectangular Cavity Reflector
4.5.1 Effect of Height of Rectangular Cavity
4.5.2 Effect of Width of Rectangular Cavity
4.6 Simulation and Measurement Results
4.7 Conclusion
Chapter 5 Design of a Planar Circularly Polarized Magneto-Electric Dipole Antenna
5.1 Introduction
5.2 Antenna Geometry and Design
5.3 Current Distribution
5.4 Simulations and Measurements
5.5 Parametric Study
5.5.1 Effect of Length of ground Plane
5.5.2 Effect of Width of ground Plane
5.5.3 Effect of Width of Feed Line
5.6 Conclusion
Chapter 6 Design of a Magneto-Electric Monopole Antenna
6.1 Introduction
6.2 Antenna Design and Geometry
6.3 Current Distribution
6.4 Analysis of Magneto-Electric Monopole Antenna
6.5 Simulation and Measurement Results Analysis
6.6 Parametric Study
6.6.1 Effect of Height of Monopole Antenna
6.6.2 Effect of length of Feed of Monopole Antenna
6.6.3 Effect of Width of Ground Plane
6.6.4 Effect of Length of Ground Plane
6.10 Conclusion
Chapter 7 Concluding Remarks
References
List of Figures
Figure
Figure 1.1 Geometry of U-slot microstrip patch antenna
Figure 1.2 Geometry of L-probe patch antenna
Figure 1.3 Geometry of half wavelength dipole element
Figure 1.4 Radiation pattern of dipole antenna
Figure 1.5 Wide-band and UWB dipole shapes
Figure 1.6 Basic design of a complementary antenna consisting of electric dipole and magnetic dipole
Figure 1.7 A complementary antenna combined with a slot and inverted-L wires
Figure 1.8 A design of monopole-slot complementary antenna
Figure 1.9 Current distribution of magneto-electric dipole antenna using L-shaped feed
Figure 1.10 Dielectric loaded ME dipole antenna
Figure 1.11 ME dipole antenna with stair cased shaped feed design
Figure 1.12 ME dipole antenna with modified ground plane
Figure 1.13 Differential-fed ME dipole antenna
Figure 1.14 Circularly polarized ME dipole antenna
Figure 1.15 Planar printed ME dipole antenna
Figure 1.16 A reconfigurable M.E dipole antenna
Figure 1.17 Feed network of ME dipole antenna
Figure 1.18 Geometry of the Substrate Integrated Waveguide (SIW) fed CP aperture-coupled ME dipole antenna
Figure 3.1a Prototype of proposed antenna with side view of feed
Figure 3.1b 3-D view of proposed feed design
Figure 3.2a Side view of the proposed antenna
Figure 3.2b Top view of the proposed antenna
Figure 3.3 The simulated and measured differential return loss, Sdd11
Figure 3.4 The simulated and measured gain of proposed antenna
Figure 3.5 The simulated and measured differential input impedance of proposed antenna
Figure 3.6 Simulated antenna efficiency and radiation efficiency of proposed antenna
Figure 3.7 Simulated and measured E-plane and H-Plane radiation patterns at: a-1GHz, b-1.5GHz, c-2GHz and d-2.5GHz
Figure 4.1 Prototype of proposed antenna
Figure 4.2 Schematic of Proposed Antenna (a) side view and (b) Top view
Figure 4.3 Current distribution of E-shaped antenna at 2.6 GHz
Figure 4.4 The simulated and measured return loss of E-shaped antenna
Figure 4.5 The simulated and measured gain of E-shaped antenna
Figure 4.6 Variation of return loss with frequency for different cavity heights
Figure 4.7 Variation of gain with frequency at different cavity heights
Figure 4.8 (a) Co-polar and (b) cross polar radiation patterns in E-plane for different cavity height at 2.4GHz
Figure 4.9 Variation of return loss with frequency for different cavity lengths
Figure 4.10 Variation of gain with frequency for different cavity length
Figure 4.11 Simulated (a) co and (b) cross polarization radiation patterns in E-plane for different cavity length at 2.4 GHz
Figure 4.12 Simulated and measured return loss vs frequency of proposed antenna
Figure 4.13 Simulated and measured gain with frequency of proposed antenna
Figure 4.14 Simulated antenna efficiency and radiation efficiency vs frequency of proposed antenna
Figure 4.15 Simulated and measured E-plane and H-plane radiation patterns at: (a) 2.4GHz, (b) 2.8GHz and (c) 3.2GHz
Figure 4.16 Simulated and measured co-polarization and cross polarization radiation patterns in E-plane at: a- 2.4GHz, b-2.8GHz, c- 3.2GHz
Figure 5.1a Top view of the proposed antenna
Figure 5.1b Side view of the proposed antenna
Figure 5.1c 3D view of the proposed antenna
Figure 5.2a Front side of the proposed antenna
Figure 5.2b Back side of the proposed antenna
Figure 5.3 Measured and simulated return loss of the proposed antenna
Figure 5.4 Measured and simulated 3-dB axial ratio of the proposed antenna
Figure 5.5 Measured and simulated gain of the proposed antenna
Figure 5.6 Current distribution indicating right-handed circular polarization at 10.5GHz
Figure 5.7 Measured and simulated E-plane and H-plane radiation patterns at: (a) 9.5GHz (b) 10GHz and (c) 10.5GHz
Figure 5.8 Variation of return loss with frequency for different length of ground plane
Figure 5.9 Variation of gain with frequency for different length of ground plane
Figure 5.10 Variation of axial ratio with frequency for different length of ground plane
Figure 5.11 Variation of return loss with frequency for different width of ground plane
Figure 5.12 Variation of gain with frequency for different width of ground plane
Figure 5.13 Variation of axial ratio with frequency for different width of ground plane
Figure 5.14 Variation of return loss with frequency for different width of feed
Figure 5.15 Variation of gain with frequency for different width of feed
Figure 5.16 Variation of axial ratio with frequency for different width of feed
Figure 6.1a Side view of the proposed antenna
Figure 6.1b Top view of proposed antenna
Figure 6.1c 3-D view of proposed antenna
Figure 6.2 Prototype of proposed antenna
Figure 6.3 Current distribution of proposed antenna at 4.7GHz
Figure 6.4a Schematic of proposed magneto-electric monopole antenna
Figure 6.4b Equivalent circuit of magneto-electric monopole antenna
Figure 6.5 Real and imaginary parts of input impedance of proposed magneto-electric monopole antenna
Figure 6.6 Measured, simulated and formula based return loss of proposed antenna
Figure 6.7 Measured and simulated gain of proposed antenna
Figure 6.8 Measured and simulated E-plane and H-plane radiation patterns at: (a) 5GHz (b) 6GHz (c) 7GHz (d) 8GHz
Figure 6.9 Measured and simulated co-polarization and cross polarization radiation patterns in E-Plane at: ( a) 5GHz (b) 6GHz (c) 7GHz (d) 8GHz
Figure 6.10 Simulated return loss of proposed antenna at different height
Figure 6.11 Simulated gain of proposed antenna at different height
Figure 6.12 Simulated return loss of proposed antenna for different length of feed
Figure 6.13 Simulated gain of proposed antenna for different length of feed
Figure 6.14 Simulated return loss of proposed antenna for different width of ground plane
Figure 6.15 Simulated gain of proposed antenna for different width of ground plane
Figure 6.16 Simulated return loss of proposed antenna for different length of ground plane
Figure 6.17 Simulated gain of proposed antenna for different length of ground plane
CHAPTER 1
INTRODUCTION
1.1 Background
With the rapid and extensive usage of mobile phones, wireless communication systems and technologies have entered into many important domains of our daily lives, which include social media, business development, medical and healthcare applications, agriculture, scientific applications and many more. The antenna plays a pivotal role in all the wireless communication applications to determine the overall system performance and the various novel applications have urged strong demands for new high performance antenna systems. Precisely, the development of numerous wireless communication systems and applications have triggered the all-time high demand for wideband, low profile and unidirectional antennas that can accommodate various wireless communication applications while exhibiting good electrical characteristics, including stable gain, wide impedance bandwidth, low cross-polarization and low back lobe radiations across the entire range of frequency operation. Many designs have been proposed in the literature to accommodate various wireless communication applications with enhanced antenna parameters. The standard L- shaped probe feed patch antenna are able to achieve an impedance bandwidth of 35% with an average gain of 7.5dBi [1]. With slot antenna an impedance bandwidth of 17% - 40% can be achieved [2-3]. But each of these unidirectional antennas exhibits an asymmetric E-plane and H-plane radiation pattern and unable to provide stable gain bandwidth product in the range of frequency of operation. Recently, a novel wideband antenna, known as the ME dipole antenna, is proposed by Luk et. al. [4-5], which has been derived from the complementary antenna. The basic structure of ME dipole antenna consists of a vertically oriented quarter wave shorted patch and a horizontal planar dipole, equivalent to a combination of a magnetic dipole and an electric dipole. This antenna has demonstrated good electrical characteristics like identical E-plane and H-plane radiation pattern, low cross polarization, low back lobe radiations and stable gain in the range of operating frequency.
The large size of this antenna, owing to the presence of magnetic dipole, is a matter of concern but it is also observed that the size of the ME dipole can be further reduced [6], and two ME dipole antenna elements can be integrated to form a wideband dual-polarized antenna with excellent electrical characteristics [7].
In the last few years, there have been tremendous increase in the demand for multifunctional antennas and many novel shaped antennas have been proposed in the literature to achieve high level of performance. These diversified antenna systems approaches have received much desired attention in wireless communication sectors. Broadly these diversified antennas have raised the platform for demand and supply in terms of capabilities of wireless communication system [8]. Different categories of diversity antennas include, the spatial diversity antenna, the frequency diversity antenna [9], the polarization diversity antenna [8, 10] and the pattern diversity antenna [11-12]. The pattern diversity antenna and polarization diversity antenna are most commonly used to analyze and compute the effects of multi-path fading and providing compatible solutions in the complex environment [9]. As compared to classical unidirectional antenna systems, the pattern diversity antennas are used to radiate and receive signals through different radiation modes and hence they are capable of providing high effective gain and maintaining the same installation space [11]. Various antenna structures that are offering pattern and polarization diversities for different applications have been proposed in the literature [11-12]. But the major drawback of these available pattern and polarization diversity antennas is that they suffer from narrow overlapped impedance bandwidth of the excitation ports [11] and incomplete pattern diversity modes [12], and hence, their implementation for various applications is limited.
In this thesis, several new classes of ME dipole antennas for various wireless communication applications are proposed, which include- end-fire radiation pattern ME dipole antenna for airborne radar application, an improved UWB ME dipole element with differential feeding structure for Monolithic Microwave Integrated Circuits (MMIC) and Radio Frequency Integrated Circuits (RFIC) applications, an UWB ME dipole antenna with modified ground structure for high gain applications, a novel planar circularly polarized ME dipole antenna for satellite communication and a novel design of ME monopole antenna for UWB applications. All these proposed antennas are based on a structure that is composed of a planar electric dipole antenna and a shorted magnetic dipole antenna, which are excited simultaneously to obtain almost symmetrical E-plane and H-plane radiation patterns, wide impedance bandwidth, constant gain and low cross-polarization radiations across the operational bandwidth.
1.2 Review of Previous Research
To get more insight on the design and development of ME dipole antenna consisting of electric dipole and magnetic dipole and having unidirectional radiation pattern, stringent research analysis, to compare ME dipole antenna with conventional characteristics of the patch antenna and a dipole antenna, has been incorporated in this thesis. Hence, the thesis begins with a discussion on conventional patch antenna and dipole antenna, along with several popular and extensively used feeding techniques to enhance the bandwidth. This review of conventional antennas and feeding techniques is followed by a wideband antenna which is excited by both electric dipole and magnetic dipole simultaneously. Truly, the wideband antennas are having huge potential to be considered as a prime candidate for the existing and future generations of wireless communications. Hence, it is necessary to design and develop advanced wideband antennas, having good electrical characteristics, considering the future demands of wireless communication industry. These desired characteristics of advanced wideband antennas are covered by ME dipole antenna.
1.2.1 Microstrip Patch Antenna (MPA)
Several researches are based on the design and development of wideband unidirectional antenna elements [23-25]. A unidirectional antenna can be implemented by placing a quarter wavelength electric dipole over a finite ground plane [23]. As the height of this antenna [23] is frequency dependent in terms of wavelength, the antenna has a serious drawback of having large gain and beam width variation over the operating bandwidth. The other popular unidirectional antenna is the MPA on which a wide variety of publication is available, which include the design of wideband patch antenna using an L-probe feed [26-30], an aperture coupled feed [31-33], stacked patches [34-38] or a U-slot patch [39-44] etc. Usually, 20% to 40% impedance bandwidth, for SWR<2, can be easily achieved by these design techniques, which are suitable for many applications catering the need of wireless communication systems. However, it has been observed that the radiation pattern changes substantially across the bandwidth while implementing these designs [26-44] and high crosspolarization in the upper frequency band is usually present. However, techniques like antiphase cancellation [45], twin L-probes coupled feed [46], M-probe feed [47-51] etc., were also suggested to suppress the cross-polarization, these antennas still suffer from the major drawbacks of having gain and beam width variations with frequency and having different E-plane and H-plane beam width.
1.2.2 Bandwidth Enhancement Techniques for Conventional Unidirectional MPA
1.2.2.1 U-slot Technique
The notion of using a U-slot on a MPA was first introduced by Lee et. al. in 1997 [14]. The basic advantage of the U-slot is that it preserves the low profile feature of the MPA. The impedance bandwidth is increased by cutting a U-shaped slot around the co-axial probe feed of the MPA. This U-shaped slot helps in manipulating the current flow to compensate for the inductance provided by the coaxial probe when the thickness of the substrate is increased. A MPA with U-slot is able to achieve an impedance bandwidth of more than 30% [14]. As compared to other broadband techniques having a multi-layer structure (like stacked patch approach and the proximity approach), the U-slot antenna with single layer structure is more desirable for commercial applications. The configuration of the U-slot antenna is shown in Figure 1.1
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Figure: 1.1 Geometry of U-slot MPA
1.2.2.2 L-shaped Probe Technique
Luk et. al. proposed an L-shaped probe technique to improve the impedance bandwidth and this technique proved to be a very effective design in 1998 [1]. While the conventional thick substrate MPA with vertical probe feed suffers from the high inductance introduced by the vertical probe feed, this L-shaped probe is an electromagnetic coupled feed, without having any electrical connection to the radiating microstrip patch Here, a copper strip is connected to the inner core of the SMA connector, andis bent into an L-shaped strip as shown in Figure 1.2. The L-shaped strip consists of a horizontal portion and a vertical portion. The capacitance provided by horizontal portion compensate for the inductance that is introduced by the vertical portion, and hence increasing the operating impedance bandwidth from 2-5% [13] to 28% [1]. Various designs which incorporated the L-shaped probe to achieve wideband impedance bandwidth of 32-49% are discussed in the literature [9, 11, 12, 15, 17].
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Figure. 1.2 (a)
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Figure: 1.2 (b)
Figure: 1.2 Geometry of L-shaped probe patch antenna from (a) perspective view; and (b)
side view
1.2.3 The Dipole Antenna
A dipole element into antenna design was initially introduced by Heinrich Rudolph Hertz. Figure 1.3 shows a classical half-wave dipole element. It consists of two quarter wavelength conductors and a center feed balun, which may be a parallel strip line, a balanced line or a slot line. The antenna is simple in structure but it also has the disadvantage of narrow impedance bandwidth and low power gain. The antenna has omnidirectional radiation pattern in H-plane, and bi-directional radiation pattern in E-plane, which is shown in Figure 1.4.
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Figure: 1.3 Geometry of half wavelength dipole element
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Figure: 1.4 Radiation pattern of dipole antenna
1.2.3.1 Bandwidth Enhancement Techniques for the Dipole Antenna
Various techniques have been proposed and implemented to increase the impedance bandwidth and gain of traditional thin wire or dipole antenna. These techniques include modification in shape of two dipole arms into diamond-shaped, circular-shaped, sectorialshaped, bowtie-shaped or elliptical-shaped dipole etc., as shown in Figure 1.5 and shows a flatter impedance response over a wider frequency band. With few modified shapes, over 100% impedance bandwidth have been achieved and are extensively used in UWB applications. The overview of these designs are available in the literature [18-22]. Although, with these modified shapes, the dipole antennas are able to achieve more than 100% impedance bandwidth, still, these antennas suffer from the disadvantages of having unstable radiation pattern and poor antenna gain [18-22].
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Figure: 1.5 Wideband and UWB dipole shapes: (a) bowtie-shaped dipole (b) diamond-
shaped dipole (c) rectangular-shaped dipole (d) elliptical-shaped dipole (e) circular dipole
1.2.4 The Complementary Antenna
The concept of complementary antenna consisting of an electric dipole and a magnetic dipole caught an attraction to achieve an equal E-plane and H-plane radiation pattern along with stable performance over the frequency range [52]. According to the law of electromagnetic theory, an electric dipole shows a figure eight radiation pattern in E-plane and a figure O pattern in the H-plane; while a magnetic dipole shows figure O radiation pattern in the E-plane and a figure eight radiation pattern in the H-plane. Simultaneous excitation of both electric dipole and magnetic dipole, with an appropriate amplitude and suitable phase difference, a unidirectional radiation pattern with equal E-plane and H-plane can be obtained. The practical implementation of this antenna was proposed in 1974 [53]. Another design, having a passive dipole, placed in front of a slot, was reported [54]. The technique, based on a slot and dipole combination, was realized by many investigators [5557], however, all of these designs [52-57] suffer from serious drawback of either having narrow bandwidth or possessing a bulky structure.
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Figure: 1.6 Basic design of a complementary antenna consisting of an electric dipole and a
magnetic dipole
Figure 1.6 shows the basic idea about two sources, having complementary radiation characteristics and oriented at right angles to each another. These two sources can be realized using an electric dipole and the open end of a waveguide [52]. The electric dipole and the magnetic dipole, are separated by a certain distance (known as spacing) and if this distance is maintained in an optimum manner, amplitude and phase of two complementary sources can be controlled to achieve equal E-plane and H-plane radiation pattern. This technique of combining two complementary sources have resulted into advantages of having symmetrical E-plane and H-plane radiation pattern, low back radiations, low cross polarization radiations and constant gain across the operating bandwidth.
1.2.4.1 Complementary Antenna Composed of Slot Antenna and Parasitic Wires
The modified concept of complementary antenna is available in [53-57] and modified slot antenna by placing a passive dipole in front of the slot has been designed [54]. To improve the various electrical parameters of complementary antenna, slot-dipole combination was used and was able to accomplish the advantages of complementary antenna [56-57]. Figure 1.7 shows one of the complementary antenna designed with a slot and inverted L wires [52]. Here two parasitic inverted L wires are placed beside a rectangular slot antenna. With this arrangement, it was proved that a slot behaves like a magnetic dipole whereas the inverted L wires can behave like an electric dipole.
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Figure: 1.7 A complementary antenna combined with a slot and inverted L wires
1.2.4.2 Complementary Antenna Composed of a Slot Antenna and a Monopole
Apart from slot-dipole antenna, few researchers introduced the concept of combining slotmonopole antennas to obtain similar E-plane and H-plane radiation patterns [58, 59]. A crossed-slot and a monopole element technique was explicitly used to obtain a steerable cardioid pattern [58], and on the other side, a conical monopole with a slot technique provided a broadband unidirectional antenna [59]. Figure 1.8 shows a conical monopole placed with an offset distance from the center of the slot. It is proved that while maintaining the amplitude and phase of these two radiating elements, a desirable unidirectional radiation pattern can be achieved.
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Figure: 1.8 A design of monopole-slot complementary antenna
1.2.5 The ME Dipole Antenna
ME dipole antenna, derived from wideband complementary antenna, was initially proposed by Luk et. al. in 2006 [4, 5]. This antenna consists of a vertically oriented quarter wave shorted patch as magnetic dipole and a planar dipole, which is equivalent to an electric dipole. The antenna possesses good electrical characteristics, including low back-lobe radiations, stable antenna operation across the bandwidth, and symmetrical E-plane and H- plane radiation patterns.
To understand the basic operating principle of ME dipole antenna, current distribution of an L-shaped probe feed is explained in Figure 1.9. When the probe feed is excited, following observations are noticed for one full cycle of current distribution.
At, t=0, the maximum current on the horizontal plates is observed in the same direction while the current on the shorted patch is minimum. This indicates that an electric dipole mode is excited.
At, t=T/4, the current on horizontal plates decreases and the current on shorted patch is maximum. Thus a magnetic dipole mode is excited.
At, t=T/2, the current on horizontal plates again reaches to its maximum but in reversed direction as compared to t=0, hence, exciting the electric dipole mode.
At, t=3T/4, the magnetic dipole mode is excited again but the direction of current is opposite to the current at t=T/4.
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Figure: 1.9 Current distribution of ME dipole antenna using L-shaped feed
On the basis of this initial L-shaped feed, novel modifications were proposed to increase the gain and impedance bandwidth. A dielectric loaded ME dipole, composed of two shorted rectangular patches and a I-shaped probe, was designed to give wide impedance bandwidth of 48.7% and gain of 8.1dBi [60]. ME dipole antenna with four I-shaped probes with dielectric loading provides high isolation between the two ports with 24.9% impedance bandwidth and gain of 8.2dBi [62], as shown in Figure 1.10. This I-shaped feed design is also used in dual-polarized ME dipole antenna [64]. The antenna provides wide impedance bandwidth of 65.9%, high isolation between the two ports and a stable gain of 9.5dBi. Novel combination of I-shaped feed and an L-shaped electric dipole has been used to design an ME dipole antenna to provide ultra-wide impedance bandwidth of 87% and stable gain of 7dBi [65]. This L-shaped feed design is also used to provide dual-band and an impedance bandwidth of 34% and 49.5% in the frequency range 0.78GHz - 1.1GHz and from 1.58GHz - 2.62GHz, respectively [68].
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Figure: 1.10 Dielectric loaded ME dipole antenna [62]
The L-shaped feed design is further modified to F-shaped feed. A dual-band, dual-polarized ME dipole antenna has been designed using F-shaped feed to provide an impedance bandwidth of 25.5% and 39.5% with average gain of 4.3dBi and 7.8dBi respectively [67]. When the antenna is excited by a stair-shaped probe feed, without implementing the need of additional balun, it gives an impedance bandwidth of 95.2%, in frequency range 1.65GHz - 4.65GHz, stable radiation pattern with low cross polarization, low back radiations, nearly identical E-plane and H-plane patterns and an antenna gain of 7.9 ±0.9 dBi [88]. This antenna is depicted in Figure 1.11.
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Figure: 1.11 ME dipole antenna with staircase-shaped feed design [67]
1.2.5.1 ME Dipole Antenna with Modified Ground Plane
While designing an antenna, the size and shape of ground plane, play a crucial role in estimating the gain of an antenna. The technique of changing shape of ground plane is also used in ME dipole antenna under the name modified ground plane . With this modified ground plane structure, the gain of antenna was increased to 13dBi and 9.3dBi respectively [61, 63], as indicated by Figure 1.12.
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Figure: 1.12 ME dipole antenna with modified ground plane [63]
1.2.5.2 ME Dipole Antenna with Differential Feed
The differentially-fed antennas are developed to cover various UWB applications specifically, in RFIC and MMIC circuits as they are designed to provide high common mode rejection, low mutual coupling, less noise, great harmonic suppression and high linearity. The advantages of using differentially fed antenna include the removal of bulky off-chip baluns, reducing size and to remove the insertion loss of baluns. In addition, the differentially-fed antennas generate a cancellation mechanism to suppress higher order modes and unwanted radiations from vertical feeding structures. Hence differentially-fed antenna has the ability to reduce the cross polarization level and to provide enhanced polarization purity. A parallel twin L-shaped differential feeding structure, as indicated in Figure 1.13, provides an ultra-wide impedance bandwidth of 111.4% and gain of 9dBi [66] as compared to antenna providing an impedance bandwidth of 68% with varied gain of 6.6dBi-9.6dBi [72]. An impedance bandwidth of 110% is achieved when the antenna is designed using horizontal bowtie electric dipole, a vertically oriented folded shorted patch antenna, a microstrip to stripline transition, and a rectangular cavity [69].
A novel differential feeding structure is realized using two slots in the ME dipole and using a rectangular box-shaped reflector, instead of a planar reflector, to provide an impedance bandwidth of 114% with gain of 8.25±1.05dBi [73]. Besides these non-planar differentially- fed ME dipole antennas, planar differentially-fed antenna are also designed, where composite right/left handed transmission line under balanced conditions is used to form high gain magnetic radiators and combined with radial electric radiators, to provide gain of 10.84dBi and 7.16dBi-14.1dBi respectively [74, 78].
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Figure: 1.13 Differential-fed ME dipole antenna [66]
1.2.5.3 Circularly Polarized ME Dipole Antenna
Circularly polarized antennas play a very important role in various wireless communication applications like radar, modern wireless communication, as they have the advantage of providing stable reception of the signal with better mobility. Single feed or dual feed designs, are the two techniques by which circular polarization can be achieved. The single feed design has the biggest advantage of having a simple structure without any significant need of polarizer, (like hybrid and power divider) which is essentially required in dual feed designs. A single feed circularly polarized antenna is designed using two crossed ME dipoles, to provide a 3-dB ARBW of 47.7% [70]. The concept of crossed ME dipole is also emphasized in the antenna, excited by two I-shaped strip probes, a 3-dB hybrid coupler and a reflecting plate to provide a 3-dB ARBW of 42.9% [75], as highlighted in Figure 1.14. A wide beam circular polarization is achieved using crossed dipole, incorporated with double printed vacant quarter rings to feed the antenna, and the antenna is backed by a metallic cavity to provide a unidirectional radiation pattern with a wide axial ratio beam width and a high front to back ratio [81]. The 3-dB ARBW of the antenna is 26.7%. ME dipole antenna is also designed for 60-GHz band using a dielectric substrate and utilizes single feed technique to provide an impedance bandwidth of 56.7% and 3-dB ARBW of 41% for the frequency band ranging from 45.8GHz -69.4GHz [80].
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Figure: 1.14 Circularly polarized ME dipole antenna [75]
1.2.5.4 Planar Printed ME Dipole Antenna
As the ME dipole antennas are designed with height 0.25ko, this poses a big challenge as various wireless communication applications require the antenna to maintain a low profile. Hence, to maintain the industrial requirements related to the low profile, the printed ME dipole antenna came into reality. A vertically oriented unidirectional antenna, composed of bowtie-shaped electric dipole and a loop-shaped magnetic dipole with a microstrip to strip line transition balun, is designed to provide an impedance bandwidth of 17.1%.and 6.5dBi gain [71]. As the orientation of planar electrical dipole was kept vertical, it is named as partial planar ME dipole antenna. A planar printed dual wideband antenna is designed using U-shaped ME dipole with a composite feeding structure [76], as mentioned in Figure 1.15. The composite feeding structure is designed using inverted U-shaped tapered line and a meandering T-shaped line. This planar antenna provides an impedance bandwidth of 35.8% and gain of 3±0.5dBi. A planar printed E-shaped ME dipole antenna with composite U- shaped tapered line along with meandering L-shaped line feeding structure, is designed to provide an impedance bandwidth of 31.6% and gain of 3.1±0.5dBi [77].
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Figure: 1.15 Planar printed ME dipole antenna [76]
1.2.5.5 Reconfigurable ME Dipole Antenna
Rapid developments in mobile communication has generated all time high demand for high data rate and highly reliable wireless communication system. At the same time, antennas with polarization diversity possesses many advantages such as avoiding the fadingloss caused by multi-path effects in the channel and doubling the frequency spectrum to realize frequency reuse. A single feed, wide band, reconfigurable ME dipole antenna is presented, with a bent cross dipole, with one P-Intrinsic-N (PIN) diode inserted into each of its four arms to excite the antenna [86]. The antenna is made reconfigurable to provide a pair of orthogonal linear polarization by controlling the ON/OFF states of four PIN diodes. An overlapped impedance bandwidth (Reflection Co-efficient <-14 dB or SWR < 1.5) of 23.2% in the frequency range 1.86GHz - 2.35GHz is achieved for both polarization. The antenna also provides a stable gain of 8 ± 0.6dBi across the operating band. A band reconfigurable antenna is proposed with one wideband mode (0.83GHz - 2.16GHz) and four narrowband modes [82]. The wideband mode is based on ME dipole technology, and the narrowband modes are realized using a length reconfigurable thin dipole. The advantage of using ME dipole technology is to provide a stable gain and well controlled radiation patterns. Two linearly polarized and dual polarized ME dipole antennas, with a reconfigurable controlled beam width in the H-plane, are presented in [84]. The antennas are designed using tunable parasitic dipoles placed on the sides of a driven ME dipole along its H-plane. Each parasitic dipole is loaded with a varactor diode to vary the strength of the mutual coupling, which will tune the overall radiation pattern produced by the driven ME dipole and the two parasitic dipoles. Two fully functional prototypes give more than 10% impedance bandwidth with the H-plane beam width with a continuously tuning range from 80° to 160° for the linearly polarized antenna and from 72° to 133° for either polarization mode of the dual polarized antenna. A three element linear ME dipole array with beam width reconfiguration for base stations [87], is highlighted in Figure 1.16. The reconfiguration between narrow and wide beam width is realized by varying the phase of the excitation current of the middle element and provides an overlapped impedance bandwidth of 15%, as indicated in Figure 1.17. The half power beam width in the E-plane maintains a mean value of 72° with 2° variation, while the beam width in the H-plane switches between the small valueof 37° and the large value of 136° across the frequency band.
Abbildung in dieser Leseprobe nicht enthalten
Figure: 1.16 A reconfigurable ME dipole antenna [87]
Abbildung in dieser Leseprobe nicht enthalten
Figure: 1.17 Feed network of ME dipole antenna [87]
1.2.5.6 Millimeter Wave ME Dipole Antenna
In recent years, investigation of the 60-GHz antenna technology has caught an attraction because of a series of emerging applications operating at the V-band frequencies. Antenna arrays with high gain characteristics are usually necessary for 60-GHz wireless applications to compensate the large propagation loss from the oxygen absorption in atmosphere. Also, the research on the propagation at 60-GHz demonstrated that the circularly polarized wave can provide more promising channel performance as compared to linearly polarized signal. A circularly polarized, ME dipole as radiating elements, an 8 x 8 high gain wideband planar antenna array is proposed for 60-GHz millimeter wave applications [83], as highlighted in Figure 1.18. The fabrication procedure utilizes conductive adhesive films to bond all Print Circuit Board (PCB) layers together, is implemented to realize the array design with a three layered geometry. The antenna gives an impedance bandwidth of 18.2% and wide 3-dB ARBW of 16.5%. The antenna provides gain up to 26.1dBi and good radiation efficiency of around 70% due to utilization of a full corporate Substrate Integrated Waveguide (SIW) feed network with low insertion loss at millimeter wave frequencies. The SIW technique is also implemented using two PCB laminates, to feed the antenna and a 2 x 2 antenna array is designed to give wide impedance bandwidth of 22% and high gain of 12.5dBi [79]. A single antenna element using low cost single layer PCB is proposed to provide an ultra-wide impedance bandwidth of 51% and 8dBi gain [80].
Abbildung in dieser Leseprobe nicht enthalten
Figure: 1.18Geometry of the SIW fed CP aperture-coupled ME dipole antenna [83]
A novel SIW fed end-fire ME dipole antenna is presented with the antenna structure comprises of an open ended SIW and a pair of electric dipoles integrated into substrates [85]. Both the open-ended SIW and the electric dipoles are effectively radiated together to provide an impedance bandwidth of 44%, symmetrical radiation pattern, which is identical in two orthogonal planes,low backward radiation, low cross polarizations, stable gain of 5dBi and wide beam width of 110°.
1.3 Objectives of the Study
After the evolution of first ever designed MPA, most of the work has been dedicated to explore and improve the electrical characteristics of this antenna. Unlike MPA, the ME dipole antenna has all the desired antenna parameters like wideband, stable and directional radiation pattern across the operating band, low cross-polarization, low back-lobe radiation and symmetrical radiation patterns in the E-plane and H-plane. This research thesis presents the several new designs of ME dipole antenna, possessing the above stated features which can be utilized in various wireless applications. The success of third generation (3G) mobile communication service, has promoted the development of the fourth generation (4G), WiFi, WiMAX, ZigBee and UWB system, which in turn has created a high demand of wideband and low profile, high gain and unidirectional antennas that can accommodate several wireless communication systems with excellent electrical characteristics. Considering these applications, the research work has been done in view of the following objectives:
a) To design of low profile linearly polarized ME dipole antenna with end-fire radiation pattern for airborne radar application.
b) To design of differentially fed ME dipole antenna for UWB applications.
c) To design of high gain ME dipole antenna with modified ground plane.
d) To design of linearly polarized ME monopole antenna.
e) To design of planar circularly polarized ME dipole antenna for X-band applications.
1.4 Structure of the Thesis
The research work gives insight to the novel designs of wideband ME dipole antenna for various wireless communication applications. The standard ME dipole consists of a planar dipole, as an electric dipole and vertically oriented quarter wave shorted patch, as a magnetic dipole. This thesis highlights the good electrical characteristics for the proposed designs of wideband ME dipole antenna, indicating low back lobe radiations, stable antenna gain across the operating frequency band and almost identical radiation patterns in the E-plane and H- plane. On the basis of designs proposed so far, for linearly polarized, circularly polarized or differentially fed ME dipole antenna, five novel structures of ME dipole antenna, are designed and fabricated to accomodate various wireless communication applications in L- band, S-band, C-band, X-band and Ku-band. After examining number of feeding techniques designed to enhance the impedance bandwidth of ME dipole antenna, which includes coaxial to patch probe feeding, transition of microstrip to parallel strip line feeding, differential feeding, co-axial probe has been used in all the designs of antenna for feeding purpose. The chapter-wise brief outline of the thesis is as follows:
Chapter 1 highlights the background information about the typical microstrip patch antenna, complementary antenna and ME dipole antenna, along with their wideband enhancement techniques, in the open literature. The chapter also gives insight into the key advantages of the ME dipole antenna over a conventional microstrip patch antenna with low impedance bandwidth and high variations in E-plane and H-plane radiation pattern.
Chapter 2 presents a novel design of wideband antenna with an ability to generate an endfireradiation pattern, composed of a magnetic dipole and an electric dipole and excited by a Z-shaped feed. This antenna is designed and analyzed, using a pair of planar inverted bowtie-shaped patches acting as an electric dipole, and two pairs of right angle corner vertical walls acting as a magnetic dipole. The proposed antenna element delivers an impedance bandwidth of 23.9% in the frequency range 12.2GHz - 15.5GHz. Due to the complementary nature of the antenna incorporated with phase shifted feed design, the proposed antenna has showcased a relatively stable end-fire radiation pattern with low cross polarization and low back lobe radiations over the entire operating frequency band.
Chapter 3 presents a highly improvised design of a differentially fed, broadside, ME dipole antenna to achieve UWB for covering various applications of wireless communication without the need of extra balun. A novel and special feeding structure is designed in sucha manner that it can provide equal excitation to magnetic dipole and electric dipole, for broadband impedance matching. This antenna achieves an exceptional impedance bandwidth of 133.3% in the frequency range 0.5GHz -2.5GHz.
Chapter 4 emphasizes the design of a novel broadband, linearly polarized ME dipole antenna with modified ground plane, which increases the gain to 10.45dBi and achieves an ultra-wide bandwidth of 68.8%, for the frequency band 2.0GHz - 4.1GHz. The radiation structure is primarily designed using vertically oriented quarter wave shorted patch as magnetic dipole and a planar E-shaped dipole as an electric dipole. A novel design of feeding structure is proposed to provide equal excitation to magnetic dipole and electric dipole, for wide impedance matching. The antenna is kept under rectangular-shaped cavity by modifying the ground plane structure to achieve high gain, low cross polarization level and a broadside radiation pattern. The chapter also showcases the parametric study of the proposed antenna with respect to the variation of dimensions of rectangular cavity and its impact on gain and impedance bandwidth.
Chapter 5 demonstrates the design of a novel broadband planar circularly polarized ME dipole antenna, first of its kind. The proposed antenna is designed and fabricated using RT Duroid 5880 substrate, having permittivity 2.2 and thickness 0.78mm and consists of dual horizontal T-shaped electric dipole, an inverted U-shaped feeding structure, a pair of shorted stubs, and a truncated rectangular shaped ground plane to achieve circular polarization in X- band. The proposed antenna has successfully achieved an impedance bandwidth of 21.1% in the frequency range 8.9GHz -11.0GHz and the axial ratio taken at (0, 0) degree, remains below3-dB from 10.0GHz -11.0GHz hence it delivers 3-dB axial ratio bandwidth (ARBW) of 9.52%. This planar, single-fed antenna also highlights the peak gain of 6.2dBi. The parametric study of the proposed antenna is also included in this chapter to demonstrate the effect of width of feed and dimensions of the ground plane on gain and impedance bandwidth of the antenna.
Chapter 6 gives special attention on efforts made to reduce the overall size of ground plane and hence, introducing a modified class of ME dipole antenna under the name ME monopole antenna . This antenna comprises of a novel unsymmetrical E-shaped electric monopole, with dual 'I-shaped feed and truncated ground plane and has been designed to cover multiple applications of wireless communication in C-band. The novel ME monopole antenna with truncated ground plane operates well in the frequency range 4.5GHz - 8.5GHz and hence achieving 61.5% ultra-wide impedance bandwidth with peak gain of 7.4dBi. Almost symmetrical radiation pattern for both E-plane and H-plane has also been shown by the antenna to indicate omnidirectional radiation characteristics of the ME monopole antenna. Mathematical modelling of ME monopole antenna has been done to measure its performance with a conventional monopole antenna has also been presented. Parametric study of the proposed antenna is also included in this chapter to emphasize the effect of ground plane dimensions on the gain and impedance bandwidth of the antenna.
Chapter 7 provides the concluding remarks of the research work. This research thesis broadly highlights and emphasizes the designs of novel complementary antenna with wide impedance bandwidth, low cross polarization and high gain. To conclude, the proposed novel wideband antenna elements are excellent in all electrical parameters. In particular, their low back radiation characteristic, make them highly attractive for the development of various kinds of indoor and outdoor base station antennas for modern cellular communications. Due to their wideband characteristics and desirable radiation patterns, they can easily find conceivable applications for the recent wireless communication systems like GSM1800/1900, 3G, 4G, Wi-Fi, WiMAX, ZigBee etc. Moreover, the proposed designed antennas are simple in structure and low in manufacturing costs. Therefore, they havegreat potential to be used as a basic element for the design of low cost high performance antenna arrays.
CHAPTER 2
DESIGN OF NEW WIDEBAND END-FIRE ME DIPOLE ANTENNA
2.1 Introduction
Antennas having end-fire radiation pattern are widely adopted in numerous applications as they possess excellent characteristics like: simple structure, ease of fabrication, low cost and low aerodynamic profile. These antennas are widely used in applications where low aerodynamic drag is essentially required and therefore, are used for special airborne activities. Now a days, more emphasis has been given on communication systems with wideband, low profile antenna to accommodate several wireless systems with various electrical characteristics such as wide impedance bandwidth, low cross polarization, low back lobe radiations, identical radiation patterns and stable gain over the frequency range for cost effectiveness and space utilization. Electric dipole and magnetic dipole, when used as an equivalent source in the analysis of antenna, unequal radiation patterns are achieved in the two principal planes. To implement equal E-plane and H-plane radiation patterns with low back radiation, A. Clavin et al . presented the concept of complementary antennas, where almost identical E-plane and H-plane radiation patterns were achieved by exciting an electric dipole and a magnetic dipole with appropriate amplitude and phase [52, 53]. Recently,K.M. Luk et al. has taken a step forward by proposing the concept of ME dipole antenna and implemented a series of wideband unidirectional antennas with stable gain and identical Eplane and H-plane radiation pattern. As confirmed from the literature, these proposed antennas are mainly composed of a vertically oriented quarter wave shorted patch and a horizontal planar dipole excited by r-shaped strip probes [91-94] or a coaxial feed [95]. However, some of the structures have been reported as complicated, especially considering their feeding designs and techniques. Hence, to make ME dipole antenna popularized into practical applicationsof wireless communication, an improved strategy to design novel and less complicated antenna, is a challenging and pressing task. The literature available for ME dipole antenna highlights only broadside radiations as a conventional radiation pattern, in L-band and S-band only. A dual-polarized ME dipole antenna, fed by four r-shaped probes, and achieved an impedance bandwidth of 24.9%, for GSM 1800 and GSM 1900 bands[96]. A broadband dual-band antenna for mobile base station has been designed using L-shaped feeding strips, to achieve an impedance bandwidth of 34% for 0.78GHz-1.1GHz and 49.5% for 1.58GHz-2.62GHz[97].
Conventionally, end-fire antennas are designed for the applications, where high directivity is essentially required. Recently, end-fire antennas have gained popularity as broadcasting and communication antennas, which are considered as middle directivity applications.
In this chapter, a novel wideband ME dipole antenna with Z-shaped feed line has been introduced, designed and fabricated. The antenna has been simulated using EM wave software IE3D. This antenna shows a rare end-fire radiation pattern suitable for airborne radar applications and shows high directivity, stable gain, wide bandwidth and low cross polarization level with 90% antenna efficiency.
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Figure: 2.1 Top view of proposed antenna
2.2 Antenna Description and Design Geometry
The proposed antenna has been designed by combining an electric dipole with a magnetic dipole. Electric dipole consists of inverted bow tie dipole antenna while quarter wave shorted patch is used as magnetic dipole.
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Figure: 2.2 Side view of the proposed antenna
The Top view and side view of the proposed antenna are shown in Figure 2.1 and Figure 2.2 respectively. It is to be noted that the primary calculations of the proposed antenna parameters have been carried out at 13GHz. The length of the ground plane has been taken as 42mm and width of the ground plane is 55mm. It is indicated from the figures that the inverted bowtie patch [115] forms an electric dipole with LD = 10mm and H=7mm and a pair of vertically oriented quarter wave shorted patch antenna, at a height of 7mm, forms a magnetic dipole. This antenna is excited by Z-shaped feed to produce sufficient magnetic current along the edges of two vertical walls. The width of Z-shaped feed has been chosen as 2.5mm. The antenna is designed using copper sheet having thickness of 0.3mm. Wherever possible, single sheet has been used to design various parts of antenna to reduce the mechanical errors. Table 1 indicates all the optimized dimensions of the proposed antenna. The Z-shaped feed has been designed considering two very important parts: a transmission line and a coupled line. The purpose of a transmission line is to carry electrical energy from SMA connector to electric dipole as well as magnetic dipole, whereas the coupled line is used to provide impedance matching in such a way that the inductive reactance introduced by shorted strip is balanced by capacitive reactance offered by open ended strip. A coaxial probe is connected to the transmission line at a height of 1mm from the ground plane. The remaining L-shaped coupling strip is designed on both sides of the transmission line, so that the feed appears to be in Z-shaped.
Table: 2.1 Optimized dimensions of the proposed antenna with Z-shaped feed
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This design of end-fire ME dipole antenna was initialized by considering appropriate dimensions of electric dipole, magnetic dipole and the feed line in such a manner that the antenna would radiate in Ku-band. The dimensions shown in Table 1 are finalized after doing multiple times simulation of antenna and subsequent analysis of the results. The prototype of the proposed antenna is shown in Figure 2.3.
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Figure: 2.3 Prototype of proposed antenna
2.3 Analysis of ME Dipole Antenna with Z-shaped Feed
According to the structure and function of the ME dipole antenna, it is observed that the input impedance of antenna, Zin, can be calculated using estimated dimensions of feed line and obtaining the equivalent impedance of each part. Equation 1 represents the equivalent input impedance of the proposed antenna with Z-shaped feed.
Z in = Zl+ Z c (CD)+(Zl(BD) I I Z c (DF))+Z o (BB’)+( Zl(B’D’) I I Z c (D’F’))+ Zl (1)
where Z o (BB') isthe characteristic impedance of the transmission line, Z c (CD) is the open circuit impedance, provided by the part CD, Zl(BD) and Zl(B D') is the impedance provided by the horizontal part of coupling strip inductance, Zc(DF) and Zc(D’F’) are the impedance provided by vertical portion of coupling strip, DF and D'F' respectively and Z L is the impedance provided of an electric dipole. In the resonant process, the horizontal part of coupling strip and electric dipole behave like an inductor while magnetic dipole, vertical portions of feeding structure, and shorted wall behave like a capacitor. The schematic of proposed MEdipole antenna along with its equivalent circuit is shown inFigure 2.4.
To calculate the input impedance Zin, the impedance of each part can be expressed as:
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Here L and C represent the inductance and capacitance respectively, whereas l represents the electrical length. On the basis of equation (1), it can be stated that the input impedance is mainly affected by the following parameters:
- horizontal length and vertical height of electric dipole,
- the horizontal length and vertical length of the coupling strip
- the gap between the electric dipole and the feed.
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Figure: 2.4(a) Schematic of proposed antenna
2.4(b) Equivalent circuit of proposed antenna
2.4 Results and Discussion
The antenna behavior has been predicted by simulating it using method of moment (MOM) based full wave electromagnetic simulator IE3D. A copper sheet with thickness 0.3mm has been used while fabricating the antenna to validate the experimental results with the simulation results. It has been observed that the predicted and measured results are matched within the acceptable limits and the slight variations shown in all the measured results would have arised due to mechanical errors such as error while performing bending, soldering or presence of supporting screws in the structure etc. The simulated and measured results of reflection coefficient, S 11, of the proposed Z-shaped feed antenna has been shown in Figure. 2.5. From the figure, it is observed that the antenna has been resonating at two frequencies, which are very close to each other and hence giving wideband characteristics to the antenna. The proposed antenna has shown the impedance bandwidth of 3.3GHz i.e. 23.9%, in the frequency range 12.2GHz - 15.5GHz.
Antenna efficiency is an important parameter to measure and analyze the effective radiation activity of the antenna. The proposed antenna has shown efficiency, greater than 85%, throughout in the frequency band of operation, as shown in Figure 2.6.
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