Table of Contents

Title Page

Copyright

Preface

Acknowledgements

Abbreviations and Acronyms

Chapter 1: Introduction to Circularly Polarized Antennas

1.1 Introduction

1.2 Antenna Parameters

1.3 Basic CP Antenna Types

1.4 Antenna Modelling Techniques

1.5 Typical Requirements and Challenges in CP Antenna Designs

1.6 Summary

References

Chapter 2: Small Circularly Polarized Antenna

2.1 Introduction

2.2 Basic Theory of Antenna Size Reduction

2.3 Small CP Patch Antennas

2.4 Small Helix, QHAs and PQHAs

2.5 Small CP Slot Antennas

2.6 Small CP DRAs

2.7 Other Small CP Antennas

2.8 Summary

References

Chapter 3: Broadband Circularly Polarized Antennas

3.1 Introduction

3.2 Broadband CP Microstrip Patch Antennas

3.3 Broadband Helix, QHAs and PQHAs

3.4 Spiral Antennas

3.5 Broadband CP Slot Antennas

3.6 Broadband CP DRAs

3.7 Broadband CP Loop Antennas

3.8 Other Broadband CP Antennas

3.9 Summary

References

Chapter 4: Multi-Band Circularly Polarized Antennas

4.1 Introduction

4.2 Multi-Band CP Microstrip Patch Antennas

4.3 Multi-Band QHAs and PQHAs

4.4 Multi-Band CP Slot Antennas

4.5 Multi-Band CP DRAs

4.6 Multi-Band CP Loop Antennas

4.7 Other Multi-Band CP Antennas

4.8 Summary

References

Chapter 5: Circularly Polarized Arrays

5.1 Introduction

5.2 CP Patch Antenna Arrays

5.3 CP Dielectric Resonator Antenna Arrays

5.4 CP Slot Array Antenna

5.5 CP Printed Reflectarrays

5.6 Integrated CP Array and Active CP Array

5.7 CP Array with Reconfigurable Beams

5.8 Other CP Arrays

5.9 Summary

References

Chapter 6: Case Studies

6.1 Introduction

6.2 Dual-Band CP Patch Array for GNSS Reflectometry Receiver on Board Small Satellites

6.3 Small Printed Quadrifilar Helix Antenna for Mobile Terminals in Satellite communications

6.4 Printed Broadband CP Rectangular Bi-Loop Antenna for RFID Readers

6.5 CP Reflectarray for Ka Band Satellite Communications

6.6 Circularly Polarized Logarithmic Spiral Antenna with a Wideband Balun

6.7 Summary

References

Index

This edition first published 2014

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Library of Congress Cataloging-in-Publication Data

Gao, Steven.

Circularly polarized antennas/Steven (Shichang) Gao, Qi Luo, and Fuguo Zhu.

pages cm

Includes bibliographical references and index.

ISBN 978-1-118-37441-2 (cloth)

1. Spiral antennas. 2. Transmitting antennas. 3. Antenna radiation patterns. 4. Polarized beams

(Nuclear physics) I. Luo, Qi, 1982- II. Zhu, Fuguo. III. Title.

TK7871.67.S65G36 2014

621.382′4—dc23

2013030500

A catalogue record for this book is available from the British Library.

ISBN: 978-1-118-37441-2

Preface

Due to the features of circular polarization, circularly polarized antennas are very useful for various wireless systems such as satellite communications, global navigation satellite systems, mobile communications, wireless sensors, radio frequency identification, wireless power transmission, wireless local area networks, wireless personal area networks, worldwide interoperability for microwave access and direct broadcasting service television reception systems. Recent decades have seen a lot of research and development activities in CP antennas from industries and institutes worldwide. There are very few books focusing on CP antenna design and principles. Thus, there is a need for a comprehensive book which presents basic principles and up-to-date developments of CP antennas and arrays.

The purpose of this book is to present a comprehensive overview of various types of CP antennas and arrays, including the basic principles, design methods, size reduction techniques, broadband techniques, multi-band techniques, array design techniques and recent developments, as well as their applications and case studies. This book can be used as a reference book for graduate students, academics, researchers and antenna engineers. It includes up-to-date developments from industry and academic research experts worldwide, and numerous CP antenna design examples are illustrated.

The book is organized into six chapters. Chapter 1 introduces the reader to basic concepts of antenna parameters and the principles of typical CP antennas. This will help readers who are not familiar with the antenna basics. Many different types of basic CP antennas including CP patch antennas, crossed dipoles, helix antennas, quadrifilar helix antennas, printed quadrifilar helix antennas, spiral antennas, slot antennas, dielectric resonator antennas, patch arrays and slot arrays are explained.

Chapter 2 discusses various techniques for designing small-size CP antennas. Size reduction of CP antennas is important for practical applications as mobile devices are getting smaller and having more functions. Design techniques of different types of small CP antennas are presented, and many design examples are illustrated.

Chapter 3 introduces various types of broadband CP antennas and their design techniques. Broadband CP antennas are important for applications such as global navigation satellite system receivers, high-speed satellite data downlink, high-speed wireless communication, and so on. Design techniques of different types of broadband CP antennas are reviewed, discussed and compared.

Chapter 4 presents the design techniques of multi-band CP antennas which are important for many applications such as satellite communications, global navigation satellite systems, radio frequency identification, wireless power transmission, wireless local area networks, and so on. Design techniques of different types of multi-band CP antennas are discussed, and their advantages and disadvantages are explained.

Chapter 5 discusses different types of CP arrays and their design techniques. These include CP patch arrays, the CP dielectric resonator array antenna, CP waveguide slot arrays, CP reflectarrays, and so on. Broadband CP active integrated arrays, beam-switching and electronica beam-steering CP arrays are also discussed.

Chapter 6 presents case studies that illustrate how to design and implement CP antennas in practical scenarios. Five case studies, including a dual-band CP array for GPS remote sensing applications, a small printed quadrifilar helix antenna for satellite communications mobile terminals at the S-band, a broadband CP antenna for a radio frequency identification reader, a CP reflectarray for Ka-band mobile satellite communications and a wideband logarithmic spiral antenna for wideband RF measurement applications, are presented. The design guidelines for each of these CP antennas are given and each design step is explained. Thus the reader is provided with a comprehensive and logical path from CP antenna basics to advanced designs of different types of CP antennas and arrays.

Acknowledgements

The authors would like to thank the support from Anna Smart, Liz Wingett, Sarah Tilley, Susan Barclay and Tiina Wigley from Wiley. Dr Kai Zhang from the University of Kent helped draw some figures.

Steven Gao would like to express his deep appreciation to his wife, Jun Li, and his daughter, Karen, for their support.

Qi Luo is very grateful for the support from his wife, Yuxin Du. He also would like to express his love to his mother, Daoyu Huang.

Fuguo Zhu would like to thank his wife, Yanfang Wang, for her continued patience and support during all his book endeavours.

Abbreviations and Acronyms

Chapter 1

Introduction to Circularly Polarized Antennas

1.1 Introduction

Circularly polarized (CP) antennas are a type of antenna with circular polarization. Due to the features of circular polarization, CP antennas have several important advantages compared to antennas using linear polarizations, and are becoming a key technology for various wireless systems including satellite communications, mobile communications, global navigation satellite systems (GNSS), wireless sensors, radio frequency identification (RFID), wireless power transmission, wireless local area networks (WLAN), wireless personal area networks (WPAN), Worldwide Interoperability for Microwave Access (WiMAX) and Direct Broadcasting Service (DBS) television reception systems. Lots of progress in research and development has been made during recent years.

The CP antenna is very effective in combating multi-path interferences or fading [1, 2]. The reflected radio signal from the ground or other objects will result in a reversal of polarization, that is, right-hand circular polarization (RHCP) reflections show left-hand circular polarization (LHCP). A RHCP antenna will have a rejection of a reflected signal which is LHCP, thus reducing the multi-path interferences from the reflected signals.

The second advantage is that CP antenna is able to reduce the ‘Faraday rotation’ effect due to the ionosphere [3, 4]. The Faraday rotation effect causes a significant signal loss (about 3 dB or more) if linearly polarized signals are employed. The CP antenna is immune to this problem, thus the CP antenna is widely used for space telemetry applications of satellites, space probes and ballistic missiles to transmit or receive signals that have undergone Faraday rotation by travelling through the ionosphere.

Another advantage of using CP antennas is that no strict orientation between transmitting and receiving antennas is required. This is different from linearly polarized antennas which are subject to polarization mismatch losses if arbitrary polarization misalignment occurs between transmitting and receiving antennas. This is useful for mobile satellite communications where it is difficult to maintain a constant antenna orientation. With CP, the strength of the received signals is fairly constant regardless of the antenna orientation. These advantages make CP antennas very attractive for many wireless systems.

This chapter serves as a basis for the chapters that follow. It will introduce some basic parameters of antennas. Different types of basic CP antennas such as CP microstrip patch antenna, helix, quadrifilar helix antenna (QHA), printed quadrifilar helix antenna (PQHA), spiral antenna, CP dielectric resonator antenna (DRA), CP slot antennas, CP horns and CP arrays will be described and basic designs illustrated. Typical requirements and challenges in CP antenna designs will be discussed at the end.

1.2 Antenna Parameters

An antenna is a device which can receive or/and transmit radio signals. As a receiving device, it can collect the radio signals from free space and convert them from electromagnetic waves (in the free space) into guided waves in transmission lines; as a transmitting device, it can transmit radio signals to free space by converting the guided waves in transmission lines into the electromagnetic waves in the free space. In some cases, an antenna can serve both functions of receive and transmit.

Figure 1.1 depicts the basic operation of a transmit antenna. As shown, the information (voice, image or data) is processed in a radio transmitter and then the output signal from the transmitter propagates along the transmission lines before finally being radiated by the antenna. The antenna converts the guided-wave signals in the transmission lines into electromagnetic waves in the free space. The operation of a receive antenna follows a reverse process, that is, collecting the radio signals by converting the electromagnetic waves in free space into guided-wave signals in the transmission lines, which are then fed into radio receivers.

Figure 1.1 Basic operations of a transmit antenna

1.2.1 Input Impedance

The input impedance Z_in is defined as the impedance presented by an antenna at its feed point, or the ratio of the voltage to current at the feed point [5]. The input impedance is usually a complex number which is also frequency dependent. It can be expressed as

1.1

The real part of the impedance, R_in, includes the radiation resistance R_r of the antenna and the loss resistance R_L. R_r relates to the power radiated by the antenna, and R_L relates to the power dissipated in the antenna due to losses in dielectric materials, antenna conductor losses, and so on.

1.2.2 Reflection Coefficient, Return Loss and Voltage Standing Wave Ratio

The antenna input impedance needs to be matched with the characteristic impedance of the transmission line connected to the feed point of the antenna. Usually a 50 Ω cable is used to feed the antenna. Thus the antenna input impedance needs to be equal to 50 Ω, otherwise there will be an impedance mismatch at the antenna feed point. In the case of impedance mismatch, there will be signal reflections, that is, some of signals fed to the antenna will be reflected back to the signal sources.

The reflection coefficient Γ denotes the ratio of the reflected wave voltage to the incident wave voltage [5]. The reflection coefficient at the feed point of the antenna can be related to the antenna input impedance by the following equation:

1.2

Here, Z_in and Z_o denote the input impedance of the antenna, and the characteristic impedance of the transmission line connected to the antenna feed point, respectively. As shown in equation (1.2), the reflection coefficient is zero if Z_in is equal to Z_o.

Return loss (in dB) is defined as:

For a well-designed antenna, the required return loss should usually be at least 10 dB, though some antennas on small mobile terminals can only achieve about 6 dB. Voltage Standing Wave Ratio (VSWR) is the ratio of the maximum voltage V_max to the minimum voltage V_min on the transmission line. It is defined as:

1.3

1.2.3 Radiation Patterns

The radiation pattern of the antenna illustrates the distribution of radiated power in the space [6–9]. It can be plotted in a spherical coordinate system as the radiated power versus the elevation angle (θ) or the azimuth angle (). Figure 1.2 shows a radiation pattern plotted as the radiated power versus the elevation angle (θ). As shown, the radiation pattern has a few lobes. The main lobe is the lobe containing the majority of radiated power. The lobe radiating towards the backward direction is the back lobe. Usually there will also be a few other small lobes called the side lobes. The 3-dB beamwidth indicated in the figure refers to the angular range between two points where the radiated power is half the maximum radiated power. Figure 1.2 shows the radiation pattern in the elevation plane. There is also the radiation pattern in the azimuth plane, which can be plotted as the radiated power versus the azimuth angle (). The antenna pattern can be isotropic, directional or omni-directional. An isotropic pattern is uniform in all directions, which does not exist in reality. The pattern in Figure 1.2 is directional. As shown in Figure 1.2, the majority of radiated power is focused at one direction and the maximum radiation is along the z axis. An omni-directional pattern is donut-shaped, as shown in Figure 1.3.

Figure 1.2 A directional radiation pattern in the elevation plane

Figure 1.3 Omni-directional pattern

1.2.4 Directivity, Gain and Efficiency

A practical antenna usually radiates in certain directions. The directivity, D(θ, ), is defined as the radiated power per unit solid angle compared to what would be received by an isotropic radiator [6–9]. It can be calculated by

1.4

where E, H, r and P_rad denote the peak value of electric field, the peak value of magnetic field, the distance between the source and test point, and the radiated power from the antenna, respectively. It is also assumed that the test point is in the far field region of the antenna, which means the distance [6–9]. Here D is the maximum dimension of the antenna and λ is the wavelength.

The gain of the antenna is similar as the directivity though it includes the efficiency η of the antenna, since some power will be lost in the antenna.

1.5

Both directivity and gain are normally expressed in dB. It is common practice to write the antenna gain in dBi, which means that it is defined relative to an isotropic radiator.

1.2.5 Linear Polarization, Circular Polarization and Axial Ratio

Polarization of an antenna is related to the orientations of electric fields radiated by the antenna. Assuming a half-wavelength dipole is vertically oriented above the Earth, it will produce radiated fields in the far field and the radiated electric fields will be dominated by E_θ(θ, ). In this case, the polarization of the dipole is called vertical polarization. On the other hand, if a half-wavelength dipole is horizontally oriented above the Earth, the radiated electric fields of the antenna will be dominated by in the far field. The polarization of the antenna is then called horizontal polarization. Both vertical and horizontal polarizations are linear polarizations. Linearly polarized antennas are commonly used in terrestrial wireless communications.

To produce circular polarization, two orthogonal components of electric fields in the far field region are required [6–10]. The electrical field radiated by an antenna can be written as

1.6

Here E_θ(θ, ) and denote the magnitudes of electric field components in the far field of the antenna. φ₁ and φ₂ denote the phase shift of each field component.

Circular polarization can be achieved only if the total electric field has two orthogonal components which have the same magnitudes and a 90^o phase difference between the two components. That is

1.7

For a circularly polarized wave, the electric field vector at a given point in space traced as a function of time is a circle. The sense of rotation can be determined by observing the direction of the field's temporal rotation as the wave is viewed along the direction of wave propagation: if the field rotation is clockwise, the wave is RHCP; if the field rotation is anti-clockwise, the wave is LHCP.

In reality, it is impossible to achieve a perfect circular polarization, thus the curve traced at a given position as a function of time is usually an ellipse, as shown in Figure 1.4. Lines a and b denote the major axis and the minor axis of polarization ellipse, respectively. The ratio of the major axis to the minor axis of the ellipse is termed as the axial ratio (AR) [6–10].

1.8

AR is a key parameter for measuring the circular polarization. Usually AR is required to be below 3 dB for a CP antenna.

Figure 1.4 Polarization ellipse traced at a certain position as a function of time

1.2.6 Bandwidth and Resonant Frequency

Usually an antenna is designed to operate within a specified frequency range. The bandwidth of an antenna is usually determined by the frequency range within which the key parameter of the antenna satisfies a certain requirement, for example, minimum return loss of 10 dB. At the resonant frequency of an antenna, the antenna input impedance is purely resistive. Often the resonant frequency is chosen as the centre of the frequency bandwidth of an antenna. The bandwidth of an antenna can be calculated by using the upper and lower edges of the achieved frequency range:

1.9

where f₁ is the lower edge of the achieved frequency range,

f₂ = is the upper edge of the achieved frequency range, and

f_o = is the centre frequency of the range.

Note that this definition is for antennas with a bandwidth below 100%. For antenna bandwidths over 100%, the bandwidth can be calculated using the ratio between the upper and lower edge of frequencies. For a linearly polarized antenna, the input impedance is usually the most sensitive parameter compared to other antenna parameters such as radiation patterns, gain and polarization. Thus the bandwidth of a linearly polarized antenna is often referred to as the ‘impedance bandwidth’, but it can also be to do with other parameters such as radiation patterns, gain and polarization.

When evaluating the bandwidth of CP antennas, one must check both the impedance bandwidth and the bandwidth of AR, that is, the frequency range within which the AR is below 3 dB. A good impedance matching does not necessarily lead to a good gain or a low AR value. The impedance bandwidth of an antenna can be broadened using suitable impedance matching networks, while the AR bandwidth can be broadened by using a broadband phase shifter network [5].

1.3 Basic CP Antenna Types

1.3.1 CP Microstrip Patch Antennas

Microstrip patch antenna is one very popular type of antennas, due to its advantages of low profile, easy fabrication, low cost and conformability to curved surfaces. Circular polarization in patch antennas can be achieved using a multi-feed technique or a single-feed technique [6–9, 11–13].

Figure 1.5 shows a simple CP microstrip patch antenna using a dual-feed technique. Both the top and side views of the antenna are shown in Figure 1.5. To produce circular polarization, a square microstrip patch is fed by two orthogonal microstrip feedlines as shown in Figure 1.5(a). Two microstrip feed lines excite the patch antenna in TM₀₁ and TM₁₀ modes so that it radiates both a horizontally polarized wave and a vertically polarized wave simultaneously [6–9, 11–13]. A microstrip hybrid is employed in Figure 1.5(a) to produce a 90^o phase difference between two orthogonally polarized waves. Figure 1.5(b) shows the side view of the antenna. The metallic patch is etched on the top of a dielectric substrate having thickness h and relative permittivity . The dielectric substrate is backed by a metallic ground plane. To design the antenna resonant at a frequency , the length L of the patch can be approximately calculated by using the following equation:

1.10

where c is the velocity of light. To achieve accurate antenna designs, full-wave electromagnetic simulators can be employed to do simulations and optimizations of the dimensions of antenna. The results from equation (1.10) can be used as an initial value for the antenna optimization.

Figure 1.5 A microstrip line-feed patch antenna with a 90^o hybrid

As shown in Figure 1.5(a), two microstrip feed lines connect the square patch and two ports of the microstrip 90^o hybrid. The microstrip line serves as an impedance transformer between the input of the antenna and the input ports of the hybrid. The length () and characteristic impedance () of the microstrip feed lines can be calculated by

1.11

where is the guided wavelength of the microstrip line,

= is the input impedance of the patch antenna, and

= is the characteristic impedance of the microstrip line at the input of hybrid circuit.

The microstrip 90^o hybrid circuit consists of four sections of microstrip lines. The following equations can be employed to determine the length and width of each section of microstrip lines:

1.12

where and denote the length of microstrip lines as indicated in Figure 1.5(a), and denote the characteristic impedance of microstrip line sections indicated in Figure 1.5(a).

The characteristic impedance of the microstrip line at the input of hybrid circuit, , is usually chosen to be 50 Ω. The microstrip hybrid circuit is easy to be fabricated and has been widely used in CP antennas. One drawback of microstrip hybrid circuit is the large size. Many techniques have been developed to reduce the size of microstrip hybrid, for example, by using ‘Π network’ with stub loading or lumped-element loading of transmission lines [5, 14, 15]. The hybrid circuit can also be implemented by using lumped elements or left-handed transmission lines [14, 15].

The CP patch antenna can also use other feeding structures, such as probe feeds, slot-coupled feeds, electromagnetically-coupled feed and coplanar waveguide (CPW) feeds. Figures 1.6 and 1.7 show a CP patch antenna using two probe feeds, and a CP patch with slot-coupled feeds, respectively. Both the top view and side view of the antenna are shown in Figure 1.6. As shown, circular polarization is obtained by feeding a square patch with a 90^o phase difference between two feed probes placed symmetrically on the two orthogonal edges of the patch. In this case, an external phase shifter is required for producing the 90^o phase difference between two feeds. The antenna in Figure 1.7 employs a microstrip hybrid circuit for producing the 90^o phase difference between two feeds. The microstrip hybrid is put at the bottom of the antenna, and coupled to the square patch on the top via two orthogonal slots cut in the ground plane. The slot-coupled CP antenna has been widely used in wireless systems during recent years, as it has many advantages compared to CP antennas using other feed structures: (1) It allows the patch antenna and the feed circuits to employ different dielectric substrate so that both the patch antenna and the feed circuits can achieve optimized performance; (2) It is easy to integrate active circuits with the feed network; (3) The parasitic radiation of feed network is reduced due to the isolation of the ground plane. The slot-coupled CP patch antenna is a popular choice for radiating elements in phased arrays for satellite communications.

Figure 1.6 A probe-feed patch antenna with an external 90° phase shifter

Figure 1.7 Slot-coupled CP patch antenna (dual feed with a hybrid coupler)

For the dual-feed antenna, the radiating patch can use different shapes, such as square, circular, annular ring, and so on. Figure 1.8 shows a circular patch integrated with a microstrip hybrid circuit. As shown, the antenna can achieve either RHCP or LHCP, depending on the choice of input port.

Figure 1.8 Circular patch integrated with a hybrid coupler

To design the CP antenna in Figure 1.8, an approximate solution to the radius of patch is given by [6, 7]:

1.13

where a is the radius of circular patch,

= is the resonant frequency of the CP antenna in TM₁₁ mode

= is the relative permittivity of the dielectric substrate, and

h = is the thickness of the substrate.

Besides the use of two feeds, it is also possible to excite circular polarization using more than two feeds. For example, circular polarization in the patch antenna can be excited using four feeds orthogonally located at four edges of a square patch and with an appropriate phase difference. Such a multi-feed technique can suppress higher order modes, and provide high polarization purity and a broad bandwidth at the expense of large size and complexity of feed network [13].

To simplify the feed network of CP patch antennas, a single-feed technique has been developed. Figure 1.9 shows six different configurations of single-feed CP microstrip patch antennas [12, 13]. Such a technique requires the perturbation of the patch shape. For example, Figure 1.9(a) shows an elliptical patch fed along a line 45^o from its major axis. The elliptical patch can be regarded as a circular patch with perturbations. The perturbation of the patch shape is used to excite two orthogonal modes with a phase difference. To design the CP antenna using a single-feed elliptical patch, the ratio between the major axis and minor axis is given by:

1.14

Figure 1.9 Single-feed CP patch antenna

The value of antenna quality factor Q can be computed using the cavity model [6, 7, 12, 13]. Alternatively, one can either measure the Q of the antenna experimentally, or use results from a full-wave electromagnetic analysis to estimate Q.

1.15

where is the resonant frequency of the antenna, and

= is the bandwidth of the antenna.

Another option for single-feed CP patch antenna design is to use the configuration shown in Figure 1.9(b), which is a nearly square patch fed at a point along the diagonal line of the patch. The length and width of the patch are L and W, respectively. The condition of circular polarization is satisfied when

1.16

The value of antenna quality factor Q can be computed using equation (1.15). The resonant frequencies f₁ and f₂ associated with the length L and width W of a rectangular microstrip patch are

1.17

where f_o is the centre frequency of the bandwidth.

Figure 1.9(c) and (d) show a square patch with two corners truncated, and a circular patch with two notches, respectively. Both antennas employ a single probe feed and the feed position is indicated in the figure. The perturbation can also take the form of a narrow slot cut in the centre of the patch, as shown in Figure 1.9(e) and (f). Other shapes of single-feed patch antennas, using pentagons, annual elliptic patches, and so on have also been reported [12, 13]. The single-feed technique does not require a complicated feed network as in the multi-feed CP patch antennas, and is compact in size. The main drawback of this technique is the narrowband AR performance, typically 1–2%.

As in the case of multi-feed CP patch antenna, the single-feed CP antenna can also use different feed structures. Figure 1.10 shows a slot-coupled single-feed patch antenna. As shown, a circular patch with two notches is put on the top of dielectric substrate. The feed network is coupled to the patch via a single slot cut in the ground plane. Due to the use of a single slot, the feed network is much simpler compared to the dual-feed CP antenna in Figure 1.7. The slot can produce bi-directional radiation. To reduce the backward radiation from the slot, another ground plane can be added at the bottom, as shown in Figure 1.10. In this case, the feed network uses the stripline instead of microstrip lines, and usually vias are added between two ground planes for avoiding the excitation of parallel-plate modes in the stripline circuits [13].

Figure 1.10 Slot-coupled single-feed patch antenna

1.3.2 CP Wire Antennas

Crossed dipoles using wires have been employed to obtain circular polarization for many years. Figure 1.11 shows a dipole antenna and a crossed dipole antenna. The half-wavelength dipole in Figure 1.11(a) is vertically polarized and has an omni-directional pattern. For a crossed dipole in Figure 1.11(b), two dipoles are mounted perpendicular to each other and fed with a 90^o phase difference between them. The 90^o phase network can employ one quarter-wavelength of coaxial cable.

Figure 1.11 A dipole and a crossed dipole antenna

1.3.3 Helix Antennas

The helix antenna is one of the most promising antenna types for CP applications [6, 9, 11]. Figure 1.12 shows a helix antenna. It is basically a conducting wire wound in the form of a screw thread. Key design parameters of a helix antenna include the diameter of one turn (D), circumference of one turn (C), vertical separation between turns (S), the number of turns (N) and pitch angle (α), which controls how far the helix antenna grows in the axial-direction per turn.

Figure 1.12 Helix antenna

The helix antenna has gained wide application because of its simple structure, wide operation bandwidth and circular polarization. The helix antenna can operate at three different modes [6, 9, 11]:

• Normal mode, which occurs when the diameter of the helix is relatively small compared to the wavelength. The radiation pattern is omni-directional as shown in Figure 1.13(a).
• Axial mode, which occurs when the circumference of the helix is of the order of one wavelength. The maximum radiation is along the axis of the helix, as shown in Figure 1.13(b).
• Higher-order radiation mode, which occurs when the dimensions of the helix exceed those required for the axial mode. The major lobe is split-up as shown in Figure 1.13(c).

Figure 1.13 Axial, normal and higher order radiation modes of helix antenna [10, 16]

The axial mode is the one of major interests for CP applications. The normal-mode helix is useful for terminals in terrestrial cellular systems but not CP applications. The following gives the design equations for key design parameters of the helix so that it can achieve optimum performance in the axial mode [6, 7, 9]:

1.18

where λ is the free-space wavelength.

To understand how circular polarization is produced by a helix antenna, the helix can be approximated by N small loops and N short dipoles connected together in series. Two orthogonally polarized fields are produced by the loops and the dipoles, respectively. Here the planes of the loops are parallel to each other and perpendicular to the axes of the vertical dipoles. A 90^o phase difference between these two orthogonal fields is obtained if the vertical separation between turns S is chosen to be one quarter-wavelength as given in equation (1.18). The helix antenna has inherently broadband properties, possessing desirable radiation patterns, impedance and polarization characteristics over a relatively wide frequency range. The axial mode pattern exists for a nearly 2 to 1 frequency range because the natural adjustment of phase velocity results in the fields from the different turns adding in phase in the axial direction. The input impedance remains almost constant because of the large attenuation to the reflected waves from the open end, and the antenna polarization remains circular because the in-phase field condition is a condition for circular polarization too [9].

1.3.4 Quadrifilar Helix Antennas and Printed Quadrifilar Helix Antennas

The QHA is one of the most commonly used antennas for satellite communications and Global Positioning Systems (GPS) applications [9, 16–19]. The QHA can produce a cardioid-shaped radiation pattern with excellent circular polarization over a wide angular range. Such a pattern is suitable for GPS as it allows more satellites to be visible. Figure 1.14 shows two typical configurations of QHA antennas. Basically, a QHA consists of four identical helices interleaved with each other. Four identical helices are fed with a separate phase quadrature network which provides 0^o, 90^o, 180^o and 270^o phases to each of four helices, respectively. The helices can be shorted circuited or open-circuited at the end, as shown in Figure 1.14(a) and (b), respectively. QHA has important features of versatility and flexibility, due to the many degrees of freedom; such as the total length of each helical element, the number of turns, radius of helix, pitch angle, axial length and so on. In addition to the cardioid-shaped pattern, optimum choice of antenna parameters in QHA can lead to other types of radiation patterns, gain and bandwidth performance so that it can be used in a variety of applications for satellite communications and terrestrial systems. The QHA is a resonant radiating structure when the total length of each helical element, L_total, is equal to an integer number of quarter wavelengths [9].

1.19

Here N is an integer number. When N is an even number, the helices should be shorted together, as shown in Figure 1.14(a); while when N is an odd number, the helices should be open-circuited, as shown in Figure 1.14(b).

Figure 1.14 Configurations of QHA

Figure 1.15 shows the simulated results of an end-shorted QHA which has helices of λ/2 long and ½ turn. Figure 1.15(a) shows the simulated radiation pattern of a QHA. As shown, QHA can achieve a wide-beam circularly polarized pattern, suitable for GPS signal reception. The simulated reflection coefficient results are shown in Figure 1.15(b). It is noted that, compared to the monofilar helical antenna, the QHA has a narrower bandwidth.

Figure 1.15 Simulated results of an end-shorted QHA

The QHA mentioned previously requires a separate phase quadrature network for providing 0^o, 90^o, 180^o and 270^o phases to each of four helices. Such a feed network increases the size and complexity of antennas. To alleviate this problem, self-phased QHA has been developed. In the self-phased approach, two bi-filars with different lengths are employed and the difference in lengths of bi-filars leads to a 90^o phase difference between them. As the self-phased QHA does not require a separate phase quadrature network, it reduces the size and complexity of QHA. QHA is a popular choice for handheld mobile terminals in mobile satellite communications due to its small size and hemi-spherical radiation patterns.

The fabrication of QHA will require accurate bending and shaping of wires which is not always easy. Figure 1.16 shows a PQHA, which is basically a printed version of QHA antennas. Four printed helices are wound around a cylinder as shown in Figure 1.16. The four printed helices are to be fed in phase quadrature in order to produce the desired hemi-spherical pattern. Compared to QHA antennas, PQHA is easier to fabricate in mass quantities and is lower cost, as the antennas can be fabricated using standard printed circuit board (PCB) technology. In addition, PQHA allows more flexibility in antenna designs. For example, it is easy to produce a PQHA with meandered-line helices patterns so as to reduce the antenna size [20]. Such a pattern will be, however, difficult to implement in wires for QHA. The use of printed technology enables accurate fabrication of PQHA with less fabrication tolerance issues as in the case of QHA. Due to the use of PCB technology, it is also possible to integrate PQHA with microwave diodes or devices.

Figure 1.16 PQHA antenna

1.3.5 Spiral Antennas

Spiral antennas belong to the class of frequency independent antennas which operate over a wide range of frequencies. Radiation pattern, polarization and impedance of such antennas remain unchanged over a wide bandwidth [21]. Figure 1.17 shows one type of spiral antenna called an Archimedean spiral antenna. It includes two conductive arms, extending from the centre outwards. The antenna has a planar structure. Each arm of the Archimedean spiral is defined by the equation:

1.20

Figure 1.17 Spiral antenna

Equation (1.20) states that the radius r of the antenna increases linearly with the angle . The parameter a is a constant which controls the rate at which the spiral flares out. Arm 2 of the spiral is the same as the arm 1, but rotated at 180^o. The direction of rotation of the spiral defines the direction of antenna polarization. Additional arms may be included to form a multi-spiral antenna. Usually the spiral is cavity backed, and the conductive cavity changes the antenna pattern to a unidirectional pattern. A two-arm spiral antenna shown in Figure 1.17 is excited in a balanced mode (that is, the same amplitude and a 180^o phase difference between the two arms). Such a two-arm spiral radiates a CP wave in the antenna-axis direction normal to the antenna plane over a wide frequency range. For exciting this antenna, a coaxial line is used with a wideband balun circuit, which transforms the unbalanced mode of the coaxial line into the balanced mode required for the spiral antenna. It is noted that designing and installing such a wideband balun circuit for the spiral antenna requires considerable effort [21–23].

1.3.6 CP Dielectric Resonator Antennas

DRA is a resonant antenna fabricated from low-loss microwave dielectric materials. The resonant frequency of a DRA is a function of its size, shape and dielectric permittivity. Compared to other antenna types, DRA offers several attractive features. For example, DRA can achieve high radiation efficiency (>95%), flexible feed arrangement, simple structure, small size and the ability to produce different types of radiation patterns using different modes. In particular, DRA avoids conductor losses in patch antennas and is useful for applications at millimetre-wave frequencies. Various shapes of resonators can be used (rectangular, cylindrical, hemispherical, etc.), and various modes can be excited, producing broadside or conical-shaped radiation patterns for different coverage requirements. A wide range of permittivity values can be used (from about 6 to 100), thus antenna engineers can have control over the antenna size and bandwidth. A wideband DRA can be achieved using low permittivity while a compact size can be achieved with high permittivity.

Figure 1.18 shows a DRA antenna for circular polarization. A square DRA is fed by two orthogonal microstrip lines connected to a 90^o microstrip hybrid. A metallic ground plane is at the bottom. This is a typical dual-feed technique as in the case of CP microstrip patch antennas. The required dual-feed network with a 90^o microstrip hybrid takes up lots of space and increases the insertion loss (hence decreasing the radiation efficiency). An alternative technique is to employ a single-feed technique. As discussed in Section 1.3.1, a single-feed square patch antenna with perturbations can excite two orthogonal modes in the patch and achieve CP operation. Similarly, in the case of DRA, it can achieve CP by using a quasi-square DRA with a single feed. Compared to the single-feed microstrip patch antenna which usually achieves narrow CP bandwidth about 1–2% for 3dB AR, single-feed DRAs can achieve up to 7% CP bandwidth [24–26].

Figure 1.18 CP dielectric resonator antenna

For a rectangular DRA, the resonant frequency can be calculated by

1.21

where ε is the permittivity,

μ = is the permeability

M, = N and K are integer numbers, and

l, = w and h are the length, width and height of the rectangular DRA, respectively.

From equation (1.21), it can be seen that the DRA can resonate at various modes, and the resonant frequency is inversely proportional to the square root of the product of material parameters. A high permittivity material will lead to a low resonant frequency of DRA. DRA can be fed using different techniques such as probe feed, aperture slot coupling, microstrip lines and CPW.

1.3.7 CP Slot Antennas

The printed slot antenna is very simple in structure: it consists of a microstrip feed that couples electromagnetic waves through the slot above and the slot radiates them. A microstrip-fed slot antenna is flexible in integration with other active and passive devices in a hybrid microwave integrated circuit (MIC) and microwave monolithic integrated circuit (MMIC) design. They are also easy to make as they can be cut into the surface of the platform they are mounted on. Slot antennas are able to achieve a broader bandwidth compared to microstrip patch antennas. Figure 1.19 shows a CP printed slot antenna. As shown, a square slot is cut in the ground plane, and fed by a feed network at the bottom. The feed network uses a Wilkinson power divider in microstrip lines. The two branches of power divider have lengths with a difference of a quarter-wavelength, leading to a 90^o phase difference between two orthogonal modes in the slot excited by two orthogonal feed lines. Thus, circular polarization is produced in the square slot antenna.

Figure 1.19 CP square slot antenna fed by a microstrip network at bottom

To design a slot antenna, the length of slot is usually chosen to be a half-wavelength. The slot can take different shapes, such as a crossed slot, circular slot, annular ring, square ring and so on. Figure 1.20 shows another CP slot antenna which employs a circular slot fed by two orthogonal feed lines at the bottom. The printed slot antenna is easy to fabricate, has a low profile and low cost. However, the use of dual feeds and a feed network at the bottom as shown in Figures 1.19 and 1.20 occupies lots of space below the ground plane. For some applications, it is necessary to simplify the feed network by using a single feed instead. Figure 1.21 shows an example, which is basically a printed square slot antenna fed by a single microstrip line at the bottom. To achieve circular polarization, an L-shaped microstrip line is employed and the line excites two orthogonal modes in the square ring slot. The L-shaped microstrip line has a length of a quarter-wavelength, thus a 90^o phase difference between two orthogonal modes in the slot is achieved. Such a CP slot antenna has a very simple feed network and is easy to implement.

Figure 1.20 CP circular slot antenna fed by a microstrip network

Figure 1.21 CP ring slot antenna fed by an L-shaped feed

In addition to the microstrip line feed shown previously, slot antennas can also be excited using other techniques such as coaxial cable or CPW. One drawback of these CP slot antenna is bi-directional radiation, as the slot will also radiate in the backward direction. Another ground plane can be added at the bottom so that the antenna can achieve broadside radiation only.

1.3.8 CP Horn Antennas

Horn antennas belong to the aperture antenna category, and their radiation performance is determined by the field distribution over the horn aperture. Horns are designed to provide a smooth transition between the feed waveguide and a wide aperture which serves to focus the main beam. Horns have found wide applications in satellite communications either as earth coverage antennas or feeds for reflector antennas. Theory and design of horn antennas with linear polarization are well documented in [7–9, 11, 27].

Most of the horn antennas reported have a single feed and can radiate linear polarization radio waves [27]. CP horn antennas can be realized by using a horn with dual orthogonal feeds and a 90^o hybrid. However, the performance of dual linear polarized horn antennas, such as the ridged horn antenna, suffers from manufacturing and assembling tolerances. Also, the use of a 90^o hybrid adds more complexity and losses in antennas. During recent years, many new techniques of CP horn antennas have been proposed [28, 29]. A typical configuration of CP horn antennas includes three major elements, that is, a wave launcher, a polarizer and a beam shaper, as shown in Figure 1.22 [28, 29].

Figure 1.22 Configuration of a CP horn antenna

The wave launcher in Figure 1.22 consists of two probes at input ports 1 and 2, which can achieve RHCP and LHCP, respectively. The input probes are located nearly one quarter-wavelength from the short-circuited end of the waveguide section 1, as shown in the figure. Waveguide 1 is a circular waveguide which allows the propagation of waves in TE11 mode [27–29].