CHAPTER 1 INTRODUCTION

Transcription

1 CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION TO ANTENNA An antenna is a device that is used to convert guided electromagnetic waves into electrical signals and vice versa (i.e. either in transmitting mode or in receiving mode of operation). Antennas are frequency dependent devices. Each antenna is designed for a certain frequency band and outside of this band, antenna rejects the signal. Therefore we can say antenna is a band pass filter and transducer. Antennas are essential part in communication systems therefore understanding their basics are important. With the advances in telecommunication, the requirement for compact antenna has increased significantly. In mobile communication, the requirement for smaller antennas is quite large, so significant developments are carried out to design compact, minimal weight, low profile antennas for both academic and industrial communities of telecommunication. The technologist focused into the design of microstrip patch antennas. Many varieties in designing are possible with microstrip antenna. The first chapter provides an introduction to microstrip patch antennas with their advantages and disadvantages. Then feeding techniques, analysis method and different parameters of microstrip patch antenna are presented. 1.2 MICROSTRIP PATCH ANTENNA Deschamps proposed the concept of microstrip patch antennas in 1953 [1]. The first patent of a microstrip antenna design was awarded to Gutton and Baissinot in France in 1955 [2]. In the early 1970 s the first practical microstrip antennas were fabricated by Munson [3] and Howell [4]. The early 1980 s was a crucial period in publications, practical realism and manufacturing of the microstrip antennas. Present-day system requirements such as compact, light weight, low profile conformal antennas that can be directly integrated into a variety of microwave circuits are an important factor in the development of printed antennas. Their low cost and ease of fabrication on printed circuit board (PCB) make 1

2 them more attractive than the traditionally used lumped element antennas. Microstrip antennas may be made of any geometrical shape and dimension. Fig. 1.1 Structure of a microstrip patch antenna [5] A microstrip patch antenna has a radiating patch on one side of a dielectric substrate having very small thickness and has an infinite ground plane on the other side as shown in figure 1.1. Dielectric materials may be considered as the backbone of microstrip antennas. The patch is generally made of conducting material such as gold or copper and the patch can take any possible shape. On the dielectric substrate the radiating patch and the feed line are usually photo etched. For simplified analysis and performance prediction, the patch is generally square, rectangular, circular, triangular, and elliptical or some other regular shape as shown in figure 1.2. Rectangular patches are the most utilized patch geometry. To design a rectangular patch, the length L of the patch is usually λo < L < 0.5 λo, where λo is the free-space wavelength. Thickness of patch is selected such that t << λo (where t is the patch thickness). The height h of the dielectric substrate is usually λo h 0.05 λo. The dielectric constant of the substrate (ε r ) is typically in the range 2.2 ε r 12 [5]. 2

3 Fringing fields between the patch edge and the ground plane is main parameter for radiation in microstrip patch antennas. A thick dielectric substrate is required for good antenna performance. Rectangular Square Circular Circular Ring Triangular Elliptical Fig. 1.2 Common shape of microstrip patch elements [5] Substrates with lower dielectric constant are preferred for antenna design for better performance. The possible shapes for conducting patch are shown in figure 1.2, but rectangular and circular configurations are most commonly used configurations. The main parameters of a microstrip antenna are its length, width, input impedance, gain and radiation pattern. The length of the antenna should be less than half wavelength in the dielectric Advantages of Microstrip Patch Antenna Microstrip patch antennas found attention of scientific community due to its inherent well-known advantages over other conventional antenna structures. Some of their principal advantages are given below: Light weight and low volume. 3

4 Low fabrication cost and readily amenable on mass production. The antennas may be easily mounted on missiles and satellites without major alteration. Linear and circular polarizations are possible with simple changes in feed position. The antennas have low scattering cross section. No cavity backing is required. Capable of dual and triple frequency operations. Feedlines and matching networks may be fabricated simultaneously with antenna structure Disadvantages of Microstrip Patch Antenna As compared to conventional antennas, microstrip patch antennas suffer from many drawbacks, some of them are Due to losses in the dielectric substrate result in low efficiency. Low gain. Low power handling capacity. Poor radiation pattern due to surface waves which travel within the substrate and scatter at surface discontinuities. Require quality substrate and good temperature tolerance Applications of Microstrip Patch Antenna Some typical system applications which employ microstrip technology are given below: Satellite communications Aircraft antennas Missiles and telemetry Missiles Guidance Systems Environmental instrumentation and remote sensing Biomedical Instruments 4

5 Radar systems Satellite navigation receiver Global positioning system 1.3 FEEDING METHODS OF ANTENNA There are several methods available in literature to feed or transmit electromagnetic energy to a microstrip patch antenna. The most famous techniques are the microstrip transmission line, coaxial probe feed, aperture coupling and proximity coupling. The simplest feeding methods to realize are those of the microstrip transmission line and coaxial probe. Both the methods utilize direct contact with the patch to induce excitation. The point of excitation is adjustable, enabling the designer to control the impedance matching between feed and antenna, polarization, mode of operation and excitation frequency. For direct contact feeds, the better impedance matching is obtained when the contact point is off the centre Microstrip Line Feed A conducting small strip is connected to the edge of the microstrip patch as shown in figure 1.3. Microstrip feed line is of rectangular shape and having width less than patch. The patch is generally made of conducting material such as gold or copper and the patch can take any possible shape viz. rectangular, square, circular, elliptical and triangular according to the requirement. The substrate is a dielectric material. Dielectric materials may be considered as the backbone of microstrip antennas. On the dielectric substrate the radiating patch and the feed line are usually photo etched. A thick dielectric substrate is required for good antenna performance. Substrates with lower dielectric constant are preferred for antenna design for better performance. The ground plane is made up of copper. The ground plane stops the back radiations. The back radiations reduces the efficiency of the antenna. 5

6 Fig. 1.3 Microstrip line feed [5] Coaxial Feed The Coaxial feed or probe feed is a very common technique used for feeding a microstrip patch antenna. As seen from figure 1.4, the inner conductor of the coaxial connector extends through the dielectric and is connected to the radiating patch, while the outer conductor is connected to the ground plane. Often a hit and trial method is used to obtain the optimum match for the direct contact feeds. Fig. 1.4 Probe fed rectangular microstrip patch antenna [5] Aperture Coupled Feed The aperture-coupled configuration consists of two parallel substrate separated by a ground plane. Excitation of the patch is accomplished by coupling energy from a microstrip line through a small aperture in the ground plane as shown in figure 1.5. With this arrangement, the microstrip feed is designed on a thin-high dielectric constant 6

7 substrate, which tightly binds the field lines while the patch is designed on a thick low dielectric constant substrate. Fig. 1.5 Aperture-coupled feed [5] The ground planes isolate the feed from the patch and thus minimize spurious radiation from the feed, which would interfere with the antenna pattern. Therefore, the design of the patch and the transmission line are independent. The major disadvantage of this feed method is that it is difficult to fabricate due to multiple layers, which increase the antenna thickness. This feeding scheme also provides narrow bandwidth Proximity Coupled Feed Proximity coupled feed technique is also known as the electromagnetic coupling scheme. As shown in figure 1.6, the proximity-coupled scheme operates in a manner similar to that of the aperture-coupled configuration except the ground plane is removed. In this method two dielectric substrates are used such that the feed line is in between the two substrate and the radiating patch is on top of the upper substrate. The main advantage of this feed technique is that it eliminates spurious feed radiation and provides very high bandwidth, due to overall increase in the thickness of the microstrip patch antenna. This method also provides choice between two different dielectric media, one for the patch and one for the feed line to optimize the individual performance. Matching can be achieved by controlling the length of the feed line. The major disadvantage of this feed 7

8 technique is that it is difficult to fabricate because of the dielectric layers need proper alignment. Fig. 1.6 Proximity-coupled feed [5] 1.4 METHODS OF ANALYSIS In common practice, microstrip antennas are evaluated using many synthesis methods such as transmission line model, the cavity model or the full-wave model. Since the inception of these techniques, several complex analytical models have been developed that account for fringing effects, special geometries, complex substrate and mutual coupling from neighboring elements in an array Transmission Line Model This model represents the microstrip antenna by two slots of width W and height h, separated by a transmission line of length L. The dimensions of patch are finite along width and length; the fields at edges of the patch undergo fringing along the length as shown in figure 1.7 and

9 Fig. 1.7 Microstrip line [6] Fig. 1.8 Electric field lines [6] The expression for ε reff [7] is ε reff = ε r ε r 1 2 (1 + 12h W ) 0.5 (1.1) Where ε reff is effective dielectric constant, ε r is dielectric constant of substrate, h is the height of dielectric substrate and W is width of the patch of antenna. Figure 1.9 below shows a rectangular microstrip patch antenna of width W, length L resting on a substrate with height h. Fig. 1.9 Microstrip patch antenna [6] In the figure 1.10 shown below, the microstrip patch antenna is represented by two slots, separated by a transmission line of length L and open circuited at both the ends. Along the width of the patch, the voltage is maximum and current is minimum due to the open 9

10 ends. The fields at the edges can be resolved into normal and tangential components with respect to the ground plane. Fig Top view of antenna [6] Fig Side view of antenna [6] It is seen from figure 1.11 that the normal components of the electric field at the two edges along the width are in opposite directions and thus out of phase and hence they cancel each other in the broadside direction. The dimensions of the patch along its length have now been extended on each end by a distance ΔL, which is given empirically by Hammerstad [8] as- L = h ε reff+0.3 The effective length of the patch L eff now becomes ε reff (W h h For a given resonant frequency f 0, the effective length is Where c is the speed of light ) (1.2) L eff = L + 2ΔL (1.3) L eff = C 2f 0 ε reff (1.4) 10

11 For a rectangular microstrip patch antenna, the resonance frequency for any TM mn mode is given by Kyungho Chung, Jaemoung Kim [9] as: f 0 = c [( m 2 ε reff L )2 + ( n 0.5 W )2 ] (1.5) Where m and n are modes along L and W respectively. The width W is [10] Cavity Model W = C 2f 0 ( ε r+1 2 ) 0.5 (1.6) In the cavity model, a patch antenna is represented as a dielectric loaded cavity. The cavity is formed via a substrate that is truncated on the top and bottom by two perfectly conducting electric boundaries, the patch and ground plane. The sidewalls are perfectly conducting magnetic boundaries that are determined by the dimension of the patch. Therefore, the electric field lines contained within the substrate (between the patch and the ground plane) are propagating perpendicular to the conducting walls, as required by Maxwell,s equations. Using these boundary conditions, the dielectric loaded cavity can be evaluated via Huygen s equivalence principle. Huygen s principle is based on the uniqueness theorem. In the case of patch antenna, when the microstrip patch is energized, a charge distribution is established on the upper and lower surfaces of the patch, as well as on the surface of the ground plane. The movement of these charges creates electric current densities on the top and bottom surfaces of the patch. Fig Charge distribution and current density creation on the microstrip patch [6] 11

12 1.5 BASIC ANTENNA PARAMETERS As studied from [5, 6], some basic parameters affect the antenna s performance. During the design process of an antenna some design parameters like frequency band of operation, polarization, input impedance, radiation patterns, gain directivity and efficiency should be adjusted as required. Some of the parameters are interrelated and not all of them are necessary to describe antenna performance. Some of the antenna parameters are described below Gain Gain is a parameter that measures the directionality of a given antenna. An antenna with low gain, emits radiation about same power in all directions, whereas a high gain antenna preferentially radiates in particular directions. Specially the gain, directive gain or power gain of an antenna is defined as the ratio of intensity of the signal radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by a hypothetical isotropic lossless antenna. Since the radiation intensity from a lossless isotropic antenna equals the power into the antenna divided by a solid angle of 4π Steradians, Gain = 4π radiation intensity total input (transmitted)power (1.8) Although the gain of an antenna is directly related to its directivity, antenna gain is a measure that takes into account: the efficiency of the antenna as well as its directional capabilities Directivity The directivity of the antenna has been defined as the radiation intensity in a given direction from the antenna divided by the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power radiated by the antenna divided by 4π. In other words, the directivity of a nonisotropic source is equal to the ratio of its radiation intensity in given direction, over that an isotropic source D = 4πU Prad (1.9) 12

13 Where, U is the radiation intensity in (W/unit solid angle) and P rad is total radiated power in (W), D is the directivity of antenna If the direction is not specified, it implies the direction of maximum radiation intensity (maximum directivity) expressed as Return Loss (S 11 ) D = 4πU max P rad (1.10) The return loss (RL) is a parameter that indicates the amount of power that is lost to the load and does not return as a reflection. Waves are reflected leading to the formation of standing waves, when the transmitter and antenna impedance do not match. Hence the RL is a parameter to indicate how well the matching between the transmitter and antenna has taken place. The RL is given by: S 11 (db) = 10 log 10 P i P r (1.11) For perfect matching between the transmitter and the antenna, the reflection coefficient Γ = 0 and hence RL= which means no power would be reflected back, whereas Γ= 1 has a RL= 0 db, which implies that all incident power is reflected back. For practical applications, a VSWR of 2 is acceptable, since this corresponds to reflection coefficient Γ= (1/3) and hence a return loss of 9.54dB or approximately 10dB is acceptable. This is the reason that during bandwidth calculations, 10dB return loss is considered. This 10dB figure carries dependency on frequency band such as the resonant frequency dip should be less than 10dB. Also we decide the bandwidth of an antenna at the two cutting point along 10dB Antenna Polarization Antenna polarization is an important consideration when selecting and installing antenna. Because radiation property of an antenna depends on the antenna polarization and if polarization is not synchronized between transmitting and receiving antenna even resonant frequency are perfectly matched still the signals will not be received by receiving antenna. The electric field or E plane determines the polarization or 13

14 orientation of the wave. An antenna is vertically linear polarized when its electric field is perpendicular to the Earth s surface. Horizontally linear polarized antenna, have their electric field parallel to the Earth s surface. In a circularly polarized antenna, the plane of polarization rotates in a corkscrew pattern making one complete revolution during each wavelength. A circularly polarized wave radiates energy in the horizontal, vertical plane as well as in every plane in between. If the rotation is clockwise looking into the direction of propagation, the sense is called right-hand-circular (RHC) polarization. If the rotation is counter clockwise, the sense is called left-hand-circular (LHC) polarization. Polarization is an important design consideration. The polarization of each antenna in a system should be properly aligned. Maximum signal strength between stations occurs when both stations are using identical polarization Voltage Standing Wave Ratio As electromagnetic waves travel through the different parts of the antenna system, from the source to the feed line to the antenna and finally to free space, they may encounter differences in impedance at each interface. Depending on the impedance match, some fraction of the wave s energy will reflect back to the source, forming a standing wave pattern in the feed line. The ratio of the maximum power to the minimum power in the wave can be measured and it is called the voltage standing wave ratio (VSWR). A VSWR of 1:1 is ideal. A VSWR of 1.5:1 is considered to be marginally acceptable in low power applications. Minimizing impedance differences at each interface will reduce VSWR and maximize power transfer through each part of the system. The VSWR can be expressed as: Bandwidth VSWR = V max = 1+ Γ V min 1 Γ (1.12) The bandwidth is the antenna operating frequency band within which the antenna performs as desired. The bandwidth of a broadband antenna can be defined as the ratio of the higher to lower frequencies of acceptable operation. In other words, the frequency over which the antenna will perform satisfactorily i.e. it s one or more characteristics have acceptable values between the bandwidth limits. 14

15 The absolute bandwidth (ABW) is defined as the difference of the two edges and the fractional bandwidth (FBW) is designated as the percentage of the frequency difference over the center frequency, as given in equation 1.13 and 1.14 respectively Axial Ratio (AR) ABW = f H f L (1.13) FBW = 2 f H f L f H +f L (1.14) Axial ratio (AR) is the most important factor in indicating the quality of the wave propagation of circular polarization. According to IEEE standard definition the term axial ratio is defined as the ratio of the major to minor axes of a polarization ellipse. While the term axial ratio pattern is the graphical representation of the axial ratio of a wave radiated by an antenna over a radiation pattern cut. For a perfect circular polarized wave the value of axial ratio will be 1 or 0 db. In case when the magnitude of the RHCP components is equal to the LHCP components, then the rotated circle formed by the electric field vector would degenerate into a line, the polarization becomes linear and the value of the axial ratio will go to infinite. 1.6 BANDWIDTH IMPROVEMENT TECHNIQUES FOR MSA Inherent narrow bandwidth is one of the major disadvantages of microstrip antennas. For wider bandwidths or multiple-frequency operation in a single element, researchers created new designs or variations to the original antenna structure. The size, height or overall volume of the single element is the main disadvantages related to the improvement in bandwidth. By increasing the thickness of the substrate the bandwidth can be increased. But more surface waves will be there due to thick substrate, which will degrade the radiation patterns as well as reduce the radiation efficiency. Decreasing the relative permittivity may be the second technique to increase bandwidth, which has an obvious limitation based on size. The inherent complexity of these techniques is apparent. In this regard we can mention multilayer structures [12], parasitic element coupled to main patch [13], microstrip antenna with different type of slots [14] and patches with non contact type feedings. 15

16 1.6.1 Multilayer Structures The overall height of the antenna increases if we use multilayer structures but the size in the plane remains same as the single patch antenna. The patches can be fabricated on different substrates and to increase the bandwidth air gaps or foam material can be introduced between these layers. Figure 1.13 shows a multi-layer antenna constructed by stacking two elements feeding the top patch by a coaxial connector through the ground plane. The second resonant frequency was highly dependent on the radius of the two patches [12]. The bandwidth achieved was up to 25% from the centre frequency and the standing wave ratio was less than 2. Varying the value of the two heights and adjusting the value of ε r2 can improve the bandwidth. If one of the layers in the multi-layer structure antenna is replaced by an air gap, the bandwidth of the antenna can be further increased [35]. Fig Dielectric stacked antenna[12] Parasitic Elements Coupled to the Main Patch In this method, an additional patch is placed close to the main radiating element. Such a patch is known as a parasitic patch. The gap between the main radiating element and the additional resonator is chosen for optimum bandwidth. If the frequencies of these two patches are close to each other, then a broad bandwidth can be obtained. Figure 1.14 (a) shows co-planar metallic strips coupled to the main patch. A bandwidth up to 6-7 times that of a single rectangular patch was reported. The bandwidth can be increased by varying the length of the parasitic patches. A bandwidth of 10.5% was obtained by this method. [13] suggested that the bandwidth of a microstrip antenna can also be enhanced 16

17 with directly coupled and parasitic patches. This configuration is shown in Figure 1.14 (b). It has a bandwidth of 12.7% with 10 db return loss. Fig. 1.14(a) Parasitic patches at edge of Fig. 1.14(b) Directly coupled and parasitic radiating patch [13] patches [13] Patches with Non-Contact Feeding Technique In this approach, the end of the feed probe is not directly in contact with the patch. This arrangement forms a series capacitance that can be controlled by adjusting the gap. This approach is to feed the patch with a coaxial probe but with a circular or linear gap Fig (a) Probe with circular or Fig (b) Aperture coupled feed [15] linear gap [15] between the patch conductor and around the feed point. This gap must be narrow to obtain sufficient coupling to the patch, to achieve large bandwidth. Figure 1.15 (a) shows this configuration. Another method for bandwidth enhancement is aperture coupled feeding technique, as shown in figure 1.15 (b). In this method, the radiating patch and the microstrip feed line are separated by a ground plane. The patch is fed through an aperture by a microstrip line, placed at bottom of the lower substrate. The bandwidth from this technique is improved by 25% [15]. 17