Fundamentals of UWB antenna

Transcription

1 Chapter 2 Fundamentals of UWB antenna An overview of UWB Technology was given in Chapter 1. The objective of the thesis as already enunciated is to design antennas for UWB applications. In this chapter a general review of wideband antennas, methodology to obtain UWB requirement and its limitations are all discussed. 2.1 INTRODUCTION The IEEE standard definitions of terms for antennas define antenna or aerial as a means for radiating and receiving radio waves. The history of antennas dates back to 1865 when Maxwell described the behavior of electric and magnetic fields. The claim of Maxwell was verified by Hertz in 1886 by first wireless system. He also constructed a loop antenna and invented parabolic cylinder reflector antenna. Marconi performed the first transatlantic transmission from Poldhu in Cornwell, England to St. Johns, Newfoundland in After Marconi, Lodge (1903) patented a syntonic radio system which embodied the first UWB antenna based on the concept of narrowband frequency domain radio. He also developed the spherical dipoles, square plate dipoles, biconical dipoles and bowtie dipoles. J. C. Bose (1888) performed pioneering work in millimeter wave systems. He demonstrated the complete transmitting and receiving system around 60 GHz which included the collecting funnel the first horn antenna. Antennas can be classified into four basic types based on the frequency of operation. First, electrically small antennas such as short dipole and loop antennas that are much less than a wavelength are used at VHF frequencies and below. Low input resistance and high input reactance are the serious disadvantages of these antennas. Also small antennas are inefficient because of the ohmic losses on the structure. Second, resonant antennas are used from HF to low GHz range. These antennas are popular because of simple structure with good input impedance, broad beam and low to moderate gain. A half wave dipole antenna, microstrip antenna and Yagi-Uda antenna falls under the category of resonant 10

2 antenna. Third, broadband antennas used at UHF and above. The broadband antennas such as spiral and log periodic antennas are characterized by an active region. Since only a portion of the antenna is responsible for radiation at a given frequency, the gain is low. But it may be an advantage to have gain that is nearly constant with frequency although low. Fourth, an aperture antenna which is used at UHF and above. These antennas are usually several wavelengths long in one or more dimensions. Aperture antennas such as horn and reflector antenna propagate EM wave through the opening. The bandwidth of these antennas is moderate with narrow beam leading to high gain. 2.2 OVERVIEW OF BROADBAND ANTENNAS According to The New IEEE Standard Dictionary of Electrical and Electronics terms, the bandwidth of an antenna is defined as the range of frequencies within which its performance, in respect to some characteristics, conforms to a specified standard. This characteristic can be taken as input impedance or radiation pattern. The input impedance is more sensitive to frequency, a common standard is the voltage standing wave ratio (VSWR) should be less than 1.5 or 2. Equivalently, the return loss should be less than 14 db or 10 db respectively. The term broadband is a relative measure of bandwidth. The bandwidth of narrowband antenna is usually expressed as a percent using the formula (2.1) f U - upper frequency f L - lower frequency Whereas for wideband antennas it is usually expressed as a ratio using (2.2) 11

3 Some examples of the most popular broadband antennas in the literature are classified in to four categories: biconical antenna, frequency independent antenna, fractal antenna and planar broadband antenna BICONICAL ANTENNA Biconical antennas constructed by Lodge are the earliest antennas used in wireless systems [Krauss (1988)]. Infinite biconical antenna is formed by increasing the thickness of the dipole antenna to increase the bandwidth and the conducting halves of the antenna are made as two infinite conical conducting surfaces with finite gap at the feed point. The infinite biconical antenna can be analyzed in the same manner as the transmission line. The impedance of the antenna can be determined from voltage and current. Voltage is obtained by integrating along the constant radius r. The total current is found by integrating current density. The impedance of the antenna is real as the antenna is infinite in extent without any discontinuities and produce pure traveling waves. To study the input impedance and radiation characteristics Schelkunoff proposed a convenient model. The biconical structure was further modified by replacing one of the cones with a disk shaped ground plane to form a discone antenna developed by Kanonian. The name is derived from the distinctive shape, which has a disc and a cone. The disc section is insulated from the cone by a block of materials which also acts as spacers keeping the two sections apart at a fixed distance. This distance is one of the factors which determine the overall frequency of the antenna FREQUENCY INDEPENDENT ANTENNA Until 1950 s, broadband antenna has been referred to as the antennas whose radiation characteristics were acceptable over a frequency range of 2 or 3:1. A breakthrough in antenna evolution was made which extended the bandwidth to 40:1. These antennas were referred to as frequency independent antennas by Rumsey (1957). He observed that the impedance and pattern properties of an antenna will be frequency independent if the antenna shape is specified by the angles. The fundamental principle for true frequency independent antennas is if all the dimensions of a perfectly conducting antenna are 12

4 changed in linear proportion to a change in wavelength, the performance of the antenna is unchanged. Although finite biconical antenna is specified by the included cone angle, it is frequency dependent because when the infinite biconical is truncated to finite biconical, impedance and pattern characteristics is modified due to the reflected waves from the ends of the cone. The second category of frequency independent antenna is based on the self complementary principle. The impedance of self complementary antenna is achieved using Babinet s principle which in optics states that when the field behind a screen with an opening is added to the field of a complementary structure the sum is equal to the field when there is no screen. The spiral and their variation conical spiral are constructed to be either exactly or nearly self-complementary yielding extremely wide bandwidth upto 40:1. The first practical frequency independent spiral antenna was constructed by Dyson (1959). The radiation takes place when the circumference is one wavelength. The other antenna with frequency independent property is the log periodic antenna introduced by DuHamel and Isabell (1957). The other forms of the log periodic structure are log periodic dipole array, two or four arm log spiral antennas and conical log spiral antennas [Mayes (1992)]. Although, frequency independent antenna has wide bandwidth, it has two limitations. Firstly, though the frequency independent antennas are infinite in principle satisfying Rumsey s requirement, in practice they are truncated which makes the antenna large in terms of wavelength. Secondly, they radiate different frequency components from different parts of the antenna. Due to these limitations the antenna can be used only where the waveform distortion is tolerated FRACTAL ANTENNAS Mandelbrot constructed recursively generated geometry which has fractional dimensions based on the concept of fractal [Balanis, (2005)]. These antennas are characterized by low profile, light weight and wide bandwidth. The self-similarity and space filling properties of fractal technology are used to realize UWB antenna. A wideband antenna based on fractal technology for wireless communication applications such as software defined radio and UWB is proposed by Cohen, et al (2003). A circular microstrip patch 13

5 antenna with triangular slot and a pentagon patch with pentagon gasket Khoch is proposed by Ding, et al (2006) and Naghshvarian, et al (2008) for UWB application. Two novel fractal antennas for both size reduction and band notch characteristics have been proposed by Lui, et al (2005), (2006). These antennas are not suitable for time domain operation due to spurious current radiation PLANAR ANTENNAS Planar Monopole (dipole) or disc antennas are characterized by small size and wide bandwidth of 60%. The earliest type of planar dipole antenna is the bow tie antenna invented by Woodward [Krauss, et al (2010)]. The planar monopole antennas are formed by radiating metal patch over finite sized ground plane. The patch shape can be rectangular, square [Ammann, (1999), Wong. et al (1997), Ammann, (2000), Thomas, (2011), Jung, et al (2005)], circular [Chen, et al (2003), Tong, et al (2005), Jinghui, et al (2006), Liang, et al (2006), Wang, et al (2007)], elliptical [Huang, et al (2005), Zhang, et al (2006), Ray, (2007), Schantz, (2002), Schantz, (2003), Paryani, et al (2011)], hexagonal or other shape [Agarwall, et al (1997), Ray, et al (2006)]. The impedance bandwidth of these antennas can be improved by three methods [Chen, et al (2006)]. First, the radiator shape beveling results in smooth impedance transition which in turn results in good impedance matching [Ammann, et al (2003), Chen, et al (2000)]. Second, etching a slot on the radiator changes the current distribution resulting in change in current path as well as impedance [Rahayu, et al (2008), Liu, (2004), Lin, (2011), Desmukh, et al (2009)]. Finally, impedance bandwidth is varied by optimizing the feed point location [Ammann, et al (2004), Ray, (2009)]. 2.3 UWB ANTENNA REQUIREMENT The design of UWB antenna is different from that of its narrowband counterpart. Firstly, the antenna should have fractional bandwidth of 0.2 or absolute bandwidth of 500 MHz. Secondly, the antenna should have consistent radiation characteristics throughout the 14

6 band. Thirdly, the antenna should be low profile and planar for easy integration into the mobile and portable devices. In addition to the above, it also depends on the type of transmission MB-OFDM or DS which was discussed earlier in Chapter 1. The electrical characteristic of a MB-OFDM are: wide bandwidth and constant gain response. In addition to the said characteristics a pulsed system requires linear phase response. The mechanical characteristic requirements such as low profile, low cost and easy integration are same for both the systems. 2.4 METHODOLOGY TO OBTAIN UWB REQUIREMENT AND ITS LIMITATIONS The first requirement of antenna covering the entire operating bandwidth is achieved by different bandwidth enhancement techniques such as i. changing the shape of the radiator by varying the ellipticity ratio or beveling the radiator to obtain good impedance matching. ii. controlling impedance bandwidth by varying the feed gap between the partial ground plane and the radiator iii. inserting slots or stubs in the radiator to vary the current distribution which in turn varies the current path length and iv. overlapping of multiple resonances. The second requirement can be achieved through the geometry of the antenna. The third requirement of low profile is not only to accommodate the antenna in miniaturized equipment but also to realize uniform radiation. The fundamental principle behind size reduction is increasing the electrical size of the antenna without increasing the physical size. This is achieved either through high contrast material loading or inductive/capacitive loading techniques [Lee, et al (2007), Kramer et al (2007)]. The main challenge in material loading is maintaining antenna weight while achieving desired 15

7 miniaturization, as the density of high contrast material and volume of material needed becomes high when a large miniaturization factor is required. The performance of the small antenna is characterized by size, quality factor (Q-factor), fractional bandwidth and gain. The Q-factor is the major limitation on small antenna [Chen, et al (2006)] where, Q of the antenna is inversely proportional to its bandwidth. The fundamental limitation on antenna was addressed a long time ago by Chu, Harrington, Wheeler, McLean and Collin. [Abbosh, (2003)]. Chu explored the fundamental limits on antenna size, bandwidth and efficiency. Harrington formalized Chu s ideas and named it the Chu-Harrington limit. This limit relates the size to the Q- factor or inverse fractional bandwidth of an ideal high efficient antenna. The size is denoted by the radius r of the boundary sphere, the smallest sphere that completely encloses the antenna. McLean pointed out the error in determining the wavelength in Chu-Harrington limit. The difference occurs in defining the wavelength of a narrowband and wideband antenna. The difference becomes more significant as the antenna becomes truly broadband [Schantz, (2003)]. Wheeler evaluated the radiation efficiency of the antenna by placing the antenna inside a closed spherical shell known as Wheeler cap with radius at the frequency of interest. But, this method is not suitable for UWB antenna [Schantz, (2003)]. A new method was developed by considering the large spherical shell. An exact expression for radiation Q was investigated by Collin, which is based on evanescent energy stored around an antenna. The exact expression for Q is formulated as (2.3) Where k - wave number a - radius of the smallest sphere enclosing the hypothetical radiator Inverse relationship exists between bandwidth and Q-factor, also the definition of Q-factor varies for narrowband and broadband antenna. 16

8 For narrowband antenna (2.4) f - resonant frequency f - ( f H - f L ) nominal bandwidth For a wideband antenna, it is difficult to give a precise relationship, but can be approximated to the geometric mean of upper and lower frequency to the nominal bandwidth. (2.5) From these equations it can be seen that a tradeoff exists between antenna size, bandwidth and efficiency. Antenna designer should optimize these parameters based on the application. 2.5 SUMMARY Several antenna types have been surveyed for the IR-UWB communication in GHz bandwidth. The biconical antenna features very wide impedance bandwidth, fair pattern bandwidth but suffers from large size. Frequency-independent antenna presents UWB, very good pattern bandwidth and large size. Fractal antennas highlight small size with wide bandwidth and fair pattern bandwidth. The biconical and frequency independent antennas are not suitable for IR-UWB because of its large size and fractal antenna introduces spurious surface current which is not suitable for time domain operation. Hence planar monopole and slot antennas are considered for investigation. In this thesis, four novel printed antennas have been developed for UWB applications. In particular the study involves the design of printed planar UWB antennas, analysis and 17

9 evaluation of their radiation characteristics for UWB communication applications. The photograph of antenna prototype is shown in Figure (2.1) (2.4) 18

10 (a) (b) (c) Figure 2.1 Antenna 1 prototype (a) printed tapered antenna (b) band notched tapered antenna front view (c) rear view (a) (b) (c) Figure 2.2 Antenna 2 prototype (a) step monopole antenna (b) band notched band notched step monopole view (c) rear view 19

11 (a) (b) (c) Figure 2.3 Antenna 3 prototype (a) pentagon antenna (b) band notched pentagon antenna front view (c) rear view (a) (b) Figure 2.4 Antenna 4 prototype (a) slot antenna (b) band notched slot antenna 20