Chapter 3 Review: UWB System and Antennas 3.1 Introduction U ltra wideband (UWB) is an emerging technology for future short-range wireless communication with high data rates, radar imaging and geolocation [1]-[4]. The word ultra-wideband commonly refers to signals or systems that have a large bandwidth. The use of a large bandwidth offers multiple benefits such as high date rates, robustness to propagation fading, accurate ranging, superior obstacle penetration, interference rejection, and coexistence with narrow bandwidth systems. A landmark patent in UWB communications was submitted by Ross in 1973. It was then in 1989 that the term Ultra Wideband appeared in a publication of the Department of Defence in the United States (U.S.) and the first patent with the exact phrase UWB antenna was filed on behalf of Hughes in 1993[4]. The first UWB signals were generated by Hertz, which radiated sparks via wideband loaded dipoles [65]. UWB communications has drawn great attention since 2000. Obstacles such as multiple access interference (MAI) and UWB emission over a large frequency range were taken into account by the regulatory body for commercial uses of UWB. In 2002, interest in UWB systems was greatly magnified by the decision of the United States frequency regulating body, the FCC. They released a report approving the use of UWB devices operating in several unlicensed frequency bands such as (0 960) MHz, (3.1 10.6) GHz, and (22 29) GHz. In April 2009, the Electronic Communications Committee (ECC) of Europe proposed two sub-bands, the lower band ranging from 3.1 GHz to 4.8 GHz and the higher band from 6 GHz to 8.5 GHz. Similarly, Japan published their proposed low and high sub-bands from 3.4 GHz to 4.8 GHz and 7.25 GHz to 10.25 GHz respectively. The upper limit for effective isotropic radiation power (EIRP) is common and is set to be -41.3 dbm/mhz. Even though the authorized frequency bands are different for the various world regions, the definition of UWB is universal. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 28
3.2 UWB Wireless System and Standards UWB describes wireless physical layer technology, which uses a bandwidth of at least 500 MHz or a bandwidth which is at least 20% of the central frequency in use. Thus those systems that have a relative bandwidth of larger than 20% are known as ultrawideband. Four methods emerged to spread the signal over large relative bandwidth, and are impulse radio (IR), direct-sequence code division multiple access (DS-CDMA), orthogonal frequency division multiplexing (OFDM) and frequency hopping [65]. There are two approaches for UWB systems: pulsed operation and multiple narrow bands. The first approach is based on traditional impulse radio (IR) method. Impulse Radio refers to the use of a series of very short duration pulses, which are modulated in position or/and amplitude. As signals are carrierless (that is only baseband signals exists) no intermediate frequency processing is needed. In IR systems, the transmitting pulse occupies the entire or partial UWB spectrum (7.5 GHz bandwidth). The second approach, the multiple narrow band is based on multiple carrier orthogonal frequency division multiplexing (OFDM) and direct sequence code division multiple access (DS-CDMA) methods. The other two competitive alternative schemes of multiband approach are multi-band orthogonal frequency division multiplexing (MB- OFDM) and multi-carrier code division multiple access (MC-CDMA). OFDM has become popular for high data rate transmission in IEEE 802.11a/g wireless standards. In MB-OFDM, the total UWB frequency band from 3.1GHz to 10.6 GHz is divided into 14 sub-bands, each of which has a bandwidth of 528 MHz and conforms to the FCC definition of UWB as shown in Figure 3.1. Each 528 MHz band comprises of 128 carriers, modulated using QPSK on OFDM tones [3]. The main difference between MB-OFDM and a traditional OFDM system is that the data transmission is not done continually on all sub-bands. MB-OFDM provides flexibility to adopt the various spectral regulations made by regulatory bodies, including multiple data rates as per the need of the end user. Due to its multiband scheme, MB-OFDM permits adaptive selection of the sub-bands so as to avoid interference with other systems at certain frequency range. Within the sub-bands, the effect of non-linearity of the phase shift on the reception performance can be ignored, because the phase varies very slowly with frequency. In this thesis the antenna designed focuses on achieving frequency response with respect to the return loss, VSWR, gain, and radiation pattern over the operating band, which fully covers the UWB of 7.5 GHz. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 29
Figure 3.1 Spectra of OFDM UWB systems compliant with the FCC s emission limit masks for indoor and outdoor UWB applications Though the communication range may be within tens of meter, pulsed or OFDM communication systems tend to use high data rates, typically in the range of 1 to 2 gigapulses per second. The use of high data rates can enable efficient transfer of data amongst various handheld devices such as digital camcorders, cell phones, personal digital audio, video players, laptops, etc. In addition with the IEEE 802.11 standard based WLAN products ( Wi-Fi ) and IEEE 802.15 standard Bluetooth-based WPAN products a variety of wireless networking products are available with high data rate, to develop digital home and commercial applications. Task Group TG3a has set out to develop a flexible standard, which will enable high data rate WPAN (110 Mbps at 10m, 200 Mbps at 4m, and 480 Mbps at an unspecified distance). The task group TG3c (formed in March 2005) developed a millimeter wave based alternative physical layer (PHY) for the existing WPAN Standard 802.15.3-2003. This millimeter wave WPAN operates in band including 57 64GHz unlicensed band defined by the FCC at 47 CFR 15.255. IEEE 802.15.3c-2009 was published on September 11, 2009. The millimetre wave WPAN application allows a high data rate of over 2 Gbit/s. Presently, several UWB devices are entering the market based on the 802.15.3a standards. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 30
3.3 Definition, Advantages and Benefits of the UWB System Definition of UWB UWB signals can be defined as signals having a fractional bandwidth of at least 20% of the center frequency or has a bandwidth of at least 500 MHz, regardless of the fractional bandwidth. The fractional bandwidth (FRB) is defined as: (3.1) Where: f 2 = the upper -10 db frequency point on the signal spectrum f 1 = the lower -10 db frequency point on the signal spectrum UWB is a wireless technology for transmitting digital data over a wide spectrum with very high data rates and low power over short distance communication. UWB technology has the ability to carry signals through doors and other obstacles. Improved channel capacity is one of the major advantages of UWB. Information is transferred through a RF spectrum channel. Shannon s capacity limit equation showed that capacity increases as a function of bandwidth (BW), faster than as a function of SNR (signal to noise ratio). (3.2) Where: C = Channel capacity (bits/sec) BW =Channel bandwidth (Hz) SNR= Signal to noise ratio Where: P = Received Signal Power (Watts) N 0 = Noise Power Spectral Density (Watts/Hz) The Shannon s equation indicates that the UWB technology is capable of transmitting very high data rates using very low power with an increase in channel Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 31
bandwidth. UWB antenna plays a very important role to increase channel capacity for high data rate communication in an indoor environment. Advantages UWB offers many advantages over narrowband technology, such as: [3] Coexistence with current narrowband and wideband radio services Large channel capacity or huge data rate Low transmit power Ability to work with low SNRs Resistance to jamming High performance in multipath channel Simple transceiver architecture Benefits Avoids expensive licensing fees High bandwidth can support real-time high definition video streaming Provides low probability of detection and interception Reliable with hostile environments Delivers higher signal strength in adverse conditions However, the number of advantages in UWB systems also gives rise to a number of challenges, such as: Pulse shaped distortion Channel estimation (difficult predicting the template signals) High frequency synchronization (very fast analog to digital converters required) Multi-access interference (hard to detect) Low transmit power (information can travel only for a short distance) Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 32
3.4 UWB spectrum Issues Many organizations and government entities around the world are grouped into regional, national and international levels to lay down rules and recommendations for UWB usage [2]. At the regional level, the Asia-Pacific Tele-community (APT) is an international body that sets recommendations and guidelines for telecommunication in the Asia-Pacific region. The European Conference of Postal and Telecommunications Administrations (CEPT) had created a task group under the Electronic Communications Committee (ECC) to draft a proposal regarding the use of UWB for Europe. At a national level, USA was the first country to legalize UWB for commercial use. In the UK, the regulatory body, called the Office of Communications (OfCom), opened consultation on UWB matters in January 2005. All the regulatory bodies set rules for protection of existing radio devices to keep the UWB out of their frequency range. 3.4.1 Frequency Regulations and Spectral Masks UWB system minimizes the interference of existing wireless systems by spreading the power over a very large bandwidth and follows the restrictions of the FCC on the emitted power spectral density as shown in Figure 3.2 [65]. The FCC and other regulatory groups have specified spectral masks for different applications and allowed power output for specific frequencies. The frequency masking depends on the applications as well as the environment in which the devices are operated. For indoor communication, a power spectral density of -41.3 dbm/mhz is allowed in the frequency band between 3.1 GHz 10.6 GHz. No intentional emissions are allowed outside the 7.5 GHz band. The admissible power spectral density (PSD) for spurious emission provides special protection for GPS and cellular services as shown in Figure 3.2. To avoid inadvertent jamming of existing systems such as GPS satellite signals, the lowest band edge of UWB for communication is set at 3.1 GHz, and the highest is set at 10.6 GHz. For outdoor communication such as wall imaging systems and ground penetrating radar, the operation is admissible either in the (3.1 10.6) GHz range, or below 960 MHz. For the through-wall and surveillance systems, a number of military UWB systems seem to operate in the frequency range from (1.99 10.6) GHz, and below 960 MHz. The frequency range from (24 29) GHz is allowed for vehicular radar systems. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 33
The emissions mask protects various other government systems in the 1.61 3.1 GHz band and satellite systems above 10.6 GHz. The FCC emission power limits for indoor and hand-held systems is illustrated in Table 3.1. The PSD review of some common wireless broadcast and communication systems is tabulated in Table 3.2. One of the benefits of low PSD is the low probability of detection, which is of particular interest for military applications, such as secret communications and radar. Figure 3.2 FCC regulated spectral mask for various indoor and outdoor applications Table 3.1 FCC emission power limits for various systems Frequency range (MHz) Indoor emission mask (dbm/mhz) Outdoor emission mask (dbm/mhz) 960-1610 -75.3-75.3 1610-1900 -53.3-63.3 1900-3100 -51.3-61.3 3100-10600 -41.3-41.3 above 10600-51.3-61.3 Table 3.2 PSD of some common wireless broadcast and communication systems System Transmission Power Bandwidth PSD (W/MHz) Classification Radio 50 KW 75 KHz 6,66,600 Narrowband Television 100 KW 6 MHz 16,700 Narrowband 2G Cellular 500 mw 8.33 KHz 600 Narrowband 802.11a 1W 20 MHz 0.05 Wideband UWB 0.5 mw 7.5 GHz 6.670 x 10-8 Ultra wideband Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 34
3.5 Spatial and Spectral Capacities Another basic property of UWB systems is their high spatial capacity, measured in bits per second per square meter [bps/m 2 ] [3]. Spatial capacity can be calculated as the maximum data rate of a system divided by the area over which that system can transmit. The transmission area can be calculated from the circular area, assuming a transmitter in the center. However, in practice the rule of thumb is to use the square of the maximum transmission distance: For narrowband systems the most popular measure of capacity is the spectral capacity, measured in bits per second per hertz (bps/hz), because the spectrum is the most limited resource. Comparison of spatial capacity of various indoor wireless systems is given in Table 3.3. Table 3.3 Comparison of the spatial capacity of various indoor wireless systems System Maximum data rate [Mbps] Transmission distance [m] Spatial Capacity[kbps/m 2 ] Spectral Capacity[bps/Hz] UWB 100 10 318.3 0.013 IEEE 802.11a 54 50 6.9 2.7 Bluetooth 1 10 3.2 0.012 IEEE 802.11b 11 100 0.350 0.1317 The transmit data rate can be increased by increasing the bandwidth occupation or transmission power, which will decrease the spectral capacity as expected, in the UWB system. For UWB systems, which operate in other licensed spectra, the power has to be kept very low. This is compensated for by the use of extremely large bandwidths. Using the traditional measure of spectral capacity (bits/hz), the UWB spectral capacity is low compared with existing systems. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 35
3.6 Speed of Data Transmission The large bandwidth of UWB systems means extremely high data rates can be achieved. As can be seen in Table 3.4 the data rates for present indoor wireless UWB transmissions are between 110 Mbps and 480 Mbps. This is fast compared with the existing wireless and wired standards. In fact, the transmission speed is presently being standardized into three different speeds: 110 Mbps with a minimum transmission distance of 10 m; 200 Mbps with a minimum transmission distance of 4 m; and 480 Mbps with no fixed minimum distance. The reasons for these particular distances lie mostly with different applications. For example, 10 m will cover an average room and may be suitable for wireless connectivity for a home theatre. A distance of less than 4 m will cover the distance between appliances, such as a home server and a television. A distance of less than 1 m will cover the appliances around a personal computer. Table 3.4 Comparison of UWB bit rate with other wired and wireless standards Standard Speed [Mbps] UWB, USB 2.0 480 UWB (4 m minimum), 1394a (4.5 m) 200 UWB (10 meter minimum) 110 Fast Ethernet 90 802.11a 54 802.11g 20 802.11b 11 Ethernet 10 Bluetooth 1 Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 36
3.7 UWB Applications 1. High-rate WPANs Wireless local area networks (WLANs) with a transmission range of about 100 m and wireless personal area networks (WPANs), with a transmission range of about 10 m or less, are rapidly being established as popular applications for wireless technology. The typical applications suggested by IEEE 802.15.3a standard for high-rate WPANs are digital home requirements, which include the following and are shown in Figure 3.3. Wireless video projectors and home entertainment systems with wireless connections between components. High-speed cable replacement, including downloading pictures from digital cameras to PCs and wireless connections between DVD players, PC, Camcorder projectors and HDTV (high-definition television). Coexistence and networking of audio, still video, and motion pictures for fixed and portable low-power devices. Wireless replacement for Universal Service Bus (USB) connections among computers and peripherals such as printer, scanner, mass storage devices in a home as well as the office indoor environment. Home network of audio and video with internet gateway. High speed data transfer for multimedia wireless distribution systems for dense user environments, such as multi-tenant units/multi-dwelling units (MTU/MDU). Office, home, auto, and wearable wireless peripheral devices. Due to the high data rate, UWB can be used as an alternative to other wireless technologies, such as Bluetooth, Wi-Fi, and Personal Area Network (PAN) applications. The UWB devices used to develop a smart digital home are illustrated in Figure 3.4 and potential UWB applications scenario is shown in Figure 3.5 [4]. 2. The FCC outlined other possible applications of this UWB technology to include radars for close range, which can be used for wall imaging systems and ground penetrating radar (GPR) systems for landmine detection in the frequency range 3.1 10.6 GHz, through-wall imaging systems (1.61 10.6 GHz), surveillance and urban warfare systems (1.99 10.6 GHz). Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 37
3. The commercial application of UWB is vehicular radar systems and communication and measurement systems (22 GHz 29 GHz). 4. Another promising application is the wireless body area network (WBAN), geolocation of nodes in a sensor network and medical systems (biological imaging) for cancer detection in the frequency range of 3.1 1 0.6 GHz. Figure 3.3 Modern digital home equipped with various UWB devices Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 38
Figure 3.4 UWB devices (a) USB storage device (b) USB hub (c) USB 2.0 networking server (d) UWB HDMI Extender (e) UWB laptop (f) Audio video Extender (g) Multimedia transmitter (h) Computer to TV Wireless Connection Kit Figure 3.5 Potential applications of the UWB system Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 39
3.8 Ultrawideband (UWB) Antennas The allocation of 7.5 GHz wide frequency spectrum with EIRP less than -41dBm / MHz for UWB applications, presents numerous exciting opportunities and challenges for antenna designers. Some of the important challenges are large operating bandwidth, segmentation of the wide bandwidth, in built band-notched design to avoid narrow band interference, and compact size. Compact size and wide impedance bandwidth are desirable features of UWB antennas for indoor applications. For practical UWB applications, planar antennas printed on various substrate materials are the capable candidates. Such planar antennas are low profile, cost less in manufacturing and can be easily integrated with MMICs of the miniaturized wireless UWB device. 3.8.1 UWB Antenna Characteristics An antenna does the important role of transmitting source signal, by converting it to electromagnetic waves into free space for communication and vice versa. An antenna is usually designed based on the need of the application in which band the radiation energy is focused, and suppressed in others at certain frequencies. A good design of the antenna can full fill system requirements and improve overall system performance for communication. The performance of an antenna is described with respect to its parameters, such as impedance bandwidth, VSWR, radiation pattern, radiation efficiency and gain. 3.8.1.1 Radiation and bandwidth The radiation pattern indicates the directions the signals will be transmitted over the wide operating bandwidth. The radiation characteristic is expected to be constant. Also across a large frequency spectrum the phase of the antenna is desired to be linear. The -10 db impedance bandwidth called as absolute bandwidth of the UWB antenna is 7.5 GHz [65]. Particularly, a UWB antenna is defined as an antenna having a fractional bandwidth (FRB) greater than 20% and a minimum bandwidth of 500 MHz, which is more when compared with the narrow (less than 1%) and wideband antenna (1 20)%. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 40
3.8.1.2 Mechanical Characteristics The mechanical requirements in antenna design are also important, such as small and compact size, low profile and low cost. The increase in electrical length will achieve miniaturization of the antenna, but the physical dimension of the antenna must be suitable to integrate it with the MMIC of short range UWB devices. 3.8.1.3 Band-Notch Characteristics The performance of an antenna designed with UWB of 7.5 GHz (3.1 10.6) GHz may get degraded due to the interference occurring from various narrow band systems. The interference of wireless systems, such as IEEE 802.11a wireless LAN in USA (5.15 5.35, 5.725 5.825) GHz and HIPERLAN/2 in Europe (5.15 5.35, 5.47 5.725) GHz, with the UWB spectrum is shown in Figure 3.6. The use of an additional filter design to reject these interferences occurring in the UWB will increase the complexity of UWB systems, whereas this task can be tackled by special antenna designs with band-stop characteristics. Therefore, to obtain dual benefits; firstly to avoid the existing band interference and secondly to achieve multiband characteristics, the antenna must be designed with single or multiple bandnotch characteristics. 3.8.1.4 Group Delay One important characteristic of the UWB antenna is its non-dispersive behaviour over the operating region. This property is quantitatively evaluated by the group delay parameter. Group delay is defined as the derivative of far field phase with respect to the frequency [4]. If the phase is linear throughout the frequency range, the group delay will be constant for the frequency range. Group delay is an important characteristic because it indicates how well a UWB pulse will be transmitted and to what degree it may be dispersed. In wideband technology, group delay is a more precise and useful measure of phase linearity and of the phase response. In short, group delay quantifies the pulse dispersion and far field phase linearity. The distortionless time domain performance of the antenna can be confirmed by small variations in the group delay [29]. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 41
Figure 3.6 Interference of WLAN with the UWB spectrum Figure 3.7 Simulation setup for group delay measurement in face-to-face orientation Figure 3.8 Co-axial feed monopole antenna with various shaped radiators Constant group delay is required in the signal bandwidth to maintain signal integrity of the pulsed wideband signal. A flat (small variation) nature of group delay Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 42
indicates UWB antennas have good transient response and fairly good dispersion in the working band. It gives an average time delay and the input signal suffers at each frequency, thus it is related to the dispersive nature of the antenna. Moreover, it is necessary to have good group delay performance, and is very important for impulseradio UWB systems. Simulation results are obtained for two identical antennas with a distance of 300mm in face-to-face configuration. That is group delay is achieved by exciting two identical antennas located in the far field. The simulation setup of the antennas in face-to-face orientation for measurement of group delay is shown in Figure 3.7. 3.8.2 Planar Broadband Monopole Antennas The planar monopoles or disk antennas show excellent radiation performance with good impedance matching over a wide spectrum [11], [24]-[26]. Because of the significantly small size, these antenna configurations are preferred for development of compact printed UWB antennas. Planar monopole antennas are a good choice to achieve wide impedance band when bandwidth enhancement techniques are applied. The planar monopole antenna is a good candidate to replace the straight wire configuration, in which the wire is replaced by a disc or by various polygon shapes. Planar disc monopole antennas yield a very large impedance BW, which can be explained in the following two ways: 1. A monopole antenna generally consists of a thin vertical wire mounted over the ground plane, whose BW increases with an increase in its diameter [12]. A planar monopole antenna can be equated to a cylindrical monopole antenna with a large effective diameter. 2. The planar monopole antenna can be viewed as a microstrip antenna on a very thick substrate with unity dielectric constant, and hence a large BW is expected. In the radiating metallic patch, various higher order modes are excited. Since all the modes will have a larger BW, these will undergo a smaller impedance variation. The shape and size of these planar antennas can be optimized to bring several modes within the VSWR = 2 circles on the Smith chart, leading to a very large-impedance BW. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 43
The monopole disc can assume various configurations such as rectangular, triangle, circular, elliptical, square, trapezoid, pentagonal, hexagonal, and so on as shown in Figure 3.8. The antenna s performance is determined by the shape and size of the planar radiator as well as the feeding section. The size and shape of the radiator mainly determine the frequency corresponding to the lower edge of the impedance bandwidth. The feed gap, location of the feed point, and the shape of the bottom of the radiator determine the impedance matching as shown in Figure 3.8(a-h). The impedance matching is determined by the impedance transition between the probe and the radiator. A broadband impedance transition will ensure impedance matching across a broad bandwidth. The bandwidth of the rectangular planar antenna can be enhanced by modifying the bottom part of radiator and the ground plane such as the trapezoidal shaped [33]-[36], [51]-[52]. 3.8.3 Lower Edge Frequency Determination As the planar monopole antenna possesses a wide impedance bandwidth because of excitation of higher order multi-modes and optimization of various polygon shape radiators, it is cumbersome to determine the resonant frequency of the wideband antenna [4], [12]. The lower edge frequency calculation for the monopole antennas are discussed as follows. 3.8.3.1 Planar Rectangular Monopole Antenna For rectangular planar monopole antenna of length L and width W, the lower frequency corresponding to VSWR = 2 can be approximately calculated by equating its area to that of an equivalent cylindrical monopole antenna of the same height L and equivalent radius r [12], as described below: (3.3) which gives: (3.4) Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 44
The input impedance of a /4 monopole antenna is half of that of the /2 dipole antenna. Thus input impedance of an infinitesimally thin monopole antenna is 36.5 + j21.25 Ω [101]. The real input impedance of 37 Ω which will match well with 50 Ω standard transmission line (with VSWR =1.35 2) is obtained with a slightly smaller length of the monopole given by; (3.5) Where: (3.6) From the above two equations λ is obtained as: (3.7) Therefore, the lower edge frequency is given as: (3.8) Considering the probe length p, the above equation (3.8) is modified as: (3.9) From the above equation (3.9) the lower edge frequency of any monopole can be obtained by the values of L and r of the effective cylindrical monopole. 3.8.3.2 Planar Hexagonal Monopole Antenna The hexagonal monopole antenna feed in the middle of the side length l, the L and r values of the equivalent cylindrical monopole antenna are obtained by equating their areas as follows: (3.10) (3.11) Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 45
For the hexagonal monopole antenna of side length l, when the feed at the vertex, the L and r values of the equivalent cylindrical monopole antennas are obtained by equating their areas as follows: (3.12) (3.13) Substituting the above value of L and r in equation (3.9), the lower edge frequencies can be calculated for both cases. 3.8.3.3 Planar Circular and Elliptical Monopole Antenna Similarly, for the planar circular monopole antenna of radius a, the values L and r of the equivalent cylindrical monopole antenna are given by: (3.14) (3.15) An elliptical monopole antenna is a generalized case of the circular monopole, wherein the major axis is not equal to the minor axis. The dimensions of the elliptical monopole (i.e., major axis length = 2a and minor axis length = 2b) are calculated, keeping its area equal with that of the circular monopole. For calculating f L of the elliptical monopole antenna, the L and r of the effective cylindrical monopole are determined by equating its area as: (3.16) For elliptical monopole antenna fed at minor axis, L = 2b and r = a/4, and for elliptical monopole antenna fed at the major axis, these parameters are L = 2a and r = b/4. The f lower is determined by equation (3.9). Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 46
3.9 Printed Planar UWB Antennas An antenna with a very small dimension and wide impedance bandwidth is the first priority when choosing an antenna for UWB wireless applications. Also, the designed antennas on a printed circuit board with capability of integration with UWB devices are attractive to system designers. These antennas are usually designed and constructed by etching the radiator onto the dielectric substrate and a ground plane near the radiator. The radiating patch and the ground plane can be printed separately on both sides of the substrate or both can be printed on one side of substrate [20]-[64]. 3.9.1 UWB Monopole Antenna The antenna can be fed by a microstrip transmission line or a coplanar waveguide (CPW) structure. The printed monopole structures are shown in Figure 3.9 in which the radiating patch can be fed by a microstrip or a CPW feed. The radiating patch of any shaped printed antenna is optimized to cover the UWB bandwidth. The radiator can be slotted for good impedance matching over a wide bandwidth. The impedance bandwidth and radiation performance can be enhanced by tuning the dimension of the ground plane and the radiating patch. The printed monopole antennas can be used for indoor wireless communication systems because of their wide impedance bandwidth, omnidirectional radiation patterns, simple structure, and low cost. Figure 3.9 Microstrip and CPW fed monopole and slot antennas Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 47
3.9.2 UWB Slot Antenna UWB slot antennas are evolved from the microstrip slot antennas. The features of slot antennas such as bidirectional radiation pattern, various types of slot geometry and feeding techniques offer an additional degree of freedom in the design of the UWB slot antennas [68], [73], [75]-[95]. Wide bandwidth slot antenna use the microstrip feed to excite the wide slot printed in the ground plane. In the CPW feed slot antenna, the ground plane and radiating patch are printed in one plane of the substrate. These slot antennas are called as uniplanar as shown in Figure 3.9(c). The feed line is terminated in a tubing stub. The desired 50 Ω impedance matching can be obtained by tuning the feed line, ground plane and tuning stub. The tuning stub used to construct the slot antenna is of various shapes such as rectangular, circular, ellipse, U-shaped, fork shaped and many more [66]-[100]. Because of their structure, CPW feed slot antennas are also called as monopole slot antennas. The slot antennas are capable of producing very wide impedance bandwidth with various impedance matching techniques. The broad bandwidth is achieved with good coupling between the slot, feed and tuning stub. Researchers have demonstrated microstrip and CPW feed slot antennas of rectangular, ellipse, and circular shapes. The desired slot antenna must have small size, omnidirectional patterns, and simple structure that produces low dispersion, but can provide large bandwidth. The size of the printed antennas can be made very small for their use in wireless applications. Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 48