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Ultra Wide Band technology has been used in communications since the past 20 years ago. Until recently, UWB technology has been to grow more rapidly as with the high data rate for wireless communication technology especially in mobile communication system applications. Ultra Wideband is carrier signals that carry less short of range communications technology which transmits the information in a very short pulse.
Ultra Wideband is a carrier less short range communications technology which transmits the information in the form of very short pulses. UWB has promised to offer high data rates at short distances with low power, primarily due to wide resolution bandwidth. The FCC in the USA has allocated a frequency band 3.1 GHz to 10.6 GHz for UWB transmission and released the power levels to keep the narrow band incumbents spectrum free from interference . In this case, it is to intend to provide an efficiency of usage for limited radio bandwidth while enabling both of high data rate for Personal Area Network (PAN) wireless connectivity for longer range, low data rate applications for radar and imaging systems.
Since now, the UWB technology has been declared as one of the most popular wireless technologies that can be archived for the high data rate transmission signal and can capture the personal area networking industry leading to new innovations and greater quality of services for end users. According to the FCC rules for any signal that covered at least 500MHz spectrum can be used in UWB systems such as in mobile communication system. So that, the UWB is not limited to impulse radio any more, it also applies to any technology application system that uses 500MHz spectrum and also complies with all other requirements for UWB range frequency.
As a beginning, the first model of UWB antennas have been invented by Artimi (2003) where obtained an array of narrowband structures. The UWB antenna has ultra wideband properties where are desired for a variety of applications, but Artimi developed an application for a pulsed radio system. Some of the antennas that proposed have non-planar structure while others have poor return loss and lower gain. Mostly of the compact UWB antenna presented in literature exhibit omni-directional radiation patterns with relatively low gain [3-5]. For those types of antennas are usually suitable for the short range frequency for indoor and outdoor communication application. However, for mobile communication system application, gain in antenna must be preferred. By the way, a microstrip array antenna was chosen due to the ease in design and fabrication and also can be used in a planar gap coupled array configuration for mobile communication system applications.
To ensure the application efficiency for ultra wide band (UWB) systems, much attention has been paid the UWB system which the UWB antennas are the very important components. As one of UWB antennas, array bowtie antennas fed by coaxial line, coplanar waveguide and strip line have many advantages, such as low profile, ultra wide impendence band, high radiation efficiency and easy to manufacture .
The characteristics of the ultra-wide-band (UWB) systems usually dependent to the frequency characteristics of the antennas and basically the frequency dependent behavior of the channel have to be considered. In other word, the UWB systems always realized in an impulse based technology, and therefore the time-domain effects and properties as well as to be known .
2.2 Advantages of Ultra Wide Band
The UWB has a several number of advantages. In this situation, that is the reasons why it presents a more elegant solution to the wireless broadband compare to the other technologies communication application. Firstly, refer to the Shannon Hartley theorem while the channel capacity is in proportion to bandwidth spectrum. While, the UWB technology application has an ultra wide band for frequency bandwidth, it can achieved to the huge capacity exceed hundreds of Mbps or in several situations to Gbps with distances of 1m to 10m .
Second, the UWB technology systems also operate at extremely low power transmission levels. By dividing the power of the signal across a wide frequency spectrum and the effect of that to upon of any frequency are below the acceptable noise floor , as shown in Figure 2.1.
Figure 2.1: The Ultra Wide Band communication spread transmitting energy across a wide spectrum of frequency .
Then, the UWB band also provides higher secure and reliable for mobile communication solutions. Due to the low energy power density, the UWB signal is known noise like, which makes unintended detection quite difficult. Furthermore, the "noise like" is the signal that has a particular shape and the real noise has no shape. So, this reason is the almost impossible for real noise to obliterate the pulse because the interference has to spread uniformly signal across the whole spectrum to obscure the pulse. Interference spectrum reduces the amount of received signal, but the pulse of the signal still can be obtained to restore and recovered the good signal. So that, the UWB technology system may be becomes the most secure and safety medium for the wireless mobile communication transmission for ever previously available.
Besides that, the UWB system also has low cost and less complexity from the basic characteristic baseband nature of the signal transmission. The UWB technology system does not modulate and demodulate a more complex carrier waveform so that, it does not require additional components for the application system such as mixers, filters, amplifiers and local oscillators.
Since, the FCC the approval of UWB range frequency which means from 3.1 GHZ to 10.6 GHZ for commercial use can prompt the industrial and the academia also to put significant effort and potential by them into this technology. In other situation, the futures of UWB technology application depend on these rules for the regulatory rulings and standards. The UWB technology would be applied in a wide and huge range of areas, especially in mobile communications system application.
2.3 Overview on Ultra Wide Band Antenna
Basically the antenna is an essential part of any wireless system. Refer to the IEEE Standard; the antenna is defined as a part of function for radiating or receiving radio waves . In others words, an antenna is a device that can transmit and receive the signal from a transmission line, converts into electromagnetic radio waves and then broadcasts into free space.
By the ways, the antenna also must serve as a directional to a transition device. In order to get the particular requirement, it must take the various forms. Then, as a result, an antenna may be can like a patch, a piece of wire, an aperture, a reflector, a lens, an arrays antenna and so on. Then, a good design of the antenna can be archived for all the system requirements and would improve the overall system performance.
2.3.1 Several Important Literatures about Parameters of Antenna
To discuss about the performance of an antenna, some of the definitions of various parameters are necessary required and must be considered. Practically, there are commonly several characteristics used antenna parameters such as frequency bandwidth, input impedance, radiation pattern, gain, directivity and others.
18.104.22.168 Frequency Bandwidth
Frequency Bandwidth (BW) defined as the range of frequency where the performance of the antenna, according to the some characteristic, with the special specified standard. The bandwidth of the frequency can be considered to become a range of the frequency in either side of the center frequency where the antenna characteristics can accept the value of the center frequency. By the way, for wireless mobile communication system application, the antenna must obtained to provide a return loss lower than -10dB over its frequency bandwidth.
22.214.171.124 Radiation Pattern
The radiation pattern is normally known as the radiation properties of the antenna signal. In the most cases, it is determined in the far-field region. The spatial angular distributions of the radiated power not depend on the range and distance. Basically, the pattern would be described as the normalized of field value respectively to the maximum values.
The radiation character normally use in condition either 2D or 3D dimensional spatial distribution of the radiated energy pattern in term of the observe position along the path or surface of the constant radius. In practical, the 3D pattern is sometimes needed and also can be constructed of 2D patterns in a series. For most practical applications, a few plots of the character as a function of Ï† for some particular values of frequency range, added a few plots of frequency for some particular values of Î¸ would be provide most of the useful information required, where Ï† and Î¸ are the two axes in a spherical coordinate.
For a linear polarized antenna, the performance is often described in terms of E-plane and H-plane patterns. The E-plane defined as a plane of containing the electric field vector and the direction of maximum radiation, while for the H-plane can be defined as the plane containing the magnetic field vector and the direction of maximum radiation .
Commonly there are three common types of the radiation patterns that are used to describe the radiation pattern for character:
A hypothetical loss less of the antenna has an equal radiation in all directions. This only valid for an ideal and unique antenna and often taken as a reference for delivered the directive of the actual antenna properties.
An antenna has the property of radiate or receives electromagnetic waves more efficiently in some of directions than in others. This is usually applicable to an antenna where the maximum directivity is greater than of a half wave dipole
An antenna has a basically non-directional pattern in a given plane and a directional pattern in an orthogonal plane.
126.96.36.199 Directivity and Gain
To discuss about the directional properties of antenna radiation pattern, the directivity; d it can be described as the ratio of the radiation intensity ; U from the antenna over that of an isotropic source. For an isotropic source, the radiation intensity; U0 means the value equal to the Total Radiated Power Prad divided by 4Ï€. So the directivity can be calculated by equation as shown as below:
Antenna gain; G is related to the directivity value, but it takes into the radiation efficiency of erad the antenna for directional properties, as given by:
G = eradD (3.2)
188.8.131.52 VSWR (Voltage Standing Wave Ratio)
The Voltage Standing Wave Ratio (VSWR) is an indication of the amount of mismatch between an antenna and the feed line connecting to it. The range of values for VSWR is from 1 to âˆž. A VSWR value under 2 is considered suitable for most antenna applications. The antenna can be described as having a good match. So when someone says that the antenna is poorly matched, very often it means that the VSWR value exceeds 2 for a frequency of interest.
When a VSWR of 1:1, it means that there is no power being reflected back to the source. This is an ideal situation that rarely, if ever, is seen. In the real world, a VSWR of 1.2:1 is considered excellent in most cases. At a VSWR of 2.0, approximately 10% of the power is reflected back to the source. Not only does a high VSWR mean that power is being wasted, the reflected power can cause problems such as heating cables or causing amplifiers to fold-back.
There are ways to improve the VSWR of a system. One way is to use impedance matching devices where a change in impedance occurs. Baluns are used extensively in antennas to not only convert from balanced to unbalanced signals but also to match the impedance of the source to the antenna. One emissions standard, for instance, specifies using an attenuator at the connector of a biconical antenna since it has a high VSWR at some frequencies. One of the conducted immunity standards suggests using a 6dB pad at the input of the coupling device, which is commonly 150 ohms. Attenuators obviously cause power loss, but they reduce VSWR by providing an apparently better termination to a signal.
VSWR can also be represented other ways, such as return loss, mismatch loss and reflection coefficient. Reflection Coefficient is common, can be calculated several ways, and ultimately used to calculate VSWR. It is important to know that for accurate VSWR measurements of devices. Any cable loss, or attenuation, will make the VSWR at the input of the cable appear much better than at the load or termination. The reason is that the cable loss or attenuation increases the return loss.
2.3.2 Requirements characteristics for UWB Antennas
In all the case in conventional wireless communication systems, such as for mobile communication system, antenna also plays a function in UWB systems. However, there are more challenges in designing a structure of UWB antenna than a narrow band antenna . It is because a special characteristic for UWB antenna from other antennas is its ultra wide frequency bandwidth. According to the FCC's rules, a suitable UWB antenna should be able to yield an absolute bandwidth no less than 500MHz or in other word in must in the frequency range from 3.1 GHz until 10.6 GHz.
Also that, the performance of a UWB antenna is required to be consistent over the entire operational band. Ideally, in this situation, the antenna radiation patterns, gains and impedance matching should be stable across the entire band. Sometimes, it is also demanded that the UWB antenna provides the band rejected characteristic to coexist with other narrowband devices and services occupying the same operational band [19, 20].
Next, the directional or omni-directional radiation properties also are needed depend on the practical application. Omni-directional patterns are normally desirable in mobile communication system. But for radar systems and other directional systems where high gain is desired, directional radiation characteristics are preferred.
A suitable antenna needs to be small enough to be compatible to the UWB unit especially in mobile communication system. It is also highly desirable that the antenna feature low profile and compatibility for integration with the printed circuit board (PCB).
Lastly, a good design of UWB antenna also must be optimal for the performance of overall system. The antenna should be designed such that the overall device in term of antenna and RF front end can complies with the mandatory power emission mask given by the FCC or other regulatory bodies. In other word, a UWB antenna is required to achieve good time domain characteristics which mean an antenna should have give the performance over the entire bandwidth and the basic parameters, such as gain and return loss, because of this characteristic of the antennas structure are also a key component in UWB systems especially in mobile communication system requirements.
2.4 Microstrip antenna
Microstrip Antennas also kwon as patch antennas recently are widely used in the microwave frequency region because of simple and compatible with printed circuit technology. It is because to make it easy to manufacture either as standalone elements or array of elements. For a simple of microstrip antenna usually consists of a patch of metal, a rectangular or circular on top of a grounded substrate, as shown in Figure 2.2
Figure: 2.2 (a) Rectangular microstrip patch antenna and (b) circular microstrip patch antenna (David R. Jackson)
A microstrip antenna is suitable for this kind of application. The conventional microstrip antenna suffers from a major disadvantage that it has a narrow bandwidth. Over the years, there have been several research efforts by various groups worldwide aiming to increase the bandwidth of these antennas.
Microstrip patch antennas are being increasingly used especially in mobile communication system. It is because they are many advantages compare to the conventional antennas, such as for example it can be small, lightweight and compact. Recently untill now, there has been increased interest in minimizing microstrip patch antennas for some specific applications, especially such as for mobile communications system .
2.5 Reviews on Several Literatures
Although UWB theory is important for any designing antenna it was equally important review the FCC rules and regulations governing UWB. These rules set the limits on the emissions from UWB systems and establish some operational rules to help avoid interference. In order also, I tried to review some literatures about the software CST Microwave for modeling. It is to make those comply with the project objectives and also to archived the scope target for successful project.