**1. Introduction**

UWB technology is advancing rapidly because of its potential to have high data rates and very low radiation power. In recent years UWB technology has been used in the areas of, sensing, radar and military communications [1]. In February 2002, the Federal Communications Commission (FCC) of the United States issued a report that UWB could be used for wireless data Communications [2]. Since then, a huge surge of research interests have occurred and this technology has been considered as one of the most reliable wireless technologies for various applications that leads to new innovations and greater quality of wireless personal area network services industry.

Reliable wireless connection between computers, portable devices and consumer electronics in short distances and data storage and transfer between these devices are new subjects of scientific and industrial competitions which require data rates much more than now a day accessible ones.

In this chapter, at the second part, some advantages of ultra-wideband technology and its progress trend will be reviewed. In the third part, antennas structures and parameters, especially wideband antennas will be studied for use in the UWB systems. To describe the performance of an antenna, various parameters must be defined. There are several important and practical parameters such as frequency bandwidth, radiation pattern, directivity, gain and input impedance which will be explained briefly. The performance of a UWB antenna is required to be stable and uniform over the ultra wide operational bandwidth. In the other word, antenna radiation pattern, gain and input impedance should be stable across the entire band. Also antenna needs to be small enough to be compatible with the other UWB system elements, especially in portable devices. In addition, basic antenna parameters, such as gain and return loss, must have little variations across the

© 2012 Malayeri, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Malayeri, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

operational band. Various methods have been employed to enhancement antenna bandwidth. In this part, frequency independent antennas will be studied for instance.

Active Integrated Antenna Design for UWB Applications 187

C BW log2 1 SNR = + ( ) (1)

Where C is the maximum data rate and BW is the channel bandwidth. Equation (1) indicates that by increasing the SNR (which is directly related to transmission power) or bandwidth, transmitting data rate can be increased. Because of power limitations, increasing the SNR is not a general solution [2, 3, 4 and 5]. Therefore to increase channel capacity and achieve high data rate, a large frequency bandwidth is needed. Considering Shannon-Nyquist Equation indicates that channel capacity can be increased more rapidly by enhancing the channel bandwidth than the SNR. Thus, the wider frequency range can lead to the greater channel capacity. This is more applicable for WPAN which works over short distances and SNR is

UWB has many satisfactory advantages which make it an interesting technology for wireless systems. It is probably the most promising technology for new wireless systems because of some advantages such as low complexity, low power consumption, low cost, high data rate and short-distance wireless connectivity. From circuit point of view, accurate power transfer between transmitter and receiver is the major challenge of UWB system design to obtain a

• Shannon-Nyquist theorem shows that channel capacity is proportionally related to bandwidth. According to the ultra-wide frequency bandwidth of UWB systems, they

• UWB systems use very low power transmission levels across an ultra-wide frequency spectrum that lead to reducing the effect of power upon each frequency element below

can achieve grate capacity in distances below 10 meters [1].

the acceptable noise level [1]. This is illustrated in figure (1),

**Figure 1.** Ultra wideband communications spread transmitting energy

more satisfactory there.

flat received power with minimum ripple.

Here some other benefits are reviewed:

**2.1. UWB benefits** 

In part 4, planner spiral antenna characteristics and features will be reviewed as a frequency independent antenna. Since without optimization, spiral antenna has some limitations for UWB applications, these limitations will be improved by using some optimization techniques. One of new methods is using active circuit in antenna structure. So in the fifth part, improving history of active antenna technology will be reviewed. Integration of active circuit into passive antenna gives a lot of advantages such as increasing the effective length of short antennas, increasing bandwidth, improving noise factor, impedance matching and sensitivity of receiver antennas and some applications such as utilizing active antenna arrays in mobile communications and beam control, solving channel capacity limitation problems by increasing data rate and improving smart antenna technologies [3] and many other advantages. Overall active antenna structure and different types and applications will be discussed in this part.

A review of distributed amplifiers characteristics will be done in the sixth part as the active part of active antenna structure. Here the aim is to design a UWB distributed amplifier with uniform and acceptable parameters such as Gain and VSWR in the 3.1- 10.6 GHz band. Calculation of the optimum load resistance and the number of amplifier stages, and then design, optimization and analysis of the circuit must be done for active circuit design completion. Adding antenna element to the active circuit and combined circuit analysis will be explained in this part too. Finally a brief analysis of design and simulation results of UWB active antennas will be shown in the seventh part and it will be favorable that active antenna parameters such as VSWR and Gain are appreciably optimized rather than passive antenna.
