**3.2. UWB antenna characteristics**

Although UWB antenna is an important part of conventional wireless communication systems, but designing a UWB antenna needs to consider important notes that some of them are listed below:

Active Integrated Antenna Design for UWB Applications 191

The first problem is the unlimited dimensions of antenna requirements according the Rumsey's principle. It is impossible to have an infinite length for example in a spiral antenna and the antenna size will be a practical challenge. Truncating each of dimensions of antenna

Second problem is that spiral antenna active zone depends on the signal frequency. For a UWB signal, they are many frequency components and each frequency component is radiating from different part. In other word the smaller parts radiate higher frequencies and the larger scale parts emit lower frequencies of antenna and this may cause dispersive and signal distortion. Therefore these antennas can be cause problems for systems which cannot tolerate dispersion. Signal detection and recovery features are needed for systems which use

New generation communication systems requirements have essential role to identifying the type of antenna which is used with them. High data rate and wider bandwidth requirements for data\video transfer and growth in number of users lead to increase operating frequencies of these systems to microwave and millimeter-wave frequencies. In these frequencies performance of standard antennas such as monopole and dipole antennas is considerably weak and it causes to choose planner antennas as a way of surmount this problem. Planner antennas have many benefits such as lower manufacturing costs, considerably smaller size and less weight in compare with the other antennas. This causes to increase their applications in for example mobile phones and communication stations.

In the other hand, their planner structure makes them desirable for use in large arrays and suitable for integrating with electronic circuits such as amplifiers and phase shifters which are the main parts of designing radar, satellite communications and etc. planner structure gives them the ability of using in some applications such as antennas printed in the airplane body which can resolve limitations of antenna size [7]. These features and many other benefits of this type of antennas caused them to become a good subject of new designs of high performance planner antennas in many different applications and raised a new type of antennas with the name of Active Integrated Antennas (AIA) which will discuss more in the

According to previous discussion about frequency independent antennas such as spiral antenna, they are classical wideband antennas which can display uniform impedance and pattern characteristics over a frequency range wider than 10:1. Thus spiral antenna seems to be a good choice for use in UWB active antenna design. So in the following part, spiral

Planner spiral antennas are one of frequency independent antennas with wide bandwidth and good pattern efficiency in compare with other antenna types. Theatrically a spiral

can cause limitations in frequency bandwidth.

this type of antennas.

**4. Spiral antenna** 

next parts.

**4.1. Features** 

antenna and its parameters are discussed in detail.


### **3.3. Frequency independent antennas**

Ultra wide operating bandwidth is the main difference and advantage of a UWB antenna. To achieve wideband characteristics for different antennas, various methods can be used. Frequency independent antennas are one important group of antennas which display wideband features. Some principles and conditions of this group are discussed here.

Frequency independent antennas can display almost uniform input impedance and radiation pattern and other radiation properties over a wide frequency bandwidth. Spiral antenna is one practical example of frequency independent antennas. These antennas were called travelling wave antennas by Johnson Wang at first.

Victor Rumsey in the 1950s and Yasuto Mushiake in the 1940s introduced some principles which explain how frequency independent characteristics can be achieved.

The first principle which is introduced by Rumsey suggests that to achieving frequency independent properties of an antenna, its shape must be specified only by angles [6]. One example for this type of antennas is spiral antenna with no limitation of its length. Infinite biconical antennas are other examples whose shapes are completely described by angles.

Self-complementarity is the second principle of frequency independent characteristics which was introduced by Yasuto Mushiake. This principle suggests that if an antenna is complement of itself, frequency independent behavior is achieved. In such an antenna, impedance is constant and equal to η2 or 188.5 Ohms [1].

Although these antennas are theatrically frequency independent, but they are some limitations which cause limited bandwidth of this type of antennas and needs some optimization to achieve unlimited bandwidth.

The first problem is the unlimited dimensions of antenna requirements according the Rumsey's principle. It is impossible to have an infinite length for example in a spiral antenna and the antenna size will be a practical challenge. Truncating each of dimensions of antenna can cause limitations in frequency bandwidth.

Second problem is that spiral antenna active zone depends on the signal frequency. For a UWB signal, they are many frequency components and each frequency component is radiating from different part. In other word the smaller parts radiate higher frequencies and the larger scale parts emit lower frequencies of antenna and this may cause dispersive and signal distortion. Therefore these antennas can be cause problems for systems which cannot tolerate dispersion. Signal detection and recovery features are needed for systems which use this type of antennas.
