**3. Antenna**

A passive antenna is an electrical conductor or array of them which radiates (transmits/ receives) electromagnetic waves. Most antennas are resonant devices, which operate over a relatively narrow frequency band. For any wireless system, antenna is an essential part which must concentrate the radiation energy in some directions at certain frequencies. Thus the antenna is also considered as a directional device too. Depending on application type, antenna must take various shapes to meet the required conditions. Therefore, antenna may be a specific length of a wire, an aperture, a patch and so on. A good designed antenna can improve system performance by complying system requirements.

#### **3.1. Antenna parameters**

To discuss the performance of an antenna, various parameters must be defined. Frequency bandwidth, gain, input impedance and radiation pattern are some of them.

Frequency bandwidth; is the range of frequencies which the antenna performance conforms specified characteristics. In other words, the bandwidth is the range of frequencies which the antenna characteristics are acceptable in compare with their values in center frequency. Frequency bandwidth can be expressed in form of absolute bandwidth (Abw) or fractional bandwidth (Fbw). Abw and Fbw can be calculated as given in Equations (2) and (3), respectively:

Active Integrated Antenna Design for UWB Applications 189

$$\mathbf{A}\mathbf{b}\mathbf{w} = \mathbf{f}\_{\mathbf{H}} / \ \mathbf{f}\_{\mathbf{L}} \tag{2}$$

$$\mathbf{Fbw} = \mathbf{Abw} / \mathbf{f\_c} \tag{3}$$

$$\mathbf{f\_c} = (\mathbf{f\_H} + \mathbf{f\_L})/2$$

188 Ultra Wideband – Current Status and Future Trends

for modulation and demodulation units.

Some of the other UWB benefits and advantages are briefly listed below:

• Improved measurement accuracy of target detection in radar

improve system performance by complying system requirements.

bandwidth, gain, input impedance and radiation pattern are some of them.

means.

• Lower cost

**3. Antenna** 

respectively:

• High data transfer rate

• Channel capacity improvement • Lower power consumption

• Coexistence possibility with 802.11/b/g • Accurate position and distance metering

• Identification of target class and type

information see references [4 and 5].

**3.1. Antenna parameters** 

• Because of low energy density of the UWB signal, it is a noise-like signal and therefore its undesirable detection is unlikely. Since it is a noise-like signal which has a particular shape, it can be detected in related receiver. In contrast, real noise has no shape, thus interference cannot distort the pulse shape completely and it can still be recovered to restore primary signal. Hence UWB communications are very secure and reliable

• Baseband nature of the UWB signal which is based on impulse radios causes low cost and low complexity of operation systems. Because it does not require system components such as mixers, filters, amplifiers and local oscillators which are necessary

And so many other interesting advantages which cannot be explained here. For more

A passive antenna is an electrical conductor or array of them which radiates (transmits/ receives) electromagnetic waves. Most antennas are resonant devices, which operate over a relatively narrow frequency band. For any wireless system, antenna is an essential part which must concentrate the radiation energy in some directions at certain frequencies. Thus the antenna is also considered as a directional device too. Depending on application type, antenna must take various shapes to meet the required conditions. Therefore, antenna may be a specific length of a wire, an aperture, a patch and so on. A good designed antenna can

To discuss the performance of an antenna, various parameters must be defined. Frequency

Frequency bandwidth; is the range of frequencies which the antenna performance conforms specified characteristics. In other words, the bandwidth is the range of frequencies which the antenna characteristics are acceptable in compare with their values in center frequency. Frequency bandwidth can be expressed in form of absolute bandwidth (Abw) or fractional bandwidth (Fbw). Abw and Fbw can be calculated as given in Equations (2) and (3), Where fH and fL express the high and low frequencies of the bandwidth respectively and fc shows the center frequency. Although sometimes the bandwidth is expressed as the ratio of the high to low frequencies of operational bandwidth for broadband antennas:

$$\text{BW} = \mathbf{f}\_{\text{H}} / \mathbf{f}\_{\text{L}} \tag{4}$$

Radiation Pattern; is the representation of the radiation properties of the antenna as a function of space coordinates. Usually radiation Pattern is determined in the farfield region to avoid effects of the distance on the spatial distribution of the radiated power [1].

The radiation pattern can be expressed in two or three-dimensional spatial distribution and it is usually in normalized form with respect to the maximum values. Radiation properties of an antenna can be described by three types of radiation patterns:


Gain and Directivity; directivity is calculated as the ratio of the radiation intensity in a given direction over an isotropic source radiation intensity, to describe the directional radiation properties of an antenna. The directivity is expressed by D letter and can be calculated by equation (5):

$$\mathbf{D} = \frac{\mathbf{U}}{\mathbf{U}\_0} \tag{5}$$

$$\mathbf{U}\_0 = \frac{\mathbb{P}\_{\text{rad}}}{4\pi}$$

Where U0 is the radiation intensity for an isotropic source, U is radiation intensity of antenna and rad P is radiated power.

Antenna gain is related to the directivity and radiation efficiency and it can be calculated by equation (6):

$$\mathbf{G} = \mathbf{e}\_{\text{rad}} \mathbf{D} \tag{6}$$

Which G is antenna gain and erad is radiation efficiency.
