**4. Spiral antenna**

190 Ultra Wideband – Current Status and Future Trends

**3.2. UWB antenna characteristics**

the UWB antenna should meet them.

**3.3. Frequency independent antennas** 

impedance is constant and equal to

optimization to achieve unlimited bandwidth.

are listed below:

0.2 respectively

regulatory bodies.

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

• Because of the ultra-wide frequency bandwidth of these systems and to comply with the FCC report, Abw and Fbw of a UWB antenna should not be less than 500MHz and

• UWB antenna parameters such as antenna radiation pattern, gain and input impedance

• Radiation pattern properties are different depending on the practical conditions which

• In many cases such as portable devices, the UWB antenna should be small enough to be compatible with the overall system. In some other applications this antenna must be

• UWB antenna should comply with the FCC power emission mask or other world

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

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

Victor Rumsey in the 1950s and Yasuto Mushiake in the 1940s introduced some principles

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,

2 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

wideband features. Some principles and conditions of this group are discussed here.

which explain how frequency independent characteristics can be achieved.

η

must be uniform and stable over the entire operational band.

compatible with printed circuit board (PCB) structures.

called travelling wave antennas by Johnson Wang at first.

• UWB antenna must have minimum effects on UWB pulse waveform.

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 next parts.

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 antenna and its parameters are discussed in detail.

#### **4.1. Features**

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

antenna with infinite number of turns and dimensions which confirms the frequency independent principles can exhibit infinite pattern efficiency and bandwidth. But practical infinite arm length is impossible and some limitations must be applied [6, 8]. This type of antennas has widespread usage because of their small substrate size and few pulse dispersions in communicating processes. The primarily used single arm spiral antenna is illustrated in the figure (2). Desirable radiation characteristics can be achieved by changing circular radius r, number of turns N and the width of them W [8].

Active Integrated Antenna Design for UWB Applications 193

which has low power emission is called transmission line zone, because it just transfers power from source to load or antenna. Although the final antenna radiation pattern is not considerably affected by this area, it is effective in the value of input impedance and therefore must be considered to have an optimum design. The area between active zone and the end of antenna arm is out of radiation circuit. In fact, whole power is emitted before reach to this area, therefore it has no effect on the radiation pattern and input impedance [8]. So it can be predicted that by increasing frequency, active region will move along spiral antenna radius such a way that electrical dimensions remain constant in different frequencies. To conforming Rumsey's principle, antenna dimensions must be infinite and such antenna can be considered as frequency independent antenna. But by eliminating antenna structure in both ends because of practical manufacturing limitations, antenna

To define active zone, at first the lowest frequency of bandwidth must be selected to calculating the exterior radius of active zone. As is discussed, by increasing frequency, active zone will move to interior parts and smaller radiuses. Then by choosing the highest operating frequency of bandwidth, its related active zone and interior structure boundary can be defined. However usually interior radius is not limited more than requirements of

Defining active zone has some benefits; because of predefined manufacturing limitations, antenna structure must be limited in both directions and missing a vast width of frequency band. Therfore active zone definition helps designers to design antenna with better performance in the practical frequencies. Another advantage of defining active zone is to understanding and calculating other effective parameters in antenna performance

The idea of using active antennas was introduced in about 1928 by using a small antenna with electron tube in radio receivers. In 1960's and 1970's, active antennas were studied more seriously due to the invention of high frequency transistors [9, 10]. Because of progresses in technology of microwave integrated circuit (MIC), active antennas became an interesting subject of researches at that time [11, 12 and 13]. Integration of active device into passive antenna gives a lot of advantages such as increasing the effective length of short antennas, increasing the bandwidth, improving the noise factor, impedance matching and sensitivity of receiver antennas and some applications such as utilizing active antenna arrays in mobile communication and beam control, using to solve channel capacity limitation problems by increasing data rate and improving smart antenna technologies [3]

Radiation element or passive antenna is a device that converts received signals from a transmission line into electromagnetic waves and radiates them into free space in a

performance will be considerably dependent on active zone properties.

implementing power source in center.

optimization [8].

**5. Active antenna**

and many other advantages.

**5.1. Active antenna structure** 

**Figure 2.** Single arm spiral antenna

If W = S, i.e. the metal and the air parts of the antenna are equal, the spiral antenna is selfcomplementary.

#### **4.2. Active zone**

In spiral antenna radiation is done from that part of antenna which its circumference is equal to or greater than 2λ, where λ is the wave length. This region is called active zone of antenna. Thus low frequency limit of antenna is related to exterior radius of antenna as expressed by (7) relation and the high frequency limitation is related to its interior radius. In the other word, circumference of the radiation zone determines the radiation frequency.

$$\mathbf{f} = \mathbf{c} / \pi \mathbf{r} \quad \rightarrow \quad \mathbf{f}\_{\text{low}} = \mathbf{c} / \pi \mathbf{r}\_2 \quad \text{and} \quad \mathbf{f}\_{\text{high}} = \mathbf{c} / \pi \mathbf{r}\_1 \tag{7}$$

Where c is the light speed, r1 is the interior radius and r2 is the exterior radius of the antenna. In fact, relation shows that higher frequencies are emitted from smaller circles and the lower frequencies will radiate from bigger circles of the antenna. However in practice, because of antennas end reflections and source effects, high and low frequencies will be a little different from the calculated values of relation (7) [8].

According to the previous discussion, active zone calculation is one of important and effective parameters to design a spiral antenna. Radius of radiation circle is defined by relation (7). In fact this region is a part of antenna in which maximum power of the device is radiated from and it can be considered as only radiating zone of antenna.

In this area, current amplitude inducted by radiation is considerably higher than the currents in other parts of structure. The region between source point and the active zone which has low power emission is called transmission line zone, because it just transfers power from source to load or antenna. Although the final antenna radiation pattern is not considerably affected by this area, it is effective in the value of input impedance and therefore must be considered to have an optimum design. The area between active zone and the end of antenna arm is out of radiation circuit. In fact, whole power is emitted before reach to this area, therefore it has no effect on the radiation pattern and input impedance [8]. So it can be predicted that by increasing frequency, active region will move along spiral antenna radius such a way that electrical dimensions remain constant in different frequencies. To conforming Rumsey's principle, antenna dimensions must be infinite and such antenna can be considered as frequency independent antenna. But by eliminating antenna structure in both ends because of practical manufacturing limitations, antenna performance will be considerably dependent on active zone properties.

To define active zone, at first the lowest frequency of bandwidth must be selected to calculating the exterior radius of active zone. As is discussed, by increasing frequency, active zone will move to interior parts and smaller radiuses. Then by choosing the highest operating frequency of bandwidth, its related active zone and interior structure boundary can be defined. However usually interior radius is not limited more than requirements of implementing power source in center.

Defining active zone has some benefits; because of predefined manufacturing limitations, antenna structure must be limited in both directions and missing a vast width of frequency band. Therfore active zone definition helps designers to design antenna with better performance in the practical frequencies. Another advantage of defining active zone is to understanding and calculating other effective parameters in antenna performance optimization [8].

## **5. Active antenna**

192 Ultra Wideband – Current Status and Future Trends

**Figure 2.** Single arm spiral antenna

from the calculated values of relation (7) [8].

complementary.

**4.2. Active zone** 

antenna with infinite number of turns and dimensions which confirms the frequency independent principles can exhibit infinite pattern efficiency and bandwidth. But practical infinite arm length is impossible and some limitations must be applied [6, 8]. This type of antennas has widespread usage because of their small substrate size and few pulse dispersions in communicating processes. The primarily used single arm spiral antenna is illustrated in the figure (2). Desirable radiation characteristics can be achieved by changing

If W = S, i.e. the metal and the air parts of the antenna are equal, the spiral antenna is self-

In spiral antenna radiation is done from that part of antenna which its circumference is equal to or greater than 2λ, where λ is the wave length. This region is called active zone of antenna. Thus low frequency limit of antenna is related to exterior radius of antenna as expressed by (7) relation and the high frequency limitation is related to its interior radius. In the other word, circumference of the radiation zone determines the radiation frequency.

Where c is the light speed, r1 is the interior radius and r2 is the exterior radius of the antenna. In fact, relation shows that higher frequencies are emitted from smaller circles and the lower frequencies will radiate from bigger circles of the antenna. However in practice, because of antennas end reflections and source effects, high and low frequencies will be a little different

According to the previous discussion, active zone calculation is one of important and effective parameters to design a spiral antenna. Radius of radiation circle is defined by relation (7). In fact this region is a part of antenna in which maximum power of the device is

In this area, current amplitude inducted by radiation is considerably higher than the currents in other parts of structure. The region between source point and the active zone

radiated from and it can be considered as only radiating zone of antenna.

low 2 high 1 f c / r f c / r and f c / r = →= *ππ π* = (7)

circular radius r, number of turns N and the width of them W [8].

The idea of using active antennas was introduced in about 1928 by using a small antenna with electron tube in radio receivers. In 1960's and 1970's, active antennas were studied more seriously due to the invention of high frequency transistors [9, 10]. Because of progresses in technology of microwave integrated circuit (MIC), active antennas became an interesting subject of researches at that time [11, 12 and 13]. Integration of active device into passive antenna gives a lot of advantages such as increasing the effective length of short antennas, increasing the bandwidth, improving the noise factor, impedance matching and sensitivity of receiver antennas and some applications such as utilizing active antenna arrays in mobile communication and beam control, using to solve channel capacity limitation problems by increasing data rate and improving smart antenna technologies [3] and many other advantages.

#### **5.1. Active antenna structure**

Radiation element or passive antenna is a device that converts received signals from a transmission line into electromagnetic waves and radiates them into free space in a transmitting antenna and vice versa in a receiving one. According to IEEE Standard antenna is a means for radiating or receiving radio waves [1]. Figure (3) shows the conventional receiving configuration of passive and active antennas.

Active Integrated Antenna Design for UWB Applications 195

**Figure 4.** Passive and active antenna assembling comparison

efficiency, and lack of high-performance phase shifters [15].

The intelligent design of the antenna and integration with active circuit leads to innovative microwave and millimeter-wave application systems and considerable achievements in compactness, low cost, small profile, low power consumption, and multiple applications. This technology caused new designs in both areas of military and industrial applications

(b)

(a)

The AIA concept has been extensively employed in the areas of power combining and quasi-

It also provides an effective solution to several fundamental problems at millimeter-wave frequencies including high transmission-line losses, limited source power, reduced antenna

AIA is also used to design high-efficiency microwave power amplifiers recently. In other word, the antenna element is used as a part of circuit to terminate the harmonics at the

Retro directive arrays are applicable in a wide range of applications such as self-steering antennas, radar transponders, search and rescue and wireless identification systems which are outcome of their omnidirectional coverage and the high level of gain. In these arrays any

amplifier output in addition to its traditional role of radiating electromagnetic waves.

such as wireless and radar communications, low cost sensors and transceivers [16].

optical power combining, beam steering and switching and retro directive arrays.

**5.2. Applications** 

**Figure 3.** a). Receiving system structure for passive antenna; b). Receiving system structure for active antenna

In 1977, Lindenmeier and Meinke suggested that if the cable length between the antenna and the amplifier is about 1-5 m, the antenna system will be considered as the passive one as shown in Figure (4a). Consequently when the radiating element is closely connected (Integrated) to the active circuit or amplifier, the structure is considered as active antenna [14]. It is important to note that the distance between radiation element and active circuit is related to the operating frequency and electrical length of cable.

As mentioned, the term "active antenna" means that the active device is coupled with the passive antenna to improve antenna performance, while the term "active integrated antenna" expresses more distinctive that the passive antenna element is integrated on the same substrate with the active circuit [9]. From the microwave theory standpoint, an active integrated antenna (AIA) can be regarded as an active circuit which its output or input ports are in free space instead of a conventional 50 Ω interface. In this case, the antenna provides certain functions such as resonating, filtering, and duplexing circuit behaviors. But from the antenna theory sight, the AIA is an antenna which exhibits radio signal generating and processing capabilities such as mixing and amplification [15].

In these systems whole systems is working with antenna and controls antenna as well as load parameters. By connecting antenna and circuit in such a way, transmission line losses are reduced considerably. It will be more important when the frequency is growing up [16]. This is significantly different and more effective than the systems in which radiating element and circuit are designed separately and then connected by a strip line or another type of transmission line. This is important to note that when antenna is consisting of a nonreciprocal circuit, it means that AIA system is non-reciprocal unlike passive antenna alone [7].

**Figure 4.** Passive and active antenna assembling comparison

## **5.2. Applications**

194 Ultra Wideband – Current Status and Future Trends

antenna

receiving configuration of passive and active antennas.

transmitting antenna and vice versa in a receiving one. According to IEEE Standard antenna is a means for radiating or receiving radio waves [1]. Figure (3) shows the conventional

(a)

**Figure 3.** a). Receiving system structure for passive antenna; b). Receiving system structure for active

(b)

In 1977, Lindenmeier and Meinke suggested that if the cable length between the antenna and the amplifier is about 1-5 m, the antenna system will be considered as the passive one as shown in Figure (4a). Consequently when the radiating element is closely connected (Integrated) to the active circuit or amplifier, the structure is considered as active antenna [14]. It is important to note that the distance between radiation element and active circuit is

As mentioned, the term "active antenna" means that the active device is coupled with the passive antenna to improve antenna performance, while the term "active integrated antenna" expresses more distinctive that the passive antenna element is integrated on the same substrate with the active circuit [9]. From the microwave theory standpoint, an active integrated antenna (AIA) can be regarded as an active circuit which its output or input ports are in free space instead of a conventional 50 Ω interface. In this case, the antenna provides certain functions such as resonating, filtering, and duplexing circuit behaviors. But from the antenna theory sight, the AIA is an antenna which exhibits radio signal generating and

In these systems whole systems is working with antenna and controls antenna as well as load parameters. By connecting antenna and circuit in such a way, transmission line losses are reduced considerably. It will be more important when the frequency is growing up [16]. This is significantly different and more effective than the systems in which radiating element and circuit are designed separately and then connected by a strip line or another type of transmission line. This is important to note that when antenna is consisting of a nonreciprocal

circuit, it means that AIA system is non-reciprocal unlike passive antenna alone [7].

related to the operating frequency and electrical length of cable.

processing capabilities such as mixing and amplification [15].

The intelligent design of the antenna and integration with active circuit leads to innovative microwave and millimeter-wave application systems and considerable achievements in compactness, low cost, small profile, low power consumption, and multiple applications. This technology caused new designs in both areas of military and industrial applications such as wireless and radar communications, low cost sensors and transceivers [16].

The AIA concept has been extensively employed in the areas of power combining and quasioptical power combining, beam steering and switching and retro directive arrays.

It also provides an effective solution to several fundamental problems at millimeter-wave frequencies including high transmission-line losses, limited source power, reduced antenna efficiency, and lack of high-performance phase shifters [15].

AIA is also used to design high-efficiency microwave power amplifiers recently. In other word, the antenna element is used as a part of circuit to terminate the harmonics at the amplifier output in addition to its traditional role of radiating electromagnetic waves.

Retro directive arrays are applicable in a wide range of applications such as self-steering antennas, radar transponders, search and rescue and wireless identification systems which are outcome of their omnidirectional coverage and the high level of gain. In these arrays any

incident signal will reflect back toward the transmitter without prior knowledge of its location [15].

Active Integrated Antenna Design for UWB Applications 197

,

Power amplifiers are essential parts of each transmitting system. They are used to amplifying signals to transmit from one point to another. Nonlinear effects of high power

In power amplifier design depending on application, higher efficiency, power and gain can be the main subjects of optimizations and linearity and noise figure are less considered.

Power amplifiers are categorized to different classes depending on active element bias and

Travelling wave structures are new methods to design of wideband amplifiers which have vast applications in wideband communications such as wideband travelling wave amplifiers, matrix amplifiers, travelling wave oscillators, mixers and power amplifiers . Concept and basic of travelling wave structures was initially originated by Percival in 1937. In 1940, this method was used to design of wideband vacuum tube amplifiers. But using GaAs MESFET in distributed amplifiers was studied at first by Moser in 1967 and Jutzi in 1969. They designed a distributed amplifier using lumped element technology and showed

Because of the ultra-wide operating band of these amplifiers, they are receiving much attention. In a general amplifier, using parallel transistors lead to increasing the gain which is caused by summation of trans-conductances. But increasing input and output capacitors cause decrease in cutoff frequency. So as is shown in figure (5a), it does not solve the problem, because the multiplication of gain and bandwidth almost remains constant. In a distributed amplifier, low or high cutoff frequencies will be modified by summation of transistors trans-condoctances and realization of additional LC transmission lines in the

One of the most important and commonly used parameters in antenna design is the radiation pattern in the space around antenna. By defining antenna radiation pattern1

radiation power in each direction and the direction which maximum power is emitted will

Since selected passive antenna is spiral antenna in this work, its radiation pattern is halfspace. So if interior and exterior radiuses be selected according to the active zone and frequency range, E-plane and H-plane radiation patterns will be uniform in all frequency bandwidth and have few variations. Changing design parameters has no effect on antenna polarization and it has always circular polarization. All simulations of this chapter are done

signals are the main differences between power amplifiers and the others.

the ability of these circuits to achieve high gains in a wide frequency band [18].

input/output sides. The result is illustrated in figure (5b) [18].

**7. Antenna design and simulation**

in ADS (Advanced Design System) software [19].

**6. Distributed amplifier**

input/output signal forms.

**6.1. Features**

be defined.

<sup>1</sup> Radiation Pattern

Transponders are circuits which can be activated by an external explorer system transmitting signal in predefined frequency. In this case, transponder will transmit a response signal to the interrogator. These small low-cost microwave transponders are used for noncontact identifications such as entry systems, toll collection, and inventory control.

Transceivers and millimeter-wave vehicle radars are some other applications which are used respectively for wireless local area networks (WLANs) and for intelligent cruise control.

Additionally AIA can be an ideal choose for designing compact transceivers and transponders for wireless applications. In this case the whole RF subsystem, including active circuit and antenna can be built on a single substrate [15].

Active antennas are categorized depending on the function of active circuit integrated with Them. The main functions of the devices in active antenna structures are generating and amplifying RF signals and frequency conversion. Based on previous discussion, the active antenna functions can be categorized into three types comprised of oscillator type, amplifier type and frequency type. This base unites can prepare possibility of more complicated functions by integrating with antenna such as transponders [16].

Some other benefits of using active antenna in microwave and millimeter wave frequencies are as below [17];


As discussed in the previous part, for UWB applications, there are some limitations in using spiral antenna for broadband applications and it must be optimized to exhibit desirable characteristics. Using active antenna technology is one of effective optimization methods which leads to reduction of return losses and improvement in bandwidth, gain and input impedance [8].

To design a UWB active antenna with desirable parameters, study and design of an appropriate active circuit is an important step. Therfore distributed amplifier structure and parameters are reviewed in the next part as the active part of active antenna.
