This chapter gives a literature review relating to the quasi-Yagi microstrip antenna design. The chapter starts by providing some general information about antenna such as radiation pattern, gain, bandwidth and others. Moreover, this chapter continued with some explanation on microstrip antenna. Here, several design formulas for quasi-Yagi microstrip antenna such calculating the effective dielectric constant, derivations for free space wavelength and guided wavelength are provided. Then, this chapter will continue with some explanation about reconfigurable antennas. At this point, multi type of reconfigurable antennas will be discussed. The advantages and disadvantages of the microstrip antennas and reconfigurable antennas are also provided in this chapter.
An antenna is a metallic conductor system capable of radiating and capturing electromagnetic energy (Tomasi, 2004). In other words the antenna is the transitional structure between free-space and a guiding device (Balanis, 2005). In addition, antennas are resonant devices which can operate efficiently over a frequency band. Each antenna in a same network should be tuned to the same operates frequency band. This is because to ensure a transmission link can be made between transmitter and receiver. Otherwise, transmission or reception process will be terminated.
There are various parameters which can be used to characterize the performance of an antenna. Some of the parameters are correspond to the other parameters. However, not all of the parameters are needed to be clarified in order to describe the performance of an antenna. Several important antenna parameters such radiation pattern, gain, directivity, beamdwidth, bandwidth and polarization will be explained below.
Radiation Pattern
A radiation pattern is a polar diagram or graph representing field strengths or power densities at various angular positions relative to an antenna (Tomasi, 2004). In other words, it is a plot of field strength at a fixed distance from an antenna as a function of angle, normally in the far field region of an antenna (Zaiki, 2006). There are two types of plots can be used to illustrate the radiation pattern of an antenna. The plots are polar and rectangular plots. However, the polar plot as shown in figure below is more commonly used in the modern wireless communication systems.
Figure 1: Polar plot of antenna's radiation pattern
Source: Stutzman & Thiele, 1981
Figure 2: Rectangular plot of antenna's radiation pattern
Source: Stutzman & Thiele, 1981
Gain
Gain of an antenna is defined as the ratio intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically (Balanis, 2005). It is one of important measurement which can be used to describe the performance of the measured antenna. In fact, the gain of an antenna is closely related to its directivity which can affect the efficiency of the antenna. There are a lot of equations can be used to calculate the gain of antenna. One of the available equations is:
(2.1)
where;
= efficiency of the antenna
D = directivity of the antenna
Directivity
Directivity is defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions (Balanis, 2005). The directivity of an antenna is relative, and is normally compared to an isotropic antenna, since the latter radiates equally in all directions (Zaiki, 2006). The average radiation intensity is equal to the total power radiated by the antenna divided by 4. If the direction is not specified, the direction of maximum radiation intensity is implied. In mathematical form, it can be described as:
(2.2)
where;
D = directivity
U = radiation intensity (W/unit solid angle)
UO = radiation intensity of isotropic source (W/unit solid angle)
Prad = total radiated power (W)
There are two pattern types of antenna directivity available in current wireless communication system which are omnidirectional and directional antennas. Omnidirectional antenna radiates and receives equally well in all horizontal directions. It is defined as one having an essentially nondirectional pattern in a given plane and a directional pattern in any orthogonal plane (Balanis, 2005). While, directional antenna will radiates or receive signals in a specific or particular direction. A directional antenna is one having the property of radiating or receiving electromagnetic waves more effectively in some directions than in others (Balanis, 2005). Directional antennas are used in some base station applications where coverage over a sector by separate antennas is desired.
Beamwidth and Bandwidth
Beamwidth and bandwidth can be measured from the obtained radiation pattern of an antenna. These two parameters are measured at half power (-3dB) operation which show the ability of the antenna. Antenna beamwidth specifies the width of the main lobe of an antenna. Generally, the half power beamwidth of an antenna is defined as the angular separation of two half power points on the main lobe (Zaiki, 2006). In addition, the beamwidth of the antenna is also used to describe the resolution capabilities of the antenna to distinguish between two adjacent radiating sources or radar targets (Balanis, 2005). Figure 3 below shows the half power beamwidth measured at the radiation pattern.
Figure 3: Three and two dimensional radiation pattern show the beamwidth
Source: Balanis, 2005
Meanwhile, the bandwidth of an antenna is refers to the frequency range over which the antenna operation is satisfied (Tomasi, 2004). Normally, the bandwidth is taken as the difference between the highest and lowest frequencies of the radiated frequency. Moreover, bandwidth can be expressed as either in percentage or hertz.
Polarization
Polarization of an antenna is referring to the direction of the electric field radiated from an antenna. In fact, the polarization of an antenna is the polarization of the wave radiated by the antenna in a given direction (Stutzman & Thiele, 1981).
There are three types of polarizations which are linear, elliptical and circular. Linear polarization will occurred when the electric field at any point in the beam and at any point of time is directed in one direction only (Zaiki, 2006). In addition, it can be further divided into vertical or horizontal polarization. Next, elliptical polarization is the combination of two linearly polarized waves of the same frequency but different magnitudes and phases. Lastly, circular polarization is the combination of two signals of elliptical polarization with same magnitude and 90° phase difference. This is means the signals are perpendicular to each other. Circular polarization only occurs to a certain types of antenna such helical antenna.
MICROSTRIP ANTENNAS
Microstrip antennas are the current sophisticated antenna technology in wireless communication systems. It is made of a special substrate material to reduce losses exhibited by high frequency signals when ordinary printed circuit board (PCB) is used (Zaiki, 2006). In actual fact, the microstrip antennas can now be considered as an attractive solution to many wireless communication scenarios due to their low-cost, low-profile, conformable and easy-to-manufacture architecture (Wayne S.T. Rowe, 2007). Beside that, these microstrip antennas are confidently used by antenna developers worldwide, especially when low-profile radiators are required.
Historical Development
The concept of microstrip antennas was first proposed by Grieg and Englemann (Grieg & Englemann, 1952) as early as 1952, Deschamps (Deschamps, 1953) in 1953. However, not much persistent researches have been carried out until 1975. Since, it took about twenty years before the initial practical microstrip antennas were fabricated in the 1970's by John Q. Howell (Howell, 1975). In his research, Howell has fabricated several rectangular patch microstrip antenna which can be used up to Ku band frequency. Beside that, he has presented the design procedures for microstrip antennas in this paper.
Subsequently, in 1977 another development in microstrip antennas has been carried out by Chatterjee, Ganesan and Nethaji (Chatterjee et al., 1977). They have done an experimental work on the radiation characteristics of microstrip antennas at microwave frequencies. The radiation patterns and gains of the antennas have been calculated and verified by experimental session in the X band frequency. In addition, research publications regarding the development of microstrip antennas were also published in a lot of organizations such as in Institute of Electrical & Electronics Engineers (IEEE) and Institute of Electrical Engineers (IEE). In fact, all these publications are still in use today.
In 1979, P.S. Hall, C. Wood and C. Garret have reported the design idea of electromagnetically coupled patch antenna which is able to acquire higher bandwidth while maintaining a simple fabrication process (Hall et al., 1979). They also have proved it by experimental practice.
The early 1980's was not only a focal point in publications but also a milestone in practical realism and ultimately manufacturing the microstrip antennas (James & Hall, 1979). In this era, many substrate manufacturers have improved the substrate capability in order to operate under extreme environment conditions. By increasing the substrate's capability, it will turn out the increasing of substrate's manufacturing costs.
Recent demands of wireless communication systems such wider bandwidth and circular polarizations (Haneishi & Suzuki, 1979) are the dominant factor in developing the microstrip antenna. Beside that, many current antenna researchers are begun to take an interest in 'array architecture', which has become as a foremost approach to the microstrip antennas industry. This is because the designed array antenna has significant advantages in terms of bandwidth and efficiency compare to single patch antenna (Timofeev et al., 1997).
Basic Microstrip Antennas
Modern developed antennas in the wireless communication systems are established base on size, weight, cost, performance, ease of installation and aerodynamic profile factors. Moreover, the low-profile microstrip antennas are preferred over conventional antennas.
The term microstrip is refers to any type of open wave guiding structure which is not only a transmission line but also used together with other circuit components like filters, couplers, resonators and others. In fact, microstrip antennas are an extension of the microstrip transmission line.
A microstrip antenna in its simplest configuration consists of a radiating patch on one side of a dielectric substrate, which has a ground plane on the other side. The patch conductors, usually made of copper or gold, can be virtually assumed to be of any shape. The radiating patch may be square, rectangular, circular, ellipse or any other shapes as shown in Figure 4 below. However, conventional shapes are normally used to simplify analysis and performance prediction. The radiating elements and the feed lines are usually photoetched on the dielectric substrate (Balanis, 2005).
Figure 4: Basic microstrip patch antenna shapes commonly used in practice
Source: Garg et al., 2001
Quasi-Yagi Microstrip Antenna Design Formulas
There are several different ways of designing microstrip antennas. The basic components of quasi-Yagi microstrip antenna are driven element, director and reflector. Hence, in order to design this quasi-Yagi microstrip antenna, there are several related equations that should be used. First equation is to calculate the length of driven element, Ldriven,
(2.3)
where;
Ldriven = driven element length
ï¬g = guided wavelength
(2.4)
where;
ï¬g = guided wavelength
ï¬o = frequency wavelength
ï¥reff = effective dielectric constant
(2.5)
where;
ï¥reff = effective dielectric constant
ï¥r = dielectric constant
= width-to-height ratio
The width-to-height (w/h) ratio is a fundamental function of characteristic impedance, Zo and dielectric constant, ï¥r. normally, the characteristic impedance is equal to 50 ohm. In addition, the characteristic impedance of a microstrip transmission line is also related to its width (Garg et al., 2001). As for the length of the line, it does not have much significance on the impedance characteristics. According to several microstrip handbooks, the ratio of width-to-height can be calculated by using following equations:
(2.6)
where;
(2.7)
Normally, the length of director, is 5% shorter than driven element. Next, the spacing between director and driven element, S can be calculated by using following equation:
(2.8)
In order to bring in the signal into the antenna, it is required for an antenna to have a port. Generally, it is possible to have matching characteristic impedance, (Zo=50ï-) between port and feeding line. Hence, the width of feeding line can be calculated by:
(2.9)
where;
w = width of feeding line
h = height of used substrate
Zo = characteristic impedance
ï¥r = dielectric constant
Next, the feeding line cannot be directly connected to the driven element. This is due to mismatch impedance between these two lines. Normally, a folded dipole driven element has impedance around 300ï-. Thus, this microstrip antenna needs a quarterwave transformer to match the impedance between feeding line and driven element. The following equations can be used to calculate the dimensions of quarter wave transformer:
(2.10)
where;
Z1 = Quarterwave transformer impedance
Advantages and Disadvantages of Microstrip Antennas
The attractiveness of the microstrip antenna is the idea of making use of printed circuit technology. Hence, it is able to have all the advantages of a printed circuit board with all of the power dividers, matching networks, phasing circuits and radiators. Moreover, microstrip antennas have several advantages compared to conventional microwave antennas. Some of the advantages of the microstrip antennas (Garg et al., 2001):
Light weight, low volume, and low profile planar configurations which can be made conformal.
Low fabrication cost, suitable for mass production.
Linear and circular polarizations are possible with simple feed.
Dual-frequency and dual-polarization antennas can be easily made
No cavity backing is required.
Feed lines and matching networks can be fabricated simultaneously with the antenna structure.
Compatible with modular designs (solid state devices such as oscillators, amplifiers, variable attenuators, switches, modulators, mixers, phase shifters and others) which can be added directly to the antenna substrate board.
However, the disadvantages of the microstrip antennas compared to conventional antennas are:
Narrow bandwidth and associated tolerance problems.
Lower gain.
Large ohmic loss in the feed structure of arrays.
Poor end-fire radiation performance except tapered slot antennas.
Poor isolation between feed lines and radiating elements.
Possibility of excitation of surface waves.
Lower power handling capability.
Complex feed structures required for high performance arrays.
There are methods which can reduce the effect of some of the disadvantages mentioned above. For example, efficiency and bandwidth of microstrip antennas can be improved by increasing the height of the substrate (Balanis, 2005).
RECONFIGURABLE ANTENNAS
Reconfigure means rearrange the elements or settings of a certain object. Thus, reconfigurable antenna can be defined as the structure of antenna can be rearranged by using certain methods or techniques to produce difference values of same parameter such as frequency and beam shaping (directivity). Currently, there are many methods are available in the wireless telecommunication system in order to design a reconfigurable antenna. For example, by using some diodes which act like flexible switches to vary the length of driven element. Hence, by using only one frequency reconfigurable antenna can cover several frequency bands, which would traditionally demand a dedicated antenna for each band or a multiband antenna with costly RF-front end (Songnan Yan et al., 2009). Following explanations are some examples of the reconfigurable antenna which have produced by researchers before.
In August 2010, Pei-Yuan Qin, Andrew R. Weily, Y. Jay Guo, Trevor S. Bird and Chang-Hong Liang have presented a frequency reconfigurable planar quasi-Yagi antenna with a folded dipole driver element. Figure 5 and Figure 6 below show the structure of the presented antenna. In their paper, it consists of two antenna designs. The difference between these two antennas is the first antenna is used PIN diodes, while the second antenna is used varactor diodes. Normally, PIN diodes are used to rectify the voltage, while the varactor diodes are used as voltage-controlled capacitor rather than rectify the voltage (Pei-Yuan Qin et al., 2010). However, all these diodes are used as switches to vary the effective electrical length of the folded dipole driven element. By varying the length of the driven element, these antennas can provide or can be used at various frequency bands. In addition, it shows the effect of different switches material (diode) to the antenna performance.
Figure 5: Structure of the quasi-Yagi folded dipole antenna
Source: Pei-Yuan Qin et al., 2010
Figure 6: Orientation of diode in folded dipole
Source: Pei-Yuan Qin et al., 2010
Beside that, there is another method to design a frequency reconfigurable antenna. In July 2010, Tawk, Constantine and Christodoulou have done a study on a frequency reconfigurable antenna with a rotatable patch. The reconfigurability is achieved via a rotational motion of a part of the antenna patch (Tawk et al., 2010). The rotating part has the form of a circle and contains four different shapes. Each shape corresponds to a different antenna structure. With every rotation, a different antenna structure is fed in order to produce a different set of resonant frequencies. Figure 7 shows the prototype of this antenna.
Figure 7: Structure of frequency reconfigurable rotatable antenna
Source: Tawk et al., 2010
Instead of having different patch size and shape, a frequency reconfigurable antenna can be design by using mechanical patch moving. This technique has been used by Hu Ajun Chen, Zhiyuan Shi, Liping Wu and Donghui Guo who have presented a frequency reconfigurable antenna with micromechanical patch moving via electrostatic force in April 2007. Figure 8 and Figure 9 below show the prototypes of this antenna. As the bias voltage between patch and antenna is changed, it makes the patch to move and the antenna operating frequency changes too (Hu Ajun Chen et al., 2007). Its operating mode is equivalent to changing the relatively permittivity of antenna's substrate.
Figure 8: Structure of the micromechanical patch antenna without bias voltage
Source: Hu Ajun Chen et al., 2007
Figure 9: Structure of the micromechanical patch antenna with bias voltage
Source: Hu Ajun Chen et al., 2007
Beside frequency reconfigurable, antenna polarization also can be reconfigured in microstrip antenna. For example, a research on a reconfigurable microstrip antenna for switchable polarization has been conducted by Y.J. Sung, T.U Jang and Y.S. Kim in 2004. In this study, the designed antenna consist of a corner-truncated square radiating patch, four small triangular conductors, and a microstrip line feed as shown in Figure 10 below. Then, by using independently biased PIN diodes on the patch, it can produce linear polarization, left-hand or right-hand circular polarization according to bias voltages (Sung et al., 2004). Polarization reconfigurable microstrip antenna has a advantage which it can provide diversity features which leads to an increased signal to noise ratio (SNR) and therefore a higher quality of service of the whole systems (Dietrich et al., 2001).
Figure 10: Configuration of the corner-truncated square microstrip antenna with switchable polarization
Source: Sung et al., 2004
Furthermore, in September 2009 T. Debogovic and J. Bartolic have presented a pattern reconfigurable compact antenna. In the presented paper, they have proposed capacitively fed annular ring microstrip antenna. However, in order to archive radiation pattern reconfigurability the antenna patch has been shorted via shorting pins on its edges using PIN diode (Debogovic & Bartolic, 2009). Depending on the state of the PIN diodes (D1 and D2), the antenna operates in normal patch mode (switches in OFF state) or unipolar wire-patch mode (switches in ON state). Figure 11 below shows the antenna layout of the presented antenna, while Figure 12 and Figure 13 show the obtained radiation pattern. In addition, the radiation pattern reconfigurable microstrip antenna can avoid noise from a known direction by redirecting the null position or provide a larger coverage by redirecting the main beam direction (Debogovic & Bartolic, 2009).
Figure 11: The antenna layout (front and rear view)
Source: Debogovic & Bartolic, 2009
Figure 12: Radiation pattern during normal patch mode (switches in OFF state)
Source: Debogovic & Bartolic, 2009
Figure 13: Radiation pattern during unipolar wire-patch mode (switches in ON state)
Source: Debogovic & Bartolic, 2009
