Antenna is one of the important elements in the RF system for receiving or transmitting signals from and into the air as medium. Antenna is designed for certain frequency band. Beyond the operating band, the antenna rejects the signal. In recent years, Dielectric resonator antenna (DRA) and microstrip patched antenna (MPA) was paid attention by many researchers and industry. These two classes of antenna are highly suitable for the development of modern wireless communications. Many characteristics of the DRA and microstrip antena are common because both of them behave like resonant cavities. For example, since the dielectric wavelength is smaller than the free-space wavelength by a factor of 1/√𝜀𝑟 , both of them can be made smaller in size by increasing εr. Moreover, virtually all excitation methods applicable to the microstrip antenna can be used for the DRA. As compared to the microstrip antenna, the DRA has a much wider impedance bandwidth. This is because the microstrip antenna radiates only through two narrow radiation slots, whereas the DRA radiates through the whole DRA surface except the grounded part. Avoidance of surface waves is another attractive advantage of the DRA over the microstrip antenna.
For Dielectric resonator (DR), oscillators and filters are most widely used
application. It regards DR as an energy storage device rather than as a radiator. For millimeter and near-millimeter range (100-300 GHz), DRA exhibit extremely low loss while conductor loss of metallic antennas becomes severe and the efficiency of the antennas is reduced significantly. On the other hand, the only loss for a DRA is that due to the imperfect dielectric material, which can be very small in practice. The first widely-applied dielectric resonator antenna was published in 19832. There are many advantage for DRA, such as high radiation efficiency, light weight, and versatility in their shape and feeding mechanisms. In addition, Long and his colleagues found the applicability of the rectangular3 and hemispherical4 DRAs. The work created the foundation for future investigations of the DRA. Other shapes were also studied, including the triangular 5, spherical-cap 6, and cylindrical-ring 7-8 DRAs.
Rectangle antenna is one of the most versatile antenna since it has two degrees of freedom. For any given resonant frequency and fixed dielectric constant, two of the three dimensions of the rectangular DRA can be chosen independently. (The cylindrical DRA has one degree of freedom while the hemispherical DRA has none), as shown in Fig. 4-1.
The electromagnetic problems of rectangular DRAs are so complex that many of them cannot obtain the analytical solutions. Therefore, various numerical technologies were developed. In addition, in order to simply the problems, many approximations
and models were developed to estimate the resonant frequency, Q- factor, and radiation patterns. There are two group of analytic methods for retangular DRAs. First group are based on equivalent magnetic dipole, such as Transmission Line model (TLM), and Dielectric Waveguide Model (DWM); The other group are based on the electric current distribution on patch conductor and the ground plane, sush as the mthod of monents (MoM), the finite-element method (FEM), etc. Usually, dielectric waveguide model (DWM) is used to analyze the retangular DRAs.
Fig. 4-1 Geometry for the dielectric resonator antenna model.
As a DRA, the field excitation properties affect the effciency of antenna. Many coupling feeds, such as aperatures, microstrip, probes and coplanar, would influence not only impedance and radiation characteristics but different excited modes within DRAs. Coupling with microstrip is better than that coupling with aperature due to low cost and easy frabrication. A microstrip line can couple the magnetic field of the DRA modes, which are accorded by electrical boundary conditions. Considering dielectric waveguide model (DWM) in Fig. 4-1 (c), the top surface and two sidewall of DRA are assumed to be perfect magnetic wall, while two other sidewalls are imperfect magnetic wall. Furthermore, the bottom surface is assume electric wall due to being mounted on ground plane. Although isolated retangular DRA can exicite TE and TM
modes, TE modes are excited only in DRA mounted on a ground plane. 𝑇𝐸111𝑋 , 𝑇𝐸111𝑦 , 𝑇𝐸111𝑍 modes are excited as resonatant frequencies are defined as
𝑓0 = 𝑐
2𝜋√𝜀𝑟𝜇𝑟√𝑘𝑥2+ 𝑘𝑦2 + 𝑘𝑧2 (4.1.1)
through solving transcendental equation:
𝑘𝑧tan (𝑘𝑧2𝑎) = √(𝜀𝑟− 1)𝑘02− 𝑘𝑧2 (4.1.2)
where 𝑘𝑥 =𝜋𝑎, 𝑘𝑦 = 𝜋𝑏, 𝑘𝑥= 𝜔𝑐0 =2𝜋𝑓0√𝜀𝑐 𝑟𝜇𝑟 and 𝑘𝑥2+ 𝑘𝑦2+ 𝑘𝑧2 = 𝑘02
It is more complex problem for Q factor. The radiation efficiency and impedence bandwidth are affected by losses and permittivity of dielectrics. Except loss orgined from materials, the coupling between microstrip and DRA is one dominant factor for
losses.
There are several properties that need to be concerned for a antenna system[9-10]: Return loss, bandwith, efficiency, and radiation pattern.
Fig. 4-2 Antenna input impedance model
Fig. 4-2 demonstrates the simple model of an antenna system. The reflection coefficient is a complex number, which is related to loading impedance (𝑍𝐿) and
charateristic impedance is defined as the ratio of voltage to current at its terminal. The
return loss are related to reflection coefficient.
Γ =𝑍𝑍𝐿−𝑍0
𝐿+𝑍0 (4.1.3)
return loss = −20 log(|Γ|) = −20 log (|𝑍𝑍𝐿−𝑍0
𝐿+𝑍0|) (𝑑𝐵) (4.1.4)
When Γ =0, no loading impendance lead to the return loss -∞. But when Γ
=1, all signal rebounded and impedance to infinitity, thus the return loss 0 dB. The commonly required specification of an antenna is that return loss is larger than 10 dB, for mobile phone antenna, it is common to specify that return loss < 6 dB.
The antenna gain is defined as the ratio of the radiation intensity in a given
direction from antenna to the total input power accepted by the antenna divided by 4π.
𝐺 =4𝜋𝑈𝑃
𝑖𝑛 (4.1.5)
Z0 ZL
where U is the radiation intersity in solid angle and 𝑃𝑖𝑛 is the total input power accepted by the antenna.
Second, the total efficiency of antenna system is the product of two efficiecies:
radiation efficiency (𝜂𝑒) and matching efficiency (𝜂𝑚). The 𝜂𝑒 is calculated by
radiated power and input power accepted by antenna; the 𝜂𝑚is defined as the ratio of the input power accepted by the antenna to the source supplied power. Thus, the total
effciency can be estimated.
𝜂𝑒 =𝑃𝑃𝑡
𝑖𝑛 ; 𝜂𝑚 = 𝑃𝑃𝑖𝑛
𝑠 ; 𝜂𝑡 = 𝜂𝑒𝜂𝑚 = 𝑃𝑃𝑡
𝑠 (4.1.6)
There are three type of radiation patterns: istropic, omni-directional and directional. antenna are three important type of antenna which are classified by their radiation patterns. The istropic antenna is ideal antenna, but most of antenna reveal the omni-directional and directional patterns.
Ususally, to design compact DRAs, which are applied in handsets or wireless tablet, there are several technologies to make DRAs more compact. Except using high dielectric permitivity enable miniaturize DRAs, adding metallic plate is one of them.
A perfect metallic wall implies that electric fields are normal and magnetic fields are tangential to this metallic plate. bandwidth and resonant frequency decrease after adding metallization of one surface.
In this study, the targeted resonant frequency region site on 2.4-2.484 GHz,
which are defined as Industrial Scientific Medical Band (ISM) by Federal Communications Committee (USA). ISM band are widely applied in wireless comuncation, such as bluetooth, WLAN (802.11),