**2.2. Common DRAs feedings**

28 Dielectric Material

rectangular.

antenna size reduction.

trade-off between the material choice and its shape.

kind of mobile handheld devices (e.g. new tablets).

**2. Overview on DRA studies** 

**2.1. Main DRAs characteristics** 

Multiple techniques to miniaturize such an antenna will be presented in the third part, supported by concrete examples. At the same time, everyone will be able to appreciate that dielectric material properties have a major role to play in designing a DRA. It should be noted that the material choice is even more critical when the targeted challenge is the

Therefore, depending on the intended applications, this part will enable to find the best

Although some wideband or multiband DRA structures have been introduced in the third part, the fourth and last part will be dedicated to a new method to design a DRA. It will address engineering design data on hybrid modes creation to enhance the bandwidth or develop multiband antennas. This part will include many references to clearly explain this research method while highlighting their contribution to expand the use of DRA in new

The design of a DRA in any geometry must satisfy various specifications including: the resonant frequency, the impedance bandwidth, the field distribution inside the resonator and also the radiated field. The intent of this part is to provide an understanding of fundamental operation of DRAs, emphasizing both design and implementation. Thus, to provide comprehensive research method, this part will start by presenting main findings of investigations on simple-shaped DRAs. Then, it will deal with the different DRA feeding methods. Finally, this part will focus on the study of two DRA shapes: cylindrical and

A non-exhaustive list of main simple-shaped DRAs characteristics is described below:

magnetic constant of the material. In a dielectric material case, 1 *<sup>r</sup>*

of antennas because of minimal conductor losses associated with a DRA.

 and *<sup>r</sup>* 

 The radiation efficiency of the DRA is highly depending on the material losses. In case of a low-loss dielectric material, DRAs allow to achieve better efficiency than other kind

 For a given dielectric constant, both resonant frequency and radiated Q-factor are defined according to the resonator dimensions. That allows having a great flexibility

 Another degree of freedom is the large spectrum of available dielectric materials. That allows doing the best trade-off between dimensions and impedance bandwidth

.  

 where <sup>0</sup> 

are respectively the dielectric and the

is the free-space

and the main

The main dimension of a DRA is proportional to <sup>0</sup> / .*r r*

and some degrees of freedom to design such an antenna.

wavelength at the resonant frequency, *<sup>r</sup>*

according to the intended application.

dimension of a DRA is proportional to <sup>0</sup> / *<sup>r</sup>*

Multiple feeding mechanisms are employed to excite different resonator modes. This subsection will summarise most widely-used excitations while giving many references in order designers to choose the most appropriate excitation.

## *2.2.1. Coaxial probe excitation*

It can be located within the DRA or adjacent to it. Within the DRA, a good coupling can be achieved by aligning the probe along the electric field of the DRA mode as shown Figure 1.

**Figure 1.** Coaxial probe coupling the E field

The adjacent position is currently used to couple the magnetic field of the DRA mode (Figure 2). In these both cases, the probe is exciting the TE111 fundamental mode of the rectangular DRA.

**Figure 2.** Coaxial probe coupling the H field

#### 30 Dielectric Material

When the excitation probe is inside the resonator, particular attention has to be paid to the air gap between the probe excitation and the dielectric material. Indeed, an air gap results in a lower effective dielectric constant, which entails both a decrease in the Q-factor and a shift of the resonance frequencies [23-24]. The probe location allows choosing the intended excited mode and the coupling of the mode can be optimized by adjusting both length and height of the probe.

Dielectric Materials for Compact Dielectric Resonator Antenna Applications 31

Microstrip line

DRA

Feedingaperture

strong magnetic area. Feeding the aperture with a microstrip line is a current approach [25-

Feedingaperture

The main dimension of the aperture needs to be around λg/2, which is highly problematic at

On top of these multiple feeding methods, the choice of different DRAs shapes represents another degree of flexibility and versatility. The next subsection will deal with the

It offers great design flexibility, where both resonant frequency and Q-factor are depending on the ratio of radius/height. Various modes can be excited within the DRA and a significant amount of literature is devoted to their field configurations, resonant frequencies and radiation properties [27-28]. This part will present a complete and concrete study of a

Like most realistic cases, the cylindrical DRA presented Figure 5, is mounted on a finite ground plane. Because dielectric material properties will be studied in very great depth in the third part, the dielectric permittivity εr of the DRA is fixed and chosen equal to 30. It is also characterized by its height d and its radius a. To keep the chapter concise while

d

Ground plane

a

remaining comprehensive, only relevant results and equations will be given.

DRA

Metallization

DRA Metallization

Microstrip line

**Figure 4.** Aperture coupling the TE111 mode of the rectangular DRA

26].

low frequencies.

cylindrical shape.

cylindrical DRA.

**Figure 5.** Cylindrical DRA

**2.3. The cylindrical DRA** 
