**3.2. Influence of dielectric material losses**

Dielectric material losses directly impact the impedance bandwidth and antenna radiation efficiency. Their influence depends on the dielectric permittivity of the material.

Because analytical considerations do not take into account losses, only electromagnetic approaches are available to complete this study. To be more readable and relevant, charts will be provided to present and take into account all effects of losses.

The chosen example is still the same: the cylindrical DRA mounted on a ground plane with a radius and a height respectively equal to 40 mm and 45 mm and excited on its fundamental mode. The study of losses is done for different dielectric permittivity values.

The impedance bandwidth is studied as a first step. The Figure 14 presents the impedance bandwidth according to both dielectric permittivity and tangent loss of the dielectric material.

Several information have to be noted:

The most important loss tangents are, the widest the impedance bandwidth is. The impedance bandwidth is the widest when for εr=10, whatever the losses.

The radiation efficiency is now investigated. Same simulations are done and the Figure 15 presents the antenna radiation efficiency according to both dielectric permittivity and tangent loss of the dielectric material.

Dielectric Materials for Compact Dielectric Resonator Antenna Applications 39

**Figure 14.** Impedance bandwidth according to the dielectric permittivity and loss tangent

**Figure 15.** Radiation efficiency according to the dielectric permittivity and loss tangent

Other information can be deduced from this new graph:

38 Dielectric Material

characteristics.

material.

under 10. For a dielectric permittivity over 10, the Q factor is increasing and therefore the

**<sup>0</sup> <sup>20</sup> <sup>40</sup> <sup>60</sup> <sup>80</sup> <sup>0</sup>**

**0**

**2**

**4**

**6**

**Impedance bandwidth (%)**

**8**

**10**

 **r**

Now that the influence of the dielectric permittivity has been shown, we can consider in the next sub-section more realistic cases by studying the impact of losses on the antenna

Dielectric material losses directly impact the impedance bandwidth and antenna radiation

Because analytical considerations do not take into account losses, only electromagnetic approaches are available to complete this study. To be more readable and relevant, charts

The chosen example is still the same: the cylindrical DRA mounted on a ground plane with a radius and a height respectively equal to 40 mm and 45 mm and excited on its fundamental mode. The study of losses is done for different dielectric permittivity values.

The impedance bandwidth is studied as a first step. The Figure 14 presents the impedance bandwidth according to both dielectric permittivity and tangent loss of the dielectric

The radiation efficiency is now investigated. Same simulations are done and the Figure 15 presents the antenna radiation efficiency according to both dielectric permittivity and

efficiency. Their influence depends on the dielectric permittivity of the material.

The most important loss tangents are, the widest the impedance bandwidth is. The impedance bandwidth is the widest when for εr=10, whatever the losses.

will be provided to present and take into account all effects of losses.

**Figure 13.** Resonant frequency according the dielectric permittivity values εr

impedance bandwidth is decreasing (see equation 5).

**0.4**

**3.2. Influence of dielectric material losses** 

Several information have to be noted:

tangent loss of the dielectric material.

**0.8**

**1.2**

**Resonant frequency (GHz)**

**1.6**

**2**


To conclude this part, a DRA designer has to choose the dielectric material according to the application for which he is aiming. If he targets a wide bandwidth application, he could choose an alumina ceramic (εr ~10). Depending on the radiation efficiency he aims, the chosen ceramic would have more or less losses.

Now, if he targets an ultra-miniature DRA, it will be in his interest to choose a dielectric material with a higher dielectric permittivity. In this case, the impedance bandwidth will be affected, even more if the losses are high.

The dielectric material choice is one of the most important degree of freedom in the DRA design. It is necessary to highlight the best tradeoff, keeping in mind the targeted application.

Dielectric Materials for Compact Dielectric Resonator Antenna Applications 41

)

Substrat (ε<sup>s</sup>

)

Thus, the metal plate insertion allows dividing by two the DRA size, while reducing the resonant frequency. However, as pointed by the Table 1, the metallic plate insertion involves

Another way to decrease the DRA size is to insert different substrate layers as illustrated

d

It allows achieving strong coupling when the first insertion has a relatively high dielectric permittivity. This technique is detailed in [12] and [31]. The Table 2 summarizes a parametrical study done in [31] for one layer inserted (Figure 17) with w=7.875 mm, d=2 mm, h=3.175 and εr=10. It is mounted on a 0.762 mm height substrate of permittivity εs=3.

t (mm) εi Measured f0(GHz) Bandwidth 0 - 15.2 21% 0.25 20 14.7 18% 0.635 20 14.5 18% 1 20 13.9 16% 0.25 40 14.7 20% 0.635 40 13.7 13% 1 40 12.9 5% 0.25 100 14.7 16% 0.635 100 13.1 7% 1 100 10.8 5%

Thus, a thin layer insertion allows improving the coupling of modes inside the DRA while decreasing the resonant frequency thanks to the decrease of the effective dielectric permittivity of the DRA. As the previous technique, the downside is the decrease of the

h t

) Insertion (ε<sup>i</sup>

w

Microstrip line DRA (ε<sup>r</sup>

The TE111 mode of the DRA is excited with a 50Ω microstrop line.

**Table 2.** A parametrical study done in [31] for one layer inserted

also the decrease of the impedance bandwidth.

**4.2. Multisegment DRA** 

**Figure 17.** Multisegment DRA

impedance bandwidth.

Figure 17.

Thus, using high dielectric permittivity values is one method for achieving a compact design, but it is not the only one. The next section will deal with different techniques to miniaturize a DRA.
