The design reference EBG antenna, shown in Figure 1, is configured with a superstrate based on a frequency selective surface (FSS) placed in front of a multilayer cylindrical dielectric resonator antenna (MCDRA), which acts as an excitation source. Table 1 summarizes the parameters of the proposed antenna.

Initially, a design method based on a detailed parametric study is presented. Two main points are described, the characterization of the appropriate excitation source as well as the development of the upper surface with the characteristics necessary to achieve the desired gain over a given bandwidth.

In order to obtain a compact EBG structure, thin and easy to manufacture while achieving an acceptable gain, a parametric study, using CST Studio Suite 2014, is performed on the geometric properties of the superstrate FSS.

Our choice for the excitation of the EBG antenna is a multilayer cylindrical dielectric resonator (Figure 5) that is recognized for broadband applications. Indeed, it is possible to stack several resonators so that each of them can resonate at a slightly different frequency and, therefore, the system generates a wider bandwidth.

The aim of this experiment is to show the increase in bandwidth when passing from the excitation of the BEG structure by a conventional cylindrical DRA to an excitation by a multilayer cylindrical DRA.

Figure 5(a) shows the homogenous cylindrical DRA geometry with a dielectric constant ε_r1 = 6.15 and a thickness H_tot = 0.508 mm. Figure 5(b) shows two element multilayer cylindrical DRA geometry with a dielectric constant ε_r1 = 6.15 and ε_r2 = 2.2 a

thickness h₁ = h₂ = 0.254 mm. Finally Figure 5(c) shows three element multilayer cylindrical DRA geometry with a dielectric constant ε_r1 = 6.15, ε_r2 = 2.2, ε_r3 = 9.8 and thickness h₁ = h₂ = h₃ = 0.254 mm.

Numerical results in Figure 6 show that the matching frequency band corresponding to the homogeneous cylindrical DRA is from 58.1 to 63 GHz, when VSWR ≤ 2, which is equivalent to a bandwidth of 8.1%. The matching band corresponding to two element multilayer cylindrical DRA is from 58.1 to 64.2 GHz which is equivalent to a bandwidth of 10.5%. For a three element multilayer cylindrical DRA, the matching band is from 57 to 66 GHz which is equivalent to a bandwidth of 15%.

The width of the adaptation band is therefore improved by 2.4% when changing the homogeneous cylindrical DRA by two elements MCDRA, and also improved by 4.5% when passing from two elements to three. It is clear that the bandwidth of the three elements MCDRA will be higher than that of the homogeneous cylindrical DRA.

After the nature and geometry of the upper interface of the EBG resonator and its source of excitation were chosen for the three elements multi-layer cylindrical DRA, the proposed antenna has been fabricated and measured. The final structure of the proposed EBG antenna is shown in Figure 7. For comparison and verification purposes, the simulations were generated by two different electromagnetic simulators (CST Studio Suite 2014 and Ansoft HFSS 13).

The EBG antenna’s measured and simulated reflection coefficient (S₁₁) is depicted in Figure 8. As it can be seen, two resonances are observed; the first is close to 60 GHz and the second is around 65 GHz. The impedance bandwidth is large enough to cover the entire ISM band (Industrial, Scientific, and Medical band), the measured matching band ranges from 57.5 to 66.5 GHz, which corresponds to an impedance bandwidth of 15% (S₁₁ < −10 dB). A good agreement is observed between the results of the numerical

simulations and the experimental ones. It is obvious that those results show a bandwidth that meets the design goal.

The antenna gain as a function of frequency is illustrated in Figure 9. The most appealing feature of this antenna resides in its high gain, around 18 dBi, with a difference of 1 dB between the lowest and the highest values. The minimum gain is about 17 dB at 65 GHz while the maximum gain is 18 dB at 59 GHz respectively. It can be observed that the gain is very stable over the frequency range of 58 GHz to 65 GHz. The radiation patterns measured and simulated in the E-plane at 60 GHz is shown in Figure 10. These results show that the diagrams of the proposed antenna have low side lobes and that the main lobe is very directive. It is found that the simulated radiation pattern is considered to be stable and in good agreement with these measured. The results correspond to the objectives set at the beginning to get a highly directional reference antenna.

In addition to the improvement of bandwidth necessary for the development of the EBG structures associated with the antennas, the challenge is to obtain reconfigurable structures. Currently, there is now a strong demand for antennas offering polarization diversity, that is to say switching between linear and circular polarization. In the previous section, a high EBG reference antenna excited with multilayer DRA has been designed to generate a wideband linear polarization. In this section, it will be demonstrated that by changing the multilayer cylindrical source by a pyramidal one and rotating it by θ = 45˚ (Figure 11), two modes having equal amplitudes and a 90˚ phase difference can be excited, resulting in a circular polarization. The geometry of the proposed EBG antenna is numerically optimized so that it will be able to generate the circular polarization. The parameters of the structure proposed in this section are summarized in Table 2.

The geometry of the proposed source of excitation DRA is optimized numerically so that the radiated fields are equal in amplitude and 90˚ out of phase. Figure 12 illustrates a parametric study of the AR (axial ratio) with different length to width ratios of

the multilayer pyramidal DRA. The optimum AR ratio is found when the length l to width w ratio is 1.9, corresponding to l₁ = 1.7 mm, w₁ = 0.89 mm, l₂ = 0.98 mm, w₂ = 0.51 mm, l₃ = 0.88 mm, and w₃ = 0.46 mm.

Simulation results such as a reflection coefficient S₁₁, gain, efficiency, radiation patterns, and axial ration for both linear and circular configurations shown here has been performed within CST Studio Suite 2014 and prove to be very attractive. Figure 13 shows the S₁₁ simulated reflection coefficient for both linear and circular structures. As expected, the bandwidth in both cases is extended over a wide band thus covering the ISM band. The simulated bandwidth is from 56.5 to 64.5 GHz, which is equivalent to an operating band of 13.3%.

Three resonance frequencies were discerned at 57, 60 and 64 GHz, for both configurations with a few hertz offset. It can be concluded that the effect of switching between the linear/circular polarizations, does not greatly affect the operation of the antenna, such as bandwidth and resonance frequencies.

The maximum gain simulated for both linear and circular configurations is shown in Figure 14. For an antenna operating in linear polarization, a maximum gain of 18.4 dBi is achieved and the radiation efficiency vary between 0.78 and 0.85 over the entire operating band, as shown in Figure 15. When circular polarization is considered, one obtains an identical gain to that of the linear polarization and the efficiency achieved becomes more stable, taking a value between 0.80 and 0.86. It can be observed that the evolution of the gain in function of the frequency is approximately the same for both linear and circular configurations, with a slight advantage for linear polarization.

Switching between the linear and circular configurations has no major influence on the radiation of the proposed antenna. Indeed, the main lobes of the radiation patterns in the E plane for both configurations are almost identical (see Figure 16). The radiation pattern of the circularly polarized antenna shows some asymmetry in the side lobes, this asymmetry is due to the asymmetrical structure caused by the rotation of the pyramidal DRA by θ = 45˚ (see Figure 11).

One can conclude that the proposed antenna switches between two linear circular polarizations, while maintaining stable radiation (Figure 17).

Figure 18 shows the axial ratio 3 dB bandwidth for the circularly polarized configuration. This allows to conclude that the effective bandwidth obtained for S₁₁ < −10 dB and the axial ratio < 3 dB when the antenna radiates in circular polarization varies from 59.2 to 64 GHz, therefore covering both targeted applications.