II. ANTENNA STRUCTURE AND DESIGN

The body of fast moving vehicles is covered with dielectric material for protection against different environmental conditions. The antennas used in such vehicles are covered with radome for protection. Generally, the radome is placed sufficiently away from the radiating aperture of the antenna. However, the provision of sufficient space between antenna and radome is not possible for aforesaid objects due to limitations on the available space. Hence, the radome is placed close to the antenna. To investigate the effects of radome on the performance of antenna, the radome is placed in close proximity to rectangular patch antenna. Furthermore, the patch antenna is surrounded by the dielectric coating to explore its effects on the radiation characteristics of antenna. The patch antenna is designed on the RT/Duroid 5880 substrate at 2.8 GHz using electromagnetic simulator high frequency structure simulator (HFSS) as shown in Fig. 1. The length and width of the patch are L and W, respectively. The thickness of the substrate is h and the dielectric constant is ε _r. The patch antenna is fed by the coaxial probe and backed by the substrate-integrated cavity made of metallic vias. Since the dimensions of the patch are finite, the fields at the edges of the patch undergo fringing. The effective dielectric constant that takes into account fringing is determined [Reference Balanis30];

(1)$$\varepsilon _{reff}=\displaystyle{{\varepsilon _r+1} \over 2}+\displaystyle{{\varepsilon _r - 1} \over 2}\left({1+\displaystyle{{12h} \over W}} \right)^{\displaystyle{{ - 1} \over 2}}\comma \; \quad \hbox{for}\; \displaystyle{W \over h}\gt 1.$$

Due to fringing, the patch of microstrip antenna looks greater electrically than its physical dimensions. The extension in the length ΔL is determined as:

(2)$$\displaystyle{{\Delta L} \over h}=0.412\displaystyle{{\lpar \varepsilon _{reff}+0.3\rpar \lpar {[ {W/h}] +0.264} \rpar } \over {\lpar \varepsilon _{reff} - 0.258\rpar \lpar {[ {W/h}] +0.8} \rpar }}.$$

Fig. 1.

SIW cavity-backed antenna (a) side view and (b) top view.

The effective length of the patch becomes (L = λ/2 for the dominant TM₀₁₀ mode with no fringing):

(3)$$L_{eff}=L+2\Delta L.$$

For the dominant TM₀₁₀ mode, the resonant frequency of the microstrip antenna is determined as:

(4)$$\lpar f_r \rpar _{010}=\displaystyle{1 \over {2L_{eff} \sqrt {\varepsilon _{reff} } \sqrt {\varepsilon _0 \mu _0 } }}=\displaystyle{1 \over {2\lpar L+2\Delta L\rpar \sqrt {\varepsilon _{reff} } \sqrt {\varepsilon _0 \mu _0 } }}.$$

For an efficient radiator, the width of the microstrip patch antenna is determined as:

(5)$$W=\displaystyle{1 \over {2f_r \sqrt {\varepsilon _0 \mu _0 } }}\sqrt {\displaystyle{2 \over {\varepsilon _r+1}}}=\displaystyle{c \over {2f_r }}\sqrt {\displaystyle{2 \over {\varepsilon _r+1}}} .$$

where c is the velocity of light in free space.

The actual length of the patch is determined as:

(6)$$L=\displaystyle{1 \over {2f_r \sqrt {\varepsilon _{reff} } \sqrt {\varepsilon _0 \mu _0 } }} - 2\Delta L.$$

The diameter of via and pitch of substrate-integrated cavity are determined as [Reference Wu, Deslandes and Cassivi31]:

(7)$$d\lt \displaystyle{{\lambda _g } \over 5}, \; $$

(8)$$p\lt 2d.$$

The antenna is embedded in dielectric coating possessing thickness T, dielectric constant ε _rc, and loss tangent tan δ _c. The antenna is covered with composite radome possessing thickness t. The proposed EBG structure consists of square patches around he radome in square configuration. The period of EBG structure is S. The dimensions of the cavity-backed patch antenna surrounded by dielectric coating are shown in Table 1.

Table 1.

Dimensions of SIW cavity-backed patch antenna with EBG structure.

III. ELECTROMAGNETIC BAND GAP STRUCTURE

EBG surfaces can be used for suppressing the surface waves and improvement of the radiation pattern of the antenna [Reference Venkateswaran32]. The performance of EBG structure can be analyzed using direct and indirect approaches with the help of full-wave electromagnetic numerical simulation software tools. The direct method involves the extraction of scattering parameters (S-parameters) between the two ports placed across the EBG structure [Reference Sievenpiper and Zhang33]. The indirect method involves extracting dispersion diagram from an extensive procedure [Reference Sievenpiper and Zhang33, Reference Li, Chen and Yuan34]. The proposed electromagnetic band gap structure is designed using the guide lines [Reference Venkateswaran32] and analyzed by the direct method [Reference Sievenpiper and Zhang33, Reference Sujatha and Vinoy35]. The transmission across the EBG structure is evaluated using two small monopoles, one for the transmission and one for the reception, for determining surface wave suppression experimentally. The measured transmission S ₂₁ between monopoles shows surface wave suppression band centered at 2.8 GHz in which the signal is strongly attenuated in the presence of the proposed EBG structure between monopoles as shown in Fig. 2. The transmission between monopoles with metal sheet is also measured.

Fig. 2.

Measured transmission characteristics of EBG structure.

IV. PARAMETRIC ANALYSIS

The cavity-backed patch antenna shows broad beam radiation characteristics (3 dB beam width in E and H planes are 105 and 66°, respectively) and good return loss (−18 dB) in the absence of dielectric coating and radome (T = 0 mm and t = 0 mm). The return loss S ₁₁ and radiation pattern of cavity-backed patch antenna embedded in the dielectric coating are analyzed for various thicknesses of dielectric coating (T = 6, 9, and 12 mm) and radome (t = T − 3 mm). The return loss of patch antenna is increased with the increase in the thickness of radome as shown in Fig. 3. The radiation pattern of the antenna is degraded with the increase in the thickness of dielectric coating as shown in Figs 4 and 5. The narrowing of radiation pattern can be understood by studying surface current distribution. The surface current is confined near the radiating patch when there is no dielectric coating on the ground plane. For dielectric coating thickness of 12 mm, the surface currents are penetrated into the dielectric coating, as shown in Fig. 6, exciting surface waves in the dielectric coating and hence the degradation (narrowing) of radiation pattern (simulated θ _E × θ _H = 50° × 6°). The gain of antenna is also reduced by 2 dB due to close proximity of radome with radiating aperture of the antenna.

Fig. 3.

Return loss S ₁₁ of antenna for various radome thicknesses. (a) t = 0, 3 mm (b) t = 6, and 9 mm.

Fig. 4.

Effect of dielectric coating on E-plane (YZ-plane) radiation pattern of antenna (f = 2.8 GHz). (a) T = 0 and 6 mm, (b) T = 9 and12 mm.

Fig. 5.

H-plane (XZ-plane) radiation pattern for various dielectric coating thicknesses (f = 2.8 GHz). (a) T = 0 and 6 mm, (b) T = 9 and 12 mm.

Fig. 6.

Current distribution of antenna. (a) No dielectric coating, (b) 12 mm dielectric coating.

V. EXPERIMENTAL RESULTS

The proposed EBG structure is placed on dielectric-coated structure to restore broad beam radiation pattern of cavity-backed patch antenna (Fig. 7). The EBG structure consisting of conducting patches acts as the barrier to reduce propagation of surface waves in the dielectric coating and confines current distribution near the radiating aperture of the antenna. Therefore the radiation pattern of the antenna is improved, i.e. 3 dB beam width in the E-plane is increased/restored as shown in Fig. 8. Figure 8 also shows the measured cross-polarization components of electric and magnetic fields confirming linear polarization characteristics of antenna. The gain of antenna is plotted in Fig. 9. After implementation of EBG structure, broad beam radiation pattern (measured θ _E × θ _H = 100 × 64°) and 6 dBi gain is achieved. The metallic strip on the top surface of radome improves return loss to −16.5 dB with 118 MHz impedance bandwidth as shown in Fig. 10.

Fig. 7.

Fabricated cavity-backed antenna. (a) Top view, (b) bottom view.

Fig. 8.

Improvement in the radiation pattern of antenna due to EBG structure (f = 2.8 GHz). (a) E-plane, (b) H-plane.

Fig. 9.

Gain of the substrate-integrated cavity-backed patch antenna.

Fig. 10.

Simulated and measured return loss S ₁₁ of the antenna after implementation of strip on radome.