I. INTRODUCTION
Conventional antennas are not suitable for high-speed objects such as jet planes, rockets, or satellites due to large profile and massive air friction faced. Cavity-backed antennas are the most suitable types for such applications where these antennas do not protrude from the body. Cavity-backed patch antenna show superior performance because of better matching and significant suppression of surface waves [Reference Karmakar1, Reference Awida, Elkhouly and Fathy2]. However, development of cavity-backed patch antennas requires two fabrication processes. The first process involves conventional printed circuit board (PCB) technology to develop microstrip patch layer and the second process requires computer numerical control machining or metal casting to fabricate metalized cavities. These two stage processes increase the total fabrication cost of antenna and make structure assembly complicated [Reference Awida, Elkhouly and Fathy3]. Substrate-integrated waveguide (SIW) has been suggested as an alternative fabrication technology for low-cost implementation of waveguide-like components and antennas using the standard PCB fabrication technology [Reference Bozzi, Perregrini, Wu and Arcioni4–Reference Awida, Suleiman and Fathy8]. SIW cavity-backed patch antennas exhibit same radiation performance as that of conventional metal cavity-backed patch antennas along with advantages of low-profile, easy fabrication, and planar integration [Reference Bohorquez, Pedraza, Pinzon, Castiblanco, Pena and Guarnizo9–Reference Saghati and Entesari12]. Electromagnetic bandgap (EBG) structures have aroused growing interest in antenna community in the last years. EBG structures can be categorized into following classes [Reference Weily, Horvath, Esselle, Sanders and Bird13]: (1) EBG substrates and high-impedance surfaces used to reduce surface waves [Reference Sievenpiper, Zhang, Broas, Alexópoulos and Yablonovitch14–Reference Yang and Rahmat-Samii18]; (2) defect resonator antennas that create high directivity over a narrow bandwidth [Reference Yang and Alexopoulos19–Reference Weily, Esselle, Sanders and Bird21]; (3) sources embedded in EBG materials that have high directivity due to the limited angular propagation allowed within the material [Reference Bulu, Caglayan and Ozbay22, Reference Enoch, Tayeb, Sabouroux, Gu and Vincent23]. EBG materials have been successfully used to improve the performance of antennas [Reference Yang and Rahmat-Samii24, Reference Frezza, Pajewski, Piuzzi, Ponti and Schettini25]. Based on EBG technology, many novel antennas have been investigated and developed by the researchers [Reference Menudier, Thevenot, Monediere and Jecko26–Reference Bendaoudi and Naoum29]. Most of the research work has been carried out on single substrate, i.e. antenna and EBG structure are fabricated on the same substrate for improving radiation performance of antennas. The use of vias for shorting patches may not be feasible in certain applications.
In this paper, planar EBG structure in the form of via less conducting patches is proposed and developed on dielectric coating surrounding substrate-integrated cavity-backed patch antenna. The performance of cavity-backed patch antenna is analyzed for various thicknesses of dielectric coating in which patch antenna is embedded. The broad beam radiation pattern is degraded severely with the increase in the thickness of dielectric coating. The proposed EBG structure reduces the propagation of surface waves in thick dielectric coating causing significant improvement in the radiation pattern of the antenna. The composite radome placed in the close proximity of radiating aperture of antenna produces input impedance mismatching. The metallic strip placed on the top surface of radome restores impedance matching of antenna. The antenna can find use in different wireless communication applications.
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];
$$\varepsilon _{reff}=\displaystyle{{\varepsilon _r+1} \over 2}+\displaystyle{{\varepsilon _r - 1} \over 2}\left({1+\displaystyle{{12h} \over W}} \right)^{\displaystyle{{ - 1} \over 2}}\comma \; \quad \hbox{for}\; \displaystyle{W \over h}\gt 1.$$Due to fringing, the patch of microstrip antenna looks greater electrically than its physical dimensions. The extension in the length ΔL is determined as:
$$\displaystyle{{\Delta L} \over h}=0.412\displaystyle{{\lpar \varepsilon _{reff}+0.3\rpar \lpar {[ {W/h}] +0.264} \rpar } \over {\lpar \varepsilon _{reff} - 0.258\rpar \lpar {[ {W/h}] +0.8} \rpar }}.$$
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):
$$L_{eff}=L+2\Delta L.$$For the dominant TM010 mode, the resonant frequency of the microstrip antenna is determined as:
$$\lpar f_r \rpar _{010}=\displaystyle{1 \over {2L_{eff} \sqrt {\varepsilon _{reff} } \sqrt {\varepsilon _0 \mu _0 } }}=\displaystyle{1 \over {2\lpar L+2\Delta L\rpar \sqrt {\varepsilon _{reff} } \sqrt {\varepsilon _0 \mu _0 } }}.$$For an efficient radiator, the width of the microstrip patch antenna is determined as:
$$W=\displaystyle{1 \over {2f_r \sqrt {\varepsilon _0 \mu _0 } }}\sqrt {\displaystyle{2 \over {\varepsilon _r+1}}}=\displaystyle{c \over {2f_r }}\sqrt {\displaystyle{2 \over {\varepsilon _r+1}}} .$$where c is the velocity of light in free space.
The actual length of the patch is determined as:
$$L=\displaystyle{1 \over {2f_r \sqrt {\varepsilon _{reff} } \sqrt {\varepsilon _0 \mu _0 } }} - 2\Delta L.$$The diameter of via and pitch of substrate-integrated cavity are determined as [Reference Wu, Deslandes and Cassivi31]:
$$d\lt \displaystyle{{\lambda _g } \over 5}, \; $$
$$p\lt 2d.$$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.
V. CONCLUSION
Substrate-integrated cavity-backed patch antenna embedded in dielectric coating has been proposed and developed. The effect of radome and dielectric coating on the performance of antenna is investigated. Increased thickness of dielectric coating and radome severely degrades radiation pattern and impedance matching of the antenna. The proposed EBG structure and strip placed on radome restore radiation characteristics and impedance matching of cavity-backed patch antenna.
M. Jaid Asad received his Bachelors in Electrical Engineering from the University of Engineering and Technology (UET) Taxila Pakistan in 1999. He did the Master degree in Electrical Engineering from the UET Taxila in 2002. His research interests are antennas, meta materials, and microwave circuits.
Dr. M. Farhan Shafique received his B. E. degree from Hamdard University, Karachi, Pakistan, in 2003, M.S. degree from the University of Paris East Marne-La-Vallée, Paris, France, in 2005 and Ph.D. degree in Electronic and Communications Engineering from The University of Leeds, Leeds, UK in 2010. His research interests involve multilayered-microwave device fabrication on LTCC and thick-film technology, electromagnetic modeling of microwave structures, RF antenna and antenna arrays, Ultra wideband diversity antennas, and MEMS packaging. In 2005, he was an intern at Conservatoire National des Arts et Métiers, Paris, France. From 2007 to 2010 he was involved in establishing the LTCC fabrication facility at Institute of Microwave and Photonics, the University of Leeds, UK. He has extensive experience of laser micromachining and multilayer LTCC device modeling and fabrication. He is working as an Assistant Professor and MCAD head at COMSATS Institute of Information Technology, Islamabad, Pakistan.
