|Catur Apriono||Antenna Propagation and Microwave Research Group (AMRG) Department of Electrical Engineering, Faculty of Engineering, Universitas Indonesia|
|Arie Pangesti Aji||Antenna Propagation and Microwave Research Group (AMRG) Department of Electrical Engineering, Faculty of Engineering, Universitas Indonesia|
|Teguh Wahyudi||Antenna Propagation and Microwave Research Group (AMRG) Department of Electrical Engineering, Faculty of Engineering, Universitas Indonesia|
|Eko Tjipto Rahardjo|
The demand for high-speed data transmission has increased significantly in the last decades. Terahertz (THz) frequency, which lies between 100 GHz to 10 THz, has been considered as the solution to the demand. However, the low gain and low efficiency of a THz antenna remain to be issues that hinder reasonable performance for various applications. This paper proposes the design of a high-gain and high-efficiency planar bow-tie antenna for applications in the THz frequency. A planar bow-tie on a high-resistivity silicon substrate is considered to obtain the broadband characteristics. To increase the gain and efficiency, a dielectric silicon lens and a matching layer based on the quarter-wavelength are applied in the design. From simulations using Computer Simulation Technology (CST) Microwave Studio, gain and radiation efficiency of up to 32.69 dB and 90.4% are obtained, respectively. This proposed design has shown high radiation performance suitable for high-speed transmission systems.
Bow-tie, High resistivity silicon lens, Matching layer, Terahertz
The terahertz (THz) wave radiation region, located between the microwave and the infrared regions, is the transition region between the electronic and photonic domains (Akyildiz et al., 2014). The THz wave is not applied as widely as the electronic and photonic domains, although there is potential for this frequency band. The availability of reasonable performance equipment, especially for THz wave sources and detectors, is one of the main issues of THz wave system development. As femtosecond lasers and photoconductive antennas have been available since the 1980s to produce THz waves (Mourou et al., 1981), some areas have begun to utilize these waves, such as medical science, pharmacology, and security (Siegel, 2002). This region has also been considered to fulfil the demand for high-speed wireless data transmission and various communication applications (Hirata et al., 2012). High-speed data transmission demand has increased significantly in the last decades. Some modifications have been made to improve data capacity by applying advanced modulation schemes and signal processing. In the future, a new spectral resource is needed to increase data speed. The large bandwidth of THz waves can be utilized to obtain higher data rates, even with the legacy data modulation technique. Therefore, a THz wave carrier frequency is necessary to increase bandwidth and fulfil requirements (Song & Nagatsuma, 2011).
Despite its advantages of high speed and large bandwidth, THz wave radiation has a significant amount of signal loss compared to the microwave frequency band. The atmospheric attenuation at THz frequencies is too large for long range outdoor wireless communications (ITU-R Recommendation, 2009). However, this system still can be used as an alternative solution for future short-range indoor wireless communication, such as WLANs or WPANs, to provide data rates of ten Gbps and beyond (Song & Nagatsuma, 2011). Theoretically, the issue can be minimized by providing a sufficient transmitted source power or a high-gain antenna, especially for outdoor application. The concept of a THz antenna is expected to be an effective method to produce THz wave radiation, which has already been applied and is mature in the electronic domain.
Among the various types of antennas, a bow-tie type is often considered as a broad bandwidth antenna. The bi-conical structure of this type of antenna is easy to design and is affordable (Kaur & Solanki, 2012). It consists of two triangles facing each other from their apex, with a specific gap to form a dipole antenna. The performance of the bow-tie antenna depends on its geometrical parameters, including bow-tie length, apex angle and gap size. In addition, a flexible bow-tie antenna is expected to be more directional than a conventional dipole antenna because of its larger radiating area (Durgun et al., 2011).
In a THz system, an antenna can be used by combining optical elements, such as a lens or mirror, to provide reasonable radiation performance for quasi-optical detectors or radiators (Hesler et al., 2008). The bow-tie antenna combined with a capacitive bar and a hemispherical silicon lens has been studied for broadband applications; however, the radiation characteristics of gain and efficiency should be improved to achieve more reasonable radiation performance (Wahyudi et al., 2017). A bow-tie photoconductive antenna on GaAs substrate has been studied for the THz frequency band, and its directivity performance can be improved by using a silicon lens (Jyothi et al., 2016). Surface wave suppression has been studied by using bow-tie THz antennas on InP substrate combined with hemisphere and bullet Si lens, which has led to higher gain and efficiency and also broader bandwidth (Li & Song, 2016). A dipole bow-tie slot antenna on Si substrate for imaging application at 94 GHz has been studied by using SiO2 as an insulating dielectric material to enhance gain and side lobe level performances (Haraz et al., 2014).
2.1. Planar Bow-tie Antenna
Figure 1 illustrates a schematic of the bow-tie antenna. It consists of two identical triangles separated by a specific gap distance. A discrete port with a 50 ? input impedance is used as the feeding method, set in the gap. High-resistivity silicon with a relative dielectric constant (
Figure 1 Schematic of the bow-tie antenna design on a high resistivity silicon substrate
Table 1 Antenna and substrate parameters
An antenna placed on a thick dielectric substrate will radiate most of its energy into the dielectric side (Rebeiz, 1992). A hemispherical shape is used to minimize the internal reflection as well as the surface waves, so that radiation efficiency is increased. Figure 2 illustrates a dielectric lens designed on top of the substrate. To form a hemisphere shape, the lens radius (R) is set to 3000 µm, or half the dimension of the Si substrate (s). The center point of the lens is at the top of the substrate, or at the point of substrate thickness (h). The lens dielectric constant is determined to be same as the substrate dielectric constant to avoid a mismatched condition between the two.
Figure 2 Schematic of a dielectric lens structure
3.1. Planar Bow-tie Antenna
This section discusses the simulation results of the planar bow-tie antenna shown in Figure 2. The discussion is conducted for two parameters, i.e. return loss and radiation pattern.
3.1.1. Return loss (S11) parameter
Figure 3 shows the S11 simulation results of the bow-tie antenna. The line is not smooth, which indicates internal reflections occurring in the substrate. This issue can be solved by adding a hemispherical lens to provide a radiation vector parallel to the hemispherical surface vector. The return loss at 1 THz is around -20 dB. The bandwidth, which is determined at the threshold of -10 dB, is 364.97 GHz. Therefore, the result shows that the antenna’s resonant frequency is 1 THz and has broad bandwidth characteristics, as expected from a conventional bow-tie antenna design.
Figure 3 S11 parameter simulation results of the initial bow-tie antenna design
3.1.2. Radiation pattern
Figure 4 shows the radiation pattern simulation results at 1 THz frequency for planes of (a) ? = 0° (E-plane) and (b) ? = 90° (H-plane), respectively. The red and green lines represent the gain levels for all directions and the side lobe level, respectively. Both figures show that the radiation patterns are not as desired. This issue is related to the internal reflections, which lead to the radiated power being trapped in the substrate or surface waves. The problem can be solved by considering infinite substrate thickness or by providing a substrate surface with normal vectors to all radiation vectors. We consider the latter solution to solve this issue. The realized gain is -4.631 dB. The radiation efficiency and total efficiency are -13.97 dB (4%) and -14.01 dB (3.9%), respectively. The radiation pattern seems to be omnidirectional. The main lobe is directed at -53° (E-plane) and 56° (H-plane), which means that the antenna cannot radiate the energy directly to the desired direction (0o) from its center point. Although the angular beam width is narrow at the main lobe, these results indicate that the antenna is not applicable for implementation in practical use, because most of the power radiated from it is reflected back into the substrate, while the radiated power to free space is very low.
Figure 4 Radiation patterns of the bow-tie antenna for: (a) ? = 0°; and (b) ? = 90°
3.2. Bow-tie Antenna Combined with a Dielectric Lens
This section discusses the simulation results of the bow-tie antenna combined with a dielectric lens, as shown in Figure 2. The discussion relates to two parameters, return loss and radiation pattern.
3.2.1. Return loss (S11) parameter
Figure 5 shows the S11 simulation results of the bow-tie antenna combined with a silicon dielectric lens. The resonant frequency of 1 THz remains at around -20 dB, while the bandwidth determined at the threshold of -10 dB is 368.76 GHz. The S11 parameter shows that the antenna is still able to resonate at 1 THz and has broadband characteristics. These results indicate that the addition of the dielectric lens changes neither resonance nor bandwidth. The return loss line in Figure 5 is smoother than that in Figure 4, implying that the surface waves have been successfully minimized by the combination of the bow-tie antenna with the dielectric lens. The plot of gain for some frequencies shows that the gain fluctuates around 10 dB, and the highest gain is not at the resonant frequency of 1 THz. We expect the highest gain to be at the resonant frequency. Therefore, this structure is not yet at the optimum condition. In subsection 3.3 we investigate the optimum gain by characterizing substrate thickness. This technique is intended to find a focal position to produce higher directivity.
Figure 5 S11 Parameter simulations of the bow-tie antenna combined with dielectric lens
3.2.2. Radiation pattern
Figure 6 shows the radiation pattern simulation results of the bow-tie antenna combined with a dielectric lens at 1 THz for a plane of (a) ? =
Figure 6 Radiation pattern of the bow-tie antenna combined with dielectric lens at: (a) ? =
3.3. Substrate Thickness Characterization
In this stage, optimization is conducted to further improve the directivity by varying the substrate thickness. The antenna and dielectric lens geometry are kept constant, as detailed in Table 1, and the iteration in thickness is conducted from 300 µm to 1100 µm. The effect of thickness will be discussed in terms of gain, efficiency and angular beam width.
Figure 7 shows the gain for different substrate thicknesses. The gain increases for thicknesses of 300 µm to 1000 µm and subsequently decreases. The maximum gain of 31.31 dB is obtained at a thickness of 1000 µm and produces a significant gain improvement until 20 dB compared to the initial thickness. High gain is necessary to mitigate the high attenuation when the radiation propagates in the atmospheric free space. The results show that the substrate thickness affects the antenna’s directivity, and the optimum gain can be obtained at a certain thickness.
Figure 7 Gain versus substrate thickness
3.3.2. Radiation efficiency
Figure 8 shows the radiation efficiency for different substrate thicknesses; a maximum efficiency of 71.8% is obtained at the substrate thickness of 900 µm, but efficiency starts to drop from a thickness of 1000 µm and beyond. From the results, we can conclude that substrate thickness has a slight effect on the antenna’s efficiency. This condition comes from the radiation wavefront conforming with the lens surface, where the optimum thickness to pass through the interface is around 900 µm.
Figure 8 Radiation efficiency versus substrate thickness
3.3.3. Angular beamwidth
Figure 9 shows the beam width at plane ? =
Figure 9 Beamwidth versus substrate thickness
3.4. Antireflection coating implementation
A common problem with lens antennas is that the incident wave is partially reflected back to the substrate at the lens-air interface, thereby affecting radiation efficiency (van der Vorst et al., 1999). Studies on canceling internal reflection in a dielectric lens have been performed since 1954 using quarter-wave antireflection coating (Morita & Cohn, 1956). The coating works according to the principle of phase changing between the reflected waves and depends on the different refractive indices of the lens and the coating. The phase changing results from ray propagation through the coating, and because of the different path lengths between subsequent rays which leave the substrate at different points, based on a far-field observation point. The coating is theoretically designed using an infinite number of internal reflections. The derivation of the infinite reflection and transmission coefficients use the model and configuration described by van Houten & Herben (1994). In this section, an antireflection coating, otherwise known as a matching layer (ML), is designed using different techniques.
3.4.1. Quarter-wave matching layer
Figure 10 shows the proposed ML structure. The dielectric constant (
Figure 10 Schematic of the matching layer design
From the design shown in Figure 10, the radiation efficiency and gain have increased to 83.1% and 32.31 dB respectively. It is clear that the matching layer has lowered the internal reflection significantly, but not completely. The efficiency improvement is not as good as that reported by van der Vorst et al. (1999), because the antenna already has a narrow beam width and good directivity. In addition, the proposed lens antenna has different angles of incident at every different surface position. Therefore, the single quarter-wavelength matching layer does not completely reduce the reflection effect on the lens antenna, because it can only reduce the internal reflection if the wave vector is parallel to the normal vector. An optimum matching layer with non-uniform thickness is more preferable, but we do not study this because it is difficult to design and not practical in real conditions. We consider a cascaded matching layer, which will be discussed in the following section.
3.4.2. Cascaded matching layer
The concept of the cascaded matching layer is the same as that of the quarter-wave in the previous subsection, but with multiple-layer implementation. The dielectric constant and thickness of each layer can still be obtained using Equations 1 to 3, derived into Equation 4.
As shown in Figure 12, the radiation efficiency shows incremental values from one ML to 15 MLs, with a maximum value of 93.7% at 15 layers. The results show that a higher number of MLs can reduce more internal reflection, thus increasing the radiation efficiency. The gain value also shows incremental values from one ML to four
A bow-tie planar antenna on a high-resistivity silicon substrate combined with a capacitive bar provided wideband bandwidth of around 368 GHz. The antenna combined with optimum substrate thickness and a hemispherical silicon lens provides gain and beam width of 31.31 dB and 3.1o, respectively. The matching layer technique of the quarter-wavelength and the five cascaded matching layers has radiation efficiency and gain of 90.4% and 32.69 dB, respectively. The proposed design can be considered to provide a high radiation performance antenna for various THz applications.
This work was supported by PITTA 2017 Grant, contract number: 754/UN2.R3.1/HKP.05.00/2017, year 2017, Universitas Indonesia.
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