Compact Dual-Frequency Slot Antenna for C-Band Applications Based on Substrate Integrate Waveguide

ABSTRACT


INTRODUCTION
The increasing needs for high gain antenna and compact size for industrial, scientific, and medical (ISM) communication system are outlined as a requirement for better wireless network performance [1].Recently, Researchers are motivated to develop a high efficiency antenna by the rising need for multiple, dual, and wideband antennas.The primary objective is to boost data speeds using wideband antennas in order to increase traffic capacity at higher bands [2].Besides, the trending between high performance antennas and its size should be taken in consideration at higher frequencies [3].In general, standard planar transmission lines like microstrip were employed in the implementation of the antennas.However, there are a number of issues with the microstrip technology, including a high loss, undesirable crosstalk between lines, and difficulty fabricating in higher bands [4].Additionally, low gain is another feature of Microstrip technology in higher bands.Substrate integrated waveguide (SIW) technology has so been suggested as a possible approach to implement the antenna.
Due to its low loss transmission line feature, (SIW) has recently been employed more frequently than conventional waveguide and microstrip technologies [5][6][7][8][9][10].However, there are two significant difficulties with SIW antennas in lower millimeterwave bands.The substrate's vias (holes), which result in a larger size of antenna, are the first problem.The interior dimensions of the common waveguide determine the vias separation distance [11].Additionally, these vias contain undesirable radiation losses that cause the antenna's gain to decrease and the sidelobes to rise [12][13][14].The waveguide component of SIW presents the second challenge.The waveguide is widely recognized for having a narrow bandwidth [15,16].Thus, lower millimeterwave bands of SIW antenna may have a small bandwidth, This is not necessary for 5G applications or millimeterwave applications [17].In the antenna systems and applications, the high quality of the antenna used is a major factor.The antenna should provide a multi-band and wideband reception points with high gain and high efficiency.Various types of antennas are proposed.However, these types suffered from low gain, and narrow bandwidth.Fractal antennas presents a good feature of repetitive, self-similar, and wideband.This is due to the main feature of the fractal antenna of procuring multi resonance frequencies.For example, a coplanar waveguide antenna in [18] is fed by triangular slot fractal shapes.The design obtained a high bandwidth of 8 GHz.Other types of fractal antennas can be seen [19].The design in [19] is based on labyrinth fractal antenna.This achieved a high gain of 8 dB and bandwidth of 4 GHz.In [20] the antenna is based on conventional rectangular slot patch antenna.While in [21,22], the antennas type is basically based on circular slot fractal shapes.Another fractal antenna also is presented in [23] which is designed with a finite ground co-planner waveguide (FG-CPW) circuit.The return losses are found better than 16 dB at 0.92 GHz and 2.45 GHz with fractional bandwidth of 3% at 10 dB return loss.In [23,25], a fractal antenna is introduced.It is reported that the AR is 35% has been achieved.An efficiency is obtained at 2.45 GHz with 54%.Then the circuit is optimized to operate at frequency of 2.45 GHz.A 2.45 GHz circular polarization fractal antenna is designed and simulated using ADS software [26].The design gives an AR of 65% and a return loss of -12 dB.An efficiency is obtained at 0.92 GHz and 2.45 GHz with 45% for each.Despite, the good performances of these antennas in capturing the waves from wide range of frequencies Consequently, the purpose of this paper is to develop a small dual-band SIW antenna for the C-band.First, the design processes, such as fractal-shaped design and SIW antenna design, are covered in section 2. Second, section 3 shows the measured performance of the suggested SIW metamaterial antenna.In section 4, the summary of this work is completed.

DESIGN OF H-FRACTAL SLOT ANTENNA WITH SIW
Conventional SIW structure is shown in Figure 1.In SIW, The dielectric substrate has two rows of metallic vias that connect the parallel metal plates on the top and bottom and act as a waveguide.As well as the top and bottom metal plates, the two rows of vias act as the waveguide's walls.The basic structure is defined by its common parameters such as vias separation distance (weff), vias dimeter (d), and the vias neighboring distance (p).The separation distance (weff) is depended on the waveguide inner size and dielectric properties.In general, other characteristics like substrate length, width, and high are determined using standard microstrip technology.The basic SIW parameters.The fractal slots are carved at waveguide centers, exited by the greatest electric field of the fundamental mode TE10, in order to produce high coupling between the SIW structures and the fractal slots.Three metamaterial unit cells and the typical SIW structure are shown together in Figure 2.The SIW structures-based metamaterial unit cells with the 1st, 2nd, and 3rd .H-shaped fractal slots are used in several iterations in order to maximize coupling and reduce the SIW structure size.The wide (top) walls of the SIW structures are above the H-fractal forms, as seen in Figure 3.However, slots can also be put in the SIW structures' foundations, producing roughly the same effects.In high-frequency radio systems, it is better to reduce noise and radiation losses while maintaining the stability of the construction.The suggested SIW-based H-fractal unit-cells and the conventional SIW's S-parameters research are shown in Figure 4. Considering each of the situations in Figure 3. Figure 4 shows that whereas conventional SIW forbids signals with evanescent modes below the cut-off frequency of the SIW structures of 10.3 GHz, the metamaterial unit cells enable signals under that cut-off frequency to flow through them.Thus, it can be defined as evanescent resonators.As 3rd iteration of H-fractal slot shape shifts the resonant frequency to lower band, the proposed SIW with 3rd iteration of H-shaped fractal cell is implement on SIW structure as shown in Figure 5.This allows a single frequency to be resonated.To study the effects of length (L) and width (W) of the H-fractal shape, a series of CST simulation are performed.By varying the length and the width of the H-shaped fractal antenna, the return loss increases and shifting to the desired frequency as shown in Figure 6. Figure 7 shows an antenna configuration consisting of two elements H-shaped fractal of 3rd iteration to ensure two modes of resonant frequencies.The two adjusted H-shaped fractal are studied with different displacement (L3) to each other.Figure 8 shows the return loss with respect to different displacement (L3) between H-shaped fractal unit cells.It can be noticed that best response is obtained when width L3= 2.7 mm. Two frequencies modes are observed in the targeted C-band at 4 GHz and 5.7 GHz.The final parameters of the proposed H-fractal shape SIW antenna for two frequency modes are depicted in Table 1.In addition, current surface distribution for the proposed antenna is shown in Figure 9.The two H-shaped fractal unit cells and the feed line are passed over by the current.Due to the altered structure of the cell unit, current flows in the opposite direction in each H-shaped fractal unit.The dual-mode frequency resonance is produced by the opposing current flowing through the two H-shaped fractal unit cells.

ANTENNA FABRICATION AND RESULTS
Figure 10 shows the picture of the fabricated H-shaped fractal SIW antenna with a total size of 23 × 11 mm².The measured refection coefficient with respect to simulated one of the designed H-shaped metamaterial SIW antenna with short ended is shown in Figure 11.A refection of -14.6 dB is obtained with bandwidth of 1.2 GHz at 4 GHz.Another frequency is resonated at 5.7 with a reflection of -17 dB and impedance bandwidth of 100 MHz, this is due to the two elements H-shaped unit cell of the SIW structure which allows to a second frequency to be resonated.Figure 10.Fabricated H-shaped fractal SIW antenna.
Figure 12 shows the measured radiation pattern with respected with simulated radiation of the printed SIW H-shaped antenna at 4 GHz and 5.7 GHz.The radiation pattern of omni-directional shape is observed with a sidelobes less than -10 dB.In that term directivity of the printed antenna are 5 dB and 3 dB at 4 GHz and 5.7 GHz respectively.Table 2 shows a comparison between this SIW dual H-shaped unit cell antennas with other related works.This antenna exploited a good bandwidth and compact size of 23 mm × 11 mm among others with total reduction in size of 20%.

CONCLUSION
In this paper, a portable dual-band SIW antenna for the C-band was introduced.H-fractal slot cell technology combined with a SIW structure is used to implement the design.Two elements of 3rd H-shaped fractal unit cell are used to control the bandwidth and achieved the dual-band property.Good results are obtained with reflection coefficient less than 10 dB at 4 GHz and 5.7 GHz.A gain of 5 dB is achieved at 4 GHz with high efficiency of 80 %. 1.2 GHz wideband impedance is also observed at the same time.This antenna provides a feasible method for assembling the best possible array of antennas, which would be useful for current wireless and lower millimeterwave applications.

Figure 7 .
Figure 7.The proposed SIW antenna with two elements of 3rd iteration H-fractal shape unit.

Figure 8 .
Figure 8.Return loss with different position of two adjusted H-fractal unit cell of SIW structure.

Figure 9 .
Figure 9.The current surface of the SIW H-fractal unit cell antenna at 4 and 5.7 GHz.

Table 2 .
Comparison of antenna with other works.