Miniaturization and Bandwidth Enhancement of Quarter-mode and Half-mode SIW Cavity- Backed Antenna for 5G Applications

Substrate Integrated Waveguide (SIW) is a special antenna technology which retains reduced copper surface waves and low dielectric loss property of classical rectangular waveguides. The antennas developed on SIW have simple configuration with an ease of fabrication in flame redundant wearable and integration in walls and floors. In this sense a Quarter-mode SIW (QMSIW) antenna has been designed, analyzed and its performance has been compared with Half-mode SIW. QMSIW has been precisely designed to operate efficiently in the range of 40 57 GHz. It aims at enhancing the bandwidth for 5G applications with a maximum return loss of -36 db at 55 GHz. HMSIW structure has Triple band resonant frequencies at 20.5 GHz, 24.7 GHz & 27 GHz with the return loss values of -34 db, -14 db & -32.5 db respectively. Both antennas operate in 20 – 72 GHz range of 5G applications.


Introduction
The different antenna technologies like Microstrip, Metamaterials and other dimensional antenna technologies suffer from surface waves and the undesired radiation. In SIW Technology the field is restricted inside the SIW structure and hence the above-mentioned losses are minimized. Substrate Integrated Waveguide Cavity Backed (SIW-CB) antenna is a fascinating technology to realize non-planar (waveguide based) circuits into planar (Microstrip) circuits and systems. Selective filters and power amplifiers are conveniently integrated with the chip-set which illustrates the SIW integrating property. SIW also has high isolation factor which makes it a perfect candidate for low-power applications. The other general features like low cost, conformability and easily mountable device are some of the constructional challenges. These challenges are efficiently handled by the SIW technology involved CB antenna. SIW-CB antenna has different modes of operation, of which Quarter-mode and Half-mode are analyzed. The design of Quarter and Half mode SIW cavity backed antenna involves the arrangement of two rows of cylinders with conducting material embedded in a dielectric substrate.
Operation Principles SIW structure is an arrangement of via holes along the length and width of the antenna. These rows of conducting cylinders are mostly present at the sides of the SIW structure as shown in Figure 1. These via holes confine the field within the SIW core and act as an effective electric wall. Since the radiations are not allowed outside the core, the undesired radiations, surface waves and other losses are reduced [1]. Due to these structural characteristics, the antenna parameters are improved with suppressed losses. These rows of conducting cylinders or via holes connect the microstrip patch and the ground plane electrically. The propagation characteristics of SIW antenna including the Advances in Computing, Communication, Automation and Biomedical Technology radiation pattern and dispersion characteristics are similar to the conventional Rectangular Waveguides with reduced radiation leakage [2]. SIW modes are same as traditional metallic waveguide, namely with the TE n0 modes, with n 1, 2 . . . etc. The gaps between metallic via holes do not support TM modes. The dominant mode of a conventional waveguide is TE 10 mode with vertical electric current density on the side walls. For primary dimensioning and structuring of SIW design the relation of cut-off frequency and dielectric loss with substrate thickness is important [3]. Thickness of the substrate does not affect the resonant frequency whereas dielectric loss decreases with the increase in substrate thickness. The equation was derived in W eff = W + (d / (0.95) s) W eff = W/√ ε r where W eff -Dielectric width W -Spacing between the metal via hole rows s -Via hole interspaces d -Via diameter ε r -Substrate's relative permittivity

Half-Mode SIW antenna design
HMSIW antenna has two rows of metallic via holes but the via-holes are present only for half of the length of the antenna as shown in the Figure 2. The proposed Half-mode SIW-CB antenna has an overall dimension of 18*18*0.8mm 3 . The length of the substrate is 18mm whereas that of the radiating patch is 9mm. The dimension of the patch is almost half of the Full-mode SIW-CB antenna. The effective dimension of HMSIW is 9*18*0.8mm 3 . For designing a Half-mode SIW antenna, few parameters which influence the performance are considered [4]. Of the few design parameters dielectric substrate and its thickness, feeding technique and frequency of operation are very important [5]. To operate in the range of 20 -28 GHz frequency the Half-mode antenna has been designed. The above-mentioned operating range appears in the 5G millimeter wave applications. To reduce the radiation losses coaxial feeding technique has been employed. The length of the feeding port is extended since the patch length is halved. This provides the required impedance matching characteristics. The ground plane is directly connected to the outer conductor. Through the dielectric the inner conductor is soldered to the radiating patch [6]. Selection of dielectric substrate is done carefully to provide the required performance characteristics, to easily fabricate and measure in practical conditions. FR4-epoxy substrate is a perfect candidate to answer the abovementioned requirements [7]. FR4 is chosen with the following specification of relative permittivity (εr) 4.4 and dielectric loss tangents 0.02. Substrate thickness is another important parameter which is to be considered for reducing losses. Thickness has to be optimized, since conductor loss is inversely proportional to substrate thickness and directly proportional to the dielectric losses. 0.8mm is the measured substrate thickness [8]. proposed Quarter-mode SIW-CB antenna dimension is 18*18*0.8mm 3 (same as Half mode). The length and width of the substrate is 18mm each whereas that of the radiating patch is 9mm each, which is a considerable reduction in patch dimension. The dimension of the patch is almost Quarter of the Full-mode SIW-CB antenna. The effective dimension of QMSIW is 9*9*0.8mm 3 . The design specifications of Quarter-mode SIW-CB is similar to Half-mode SIW-CB with the importance being given to working frequency, technique of feeding, dielectric substrate material and substrate thickness [9]. The proposed Quarter-mode antenna has been structured to operate in 40 -57 GHz frequency range which comes under millimeter wave for 5G applications. Comparing the bandwidth of QMSIW it is nearly double that of HMSIW. The same coaxial or probe feeding technique has been carefully used [10]. The width of the feeding line is halved for proper impedance matching.
FR4-epoxy substrate has been chosen with the same specifications, relative permittivity of (εr) 4.4 and dielectric loss tangents of 0.02 to provide the required performance characteristics. The other reasons that support the selection of FR4 are easy to fabricate and measure in practical conditions [11].
This similar substrate used is helpful to compare with Half-mode characteristics. Substrate thickness is another important parameter which is to be considered for reducing losses. Thickness has to be optimized, since conductor loss is inversely proportional to substrate thickness whereas directly proportional to the dielectric losses [12]. Substrate thickness used is measured as 0.8mm.

Results and Comparison
The HMSIW and QMSIW antennas have the key feature of large bandwidth based on the mode of operation of antenna in the millimeter wave range for 5G applications. The parameters such as resonant frequency and return loss for the respective modes are effectively compared [13]. Both the Half and Quarter mode antenna have three and four resonant frequencies respectively in different GHz frequency ranges. The detailed comparisons of the results are as follows.

Half-Mode SIW -CB antenna
HMSIW-CB antenna is characterized by the rows of metallic via holes along the entire width of the antenna but present only for half of the length of the antenna. It has three resonant frequencies at 20.5 GHz, 24.7 GHz & 27 GHz with the return loss values of -34 db, -14 db and -32.5 db respectively. The HMSIW structure has good enough return loss to operate in the three resonant frequencies (20.5 GHz, 24.7 GHz & 27 GHz). Though the return loss is in the higher side for the above-mentioned frequencies, the antenna has a very small operating bandwidth. The entire radiation energy has been used by the three resonant frequencies with high return loss values instead of producing more resonant frequencies with low return loss values [14]. These frequencies support the operation of different electronic components and gadgets through internet for 5G applications. The simulated and measured return loss characteristics are shown in Figure 4.

Fig. 6: Half-mode Return Loss Characteristics
The VSWR measures 1 over the three operating frequency ranges which is an excellent value to achieve. The Standing waves have been reduced considerably which is obvious from the overall VSWR results shown in Figure 5. Ansoft High Frequency Structure Simulator (HFSS) is the simulator used for generating simulated results.

Fig. 7: Half-mode VSWR characteristics
The propagation characteristics of HMSIW antenna is obtained for 20 GHz frequency. The E-plane and the H-plane radiation characteristics are shown in Figure 6. The simulated radiation patterns are almost similar for both E and H planes. In spite of these features the antenna has the drawback of limited bandwidth [15]. From the pattern it is clear that the antenna shows unidirectional radiation characteristics. GHz. With increased no. of resonant frequencies and enhanced bandwidth, the performance of QMSIW-CB is comparatively better than HMSIW. It covers most of the mm wave arrange to operate for 5G applications. Though the two antennas operate in different range it works for the same application. These frequencies support the operation of different electronic components and gadgets through internet. This SIW-CB structure has the ability to works in four various frequencies. The simulated and measured return loss characteristics are compared in Figure 7. The VSWR value is well below 1.4 over the mm wave range, which is an excellent value to achieve. The Standing waves have been reduced considerably which is obvious from the overall VSWR characteristics as shown in Figure 8. The radiation pattern of QMSIW antenna is obtained for the frequency of 50GHz. The E-plane and the H-plane radiation characteristics are shown in Figure 9. The simulated radiation patterns are almost similar for both E and H planes. From the pattern it is clear that the antenna shows unidirectional radiation characteristics. Quarter-mode antenna has the advantage of large bandwidth is shown in Figure 10.

Comparison
The design parameters that are considered for the HMSIW and QMSIW antennas are given below in Table 1.
It is found that even though most of the parameters (via-hole diameter, substrate centre to viahole centre distance and distance between two via-hole centre) are similar for both the designs, the patch width and no. of via are different for the structures. With these design parameters the performances of the SIW-CB antennas have been compared. Simulated return loss, VSWR and radiation pattern results present that the proposed antennas are fulfilling the requirements of 5G applications is shown in Figure 11. The performance parameters of SIW-CB in Half mode and Quarter mode are analyzed and compared in Table 2 given below. frequencies with limited band of operation ranging from 20 -21 GHz, 24.25 -25 GHz and 26.5 -28 GHz. The effective bandwidth is 3.25 GHz. Quarter-mode produces an enhanced bandwidth of 16.5 GHz from 40.5 -57 GHz. Half-mode antenna has acceptable VSWR of 1 (for the resonant frequencies) whereas Quarter-mode antenna has excellent VSWR value of Less than 1.4 (for the entire operating range). The radiation characteristics of both the structures show unidirectional pattern with minor variations in E and H plane pattern. The return loss values are also on the higher side for both the antennas. Hence from the performance parameters it is obvious that for the application where a strong signal (return loss) in a particular resonant frequency is required, Half-mode structured antenna can be utilized. Quarter-mode SIW-CB structured antenna can be perfectly suited for the applications where a range of operating frequency is required.

Conclusion
Miniaturizations of SIW technology involved cavity backed antennas are proposed with two modes of operation to generate two different performance characteristics. The advantages of SIW technology are combined with FR4 substrate. A systematic investigation of HMSIW and QMSIW is carried out based on cavity resonator concepts. These antennas exhibit a reduced patch size when compared to a conventional SIW design. Proper comparisons of design and performance parameters are reported in Table 1 and Table 2 respectively. The designed, simulated, fabricated, measured, compared and analyzed SIW-CB antennas demonstrate a low-profile characteristic to covers the 5G frequency range of 20 -72 GHz.