Low-Loss and Light Substrate Integrated Waveguide Using 3D Printed Honeycomb Structure

This article proposes a low-loss and light 3D-printed substrate-integrated waveguide (SIW). Despite the use of lossy polylactic acid (PLA) material, insertion loss is reduced, and bandwidth is increased due to a honeycomb substrate similar to air. To demonstrate the proposed concept, we fabricated microstrip-fed SIWs with solid PLA and honeycomb substrates, and compared their performance numerically and experimentally. Average measured insertion loss from 3.4 to 5.5 GHz for the honeycomb SIW is 1.38 dB, whereas SIW with solid PLA is 3.15 dB. Light weight is an additional advantage of the proposed structure.

In this section, the electrical properties of the PLA honeycomb substrate in terms of the thickness of the structure and the frequency are verified for the purpose of designing the SIW. The hollow honeycomb geometry is well known as a structure with high mechanical strength. The minimum thickness and Th and a larger Lh are required to achieve the lowest effective permittivity and lowest tangential loss of the honeycomb substrate. We set Th as 0.85 mm, which is the minimum thickness for stable 3D printing. We set Lh as 2.5 mm for stable supporting of the copper tape. The ANSYS highfrequency structure simulator (HFSS, version 17.2, Pittsburgh, PA, USA) was used for electromagnetic (EM) analysis. A PLA filament provided by ColorFabb ® (Belfeld, The Netherlands) was used to fabricate the 3D printed substrates.
To design the SIW using PLA material, PLA electrical properties need to be characterized for EM analysis. An infinitely large and open-ended artificial substrate can be analyzed using spectral Green's functions [23]. However, we used the transmission line technique to characterize the effective permittivity and permeability for simplicity [24]. The dielectric constant εr and tan δ of solid PLA substrate are 2.2 and 0.05 at 3.5 GHz, respectively. Based on these characteristics of the solid PLA substrate, we designed a honeycomb substrate, as shown in Figure 1. A two-port simulation for microstrip line design was developed to analyze the dielectric constant of the honeycomb substrate, as shown in Figure 2. Microstrip line width Wm and honeycomb substrate length Lm are fixed at 3.85 mm and 45 mm for parametric study with regard to the honeycomb unit cell size Lh and thickness Th, and substrate height, hsub.  This article proposes a honeycomb substrate design to implement a low-loss SIW, similar to airfilled SIW, using common PLA filaments for easy and environmental fabrication. Electrical properties were characterized to verify the SIW characteristics on the honeycomb substrate, and performance was compared with S-parameters for air and solid PLA filled SIW. Finally, microstrip-fed SIWs [20][21][22] with solid PLA and a honeycomb substrate are fabricated, measured, and compared.

Honeycomb Substrate Design
In this section, the electrical properties of the PLA honeycomb substrate in terms of the thickness of the structure and the frequency are verified for the purpose of designing the SIW. The hollow honeycomb geometry is well known as a structure with high mechanical strength. The minimum thickness and Th and a larger Lh are required to achieve the lowest effective permittivity and lowest tangential loss of the honeycomb substrate. We set Th as 0.85 mm, which is the minimum thickness for stable 3D printing. We set Lh as 2.5 mm for stable supporting of the copper tape. The ANSYS highfrequency structure simulator (HFSS, version 17.2, Pittsburgh, PA, USA) was used for electromagnetic (EM) analysis. A PLA filament provided by ColorFabb ® (Belfeld, The Netherlands) was used to fabricate the 3D printed substrates.
To design the SIW using PLA material, PLA electrical properties need to be characterized for EM analysis. An infinitely large and open-ended artificial substrate can be analyzed using spectral Green's functions [23]. However, we used the transmission line technique to characterize the effective permittivity and permeability for simplicity [24]. The dielectric constant εr and tan δ of solid PLA substrate are 2.2 and 0.05 at 3.5 GHz, respectively. Based on these characteristics of the solid PLA substrate, we designed a honeycomb substrate, as shown in Figure 1. A two-port simulation for microstrip line design was developed to analyze the dielectric constant of the honeycomb substrate, as shown in Figure 2. Microstrip line width Wm and honeycomb substrate length Lm are fixed at 3.85 mm and 45 mm for parametric study with regard to the honeycomb unit cell size Lh and thickness Th, and substrate height, hsub.  Substrate infill percentage depends on the honeycomb unit cell and size and determines the dielectric constant of substrate. As the size increases and the thickness decreases, the substrate dielectric constant becomes similar to that of air, as shown in Figure 3a-d. As L h increases from 2.4 to 3.2 mm, the infill percentage of honeycomb substrate decreases from 55% to 46%, and the effective dielectric constant is reduced from 1.45 to 1.43, as depicted in Figure 3a. The increase in L h also leads to a decrease in dielectric constant whose range is 1.61-1.55, as shown in Figure 3b. Figure 3c,d shows the effect of the T h . As T h increases from 0.8 to 1.2 mm, the effective dielectric constant and dielectric constant increase from 1.36 to 1.52 and from 1.47 to 1.69, respectively. According to the increase of T h , substrate infill percentage increases from 46 to 59 %, which is slightly larger than the change of infill percentage for the change in L h . Thus, the dielectric constant changes over a wider range by the change of T h . The dielectric constant is influenced by substrate height. Since the effect of the fringing field between the microstrip line and ground increases as height increases, the effective dielectric constant is decreased. As h sub increases from 0.75 to 1.25 mm, the effective dielectric constant and dielectric constant decreases from 1.48 to 1.42 and from 1.595 to 1.575, as shown in Figure 3e,f, respectively. The relation between the effective dielectric constant and dielectric constant of the microstrip line is given approximately by [25]: where H is the effective height of substrate and W is the effective width of microstrip line. Substrate infill percentage depends on the honeycomb unit cell and size and determines the dielectric constant of substrate. As the size increases and the thickness decreases, the substrate dielectric constant becomes similar to that of air, as shown in Figure 3a-d. As Lh increases from 2.4 to 3.2 mm, the infill percentage of honeycomb substrate decreases from 55% to 46%, and the effective dielectric constant is reduced from 1.45 to 1.43, as depicted in Figure 3a. The increase in Lh also leads to a decrease in dielectric constant whose range is 1.61-1.55, as shown in Figure 3b. Figure 3c,d shows the effect of the Th. As Th increases from 0.8 to 1.2 mm, the effective dielectric constant and dielectric constant increase from 1.36 to 1.52 and from 1.47 to 1.69, respectively. According to the increase of Th, substrate infill percentage increases from 46 to 59 %, which is slightly larger than the change of infill percentage for the change in Lh. Thus, the dielectric constant changes over a wider range by the change of Th. The dielectric constant is influenced by substrate height. Since the effect of the fringing field between the microstrip line and ground increases as height increases, the effective dielectric constant is decreased. As hsub increases from 0.75 to 1.25 mm, the effective dielectric constant and dielectric constant decreases from 1.48 to 1.42 and from 1.595 to 1.575, as shown in Figure 3e,f, respectively. The relation between the effective dielectric constant and dielectric constant of the microstrip line is given approximately by [25]: where H is the effective height of substrate and W is the effective width of microstrip line.   To determine the dielectric loss for the honeycomb substrate, the T-resonator method is used [26,27]. The stub length of the T-resonator can be obtained from: where n is the resonance index (n = 1, 3, 5, …), c is the speed of light in a vacuum, fn is the resonant frequency, and Lstub is the effective physical length of the resonating stub. Figure 4 shows the T-resonator with stub Lstub = 17.1 mm, microstrip feed length Lms = 70 mm, and width Wms = 3.8 mm. Lh = 2.5 mm, Th = 1 mm and hsub = 1 mm are used for T-resonator design on a honeycomb substrate. In EM simulations, the effective dielectric constant and tan δ of the honeycomb substrate are characterized. Figure 5a,b shows the transmission and reflection coefficients of the Tresonator on the substrate whose tan δ varies from 0.01 to 0.05, respectively. Thus, the effective dielectric constant and tan δ for the specified honeycomb substrate are determined to 1.6 and 0.035 at 3.5 GHz, respectively.   To determine the dielectric loss for the honeycomb substrate, the T-resonator method is used [26,27]. The stub length of the T-resonator can be obtained from: where n is the resonance index (n = 1, 3, 5, . . . ), c is the speed of light in a vacuum, f n is the resonant frequency, and L stub is the effective physical length of the resonating stub. Figure 4 shows the T-resonator with stub L stub = 17.1 mm, microstrip feed length L ms = 70 mm, and width W ms = 3.8 mm. L h = 2.5 mm, T h = 1 mm and h sub = 1 mm are used for T-resonator design on a honeycomb substrate. In EM simulations, the effective dielectric constant and tan δ of the honeycomb substrate are characterized. Figure 5a,b shows the transmission and reflection coefficients of the T-resonator on the substrate whose tan δ varies from 0.01 to 0.05, respectively. Thus, the effective dielectric constant and tan δ for the specified honeycomb substrate are determined to 1.6 and 0.035 at 3.5 GHz, respectively. To determine the dielectric loss for the honeycomb substrate, the T-resonator method is used [26,27]. The stub length of the T-resonator can be obtained from: where n is the resonance index (n = 1, 3, 5, …), c is the speed of light in a vacuum, fn is the resonant frequency, and Lstub is the effective physical length of the resonating stub. Figure 4 shows the T-resonator with stub Lstub = 17.1 mm, microstrip feed length Lms = 70 mm, and width Wms = 3.8 mm. Lh = 2.5 mm, Th = 1 mm and hsub = 1 mm are used for T-resonator design on a honeycomb substrate. In EM simulations, the effective dielectric constant and tan δ of the honeycomb substrate are characterized. Figure 5a,b shows the transmission and reflection coefficients of the Tresonator on the substrate whose tan δ varies from 0.01 to 0.05, respectively. Thus, the effective dielectric constant and tan δ for the specified honeycomb substrate are determined to 1.6 and 0.035 at 3.5 GHz, respectively.

Honeycomb SIW Design
Parametric studies regarding Lh, Th, hsub (see Figure 1) were performed to investigate insertion loss for SIW on the honeycomb substrate. Figure 6a shows the SIW geometry, with the SIW width ad = 47.3 mm and length Ld = 75 mm. Figure 6b shows the insertion losses for the SIW on honeycomb substrate regrading Lh and when Lh was 2.5, 2.8, and 3.1 mm. It is observed that the average insertion losses of the SIW were 1.69, 1.62, and 1.54 dB when Lh was 2.5, 2.8, and 3.1 mm, respectively. Figure  6c shows the insertion losses of the SIW with respect to Th. When Th was 0.8, 1.0, and 1.2 mm, the average insertion losses of the SIW were 1.66, 1.74 and 1.86 dB, respectively. Since the honeycomb unit cell's larger Lh and thinner Th (see Figure 1) encompasses more empty space, larger Lh and smaller Th were preferred for lower insertion loss. When hsub was 0.75, 1.0, and 1.25 mm, the average insertion losses of the SIW were 1.7, 1.69, and 1.68 dB, as shown in Figure 6d. Substrate height, hsub, did not significantly affect SIW insertion loss compared to size Lh and thickness Th.

Honeycomb SIW Design
Parametric studies regarding L h , T h , h sub (see Figure 1) were performed to investigate insertion loss for SIW on the honeycomb substrate. Figure 6a shows the SIW geometry, with the SIW width a d = 47.3 mm and length L d = 75 mm. Figure 6b shows the insertion losses for the SIW on honeycomb substrate regrading L h and when L h was 2.5, 2.8, and 3.1 mm. It is observed that the average insertion losses of the SIW were 1.69, 1.62, and 1.54 dB when L h was 2.5, 2.8, and 3.1 mm, respectively. Figure 6c shows the insertion losses of the SIW with respect to T h . When T h was 0.8, 1.0, and 1.2 mm, the average insertion losses of the SIW were 1.66, 1.74 and 1.86 dB, respectively. Since the honeycomb unit cell's larger L h and thinner T h (see Figure 1) encompasses more empty space, larger L h and smaller T h were preferred for lower insertion loss. When h sub was 0.75, 1.0, and 1.25 mm, the average insertion losses of the SIW were 1.7, 1.69, and 1.68 dB, as shown in Figure 6d. Substrate height, h sub , did not significantly affect SIW insertion loss compared to size L h and thickness T h .

Honeycomb SIW Design
Parametric studies regarding Lh, Th, hsub (see Figure 1) were performed to investigate insertion loss for SIW on the honeycomb substrate. Figure 6a shows the SIW geometry, with the SIW width ad = 47.3 mm and length Ld = 75 mm. Figure 6b shows the insertion losses for the SIW on honeycomb substrate regrading Lh and when Lh was 2.5, 2.8, and 3.1 mm. It is observed that the average insertion losses of the SIW were 1.69, 1.62, and 1.54 dB when Lh was 2.5, 2.8, and 3.1 mm, respectively. Figure  6c shows the insertion losses of the SIW with respect to Th. When Th was 0.8, 1.0, and 1.2 mm, the average insertion losses of the SIW were 1.66, 1.74 and 1.86 dB, respectively. Since the honeycomb unit cell's larger Lh and thinner Th (see Figure 1) encompasses more empty space, larger Lh and smaller Th were preferred for lower insertion loss. When hsub was 0.75, 1.0, and 1.25 mm, the average insertion losses of the SIW were 1.7, 1.69, and 1.68 dB, as shown in Figure 6d. Substrate height, hsub, did not significantly affect SIW insertion loss compared to size Lh and thickness Th. We used the Fused Deposition Modeling (FDM) Ultimaker 2 plus (Geldermalsen, The Netherlands) 3D printer to print the honeycomb substrate. The diameter of the 3D printer filament extrusion nozzle is 0.8 mm, and the layer resolution for the quick draft is 0.6 mm. Taking into consideration the printing limitations and the advantage of stable fabrication with the 3D printer Ultimaker 2, including the results for Lh and Th for the insertion loss, Lh = 2.5 mm and Th = 0.85 mm were used. Substrate height, hsub, was also considered to specify the honeycomb substrate dimension, since hsub was the effect on determining the characteristic impedance of the feeding line. In addition, a thinner SIW is preferred for the planar configuration. Therefore, hsub was set to 0.97 mm after considering the printing resolution. The final dimension of the honeycomb substrate provides an effective dielectric constant = 1.47 and tan δ = 0.03 at 3.5 GHz. Therefore, both dielectric constant and tangential loss were reduced compared to the PLA-filled substrate.
Based on the honeycomb substrate in Figure 1, we designed the SIW with a cut-off frequency of 2.53 GHz. To verify the insertion loss of the proposed SIW with the honeycomb, the transmission coefficient was simulated and compared with that of the SIW filled with air (empty) and solid PLA. Figure 7 shows that the average insertion losses from 3.4 GHz to 5.5 GHz were 0.04 dB, 2.96 dB, and 1.64 dB for air-filled, solid PLA, and honeycomb SIW, respectively. Table 1 compares the results of several simulated SIWs. The results demonstrate that the insertion loss can be reduced with a honeycomb structure. The insertion loss can be further reduced by minimizing the PLA frame thickness.  We used the Fused Deposition Modeling (FDM) Ultimaker 2 plus (Geldermalsen, The Netherlands) 3D printer to print the honeycomb substrate. The diameter of the 3D printer filament extrusion nozzle is 0.8 mm, and the layer resolution for the quick draft is 0.6 mm. Taking into consideration the printing limitations and the advantage of stable fabrication with the 3D printer Ultimaker 2, including the results for L h and T h for the insertion loss, L h = 2.5 mm and T h = 0.85 mm were used. Substrate height, h sub , was also considered to specify the honeycomb substrate dimension, since h sub was the effect on determining the characteristic impedance of the feeding line. In addition, a thinner SIW is preferred for the planar configuration. Therefore, h sub was set to 0.97 mm after considering the printing resolution. The final dimension of the honeycomb substrate provides an effective dielectric constant = 1.47 and tan δ = 0.03 at 3.5 GHz. Therefore, both dielectric constant and tangential loss were reduced compared to the PLA-filled substrate.
Based on the honeycomb substrate in Figure 1, we designed the SIW with a cut-off frequency of 2.53 GHz. To verify the insertion loss of the proposed SIW with the honeycomb, the transmission coefficient was simulated and compared with that of the SIW filled with air (empty) and solid PLA. Figure 7 shows that the average insertion losses from 3.4 GHz to 5.5 GHz were 0.04 dB, 2.96 dB, and 1.64 dB for air-filled, solid PLA, and honeycomb SIW, respectively. Table 1 compares the results of several simulated SIWs. The results demonstrate that the insertion loss can be reduced with a honeycomb structure. The insertion loss can be further reduced by minimizing the PLA frame thickness.  To measure the SIW, a microstrip-fed SIW was designed with a tapered transition, as shown in Figure 8. We designed a mode SIW that has , , field components. Since surface currents in transverse magnetic mode (TM) are interrupted by the via, only transverse electric mode ( ) can be supported in the SIW. Figure 9a-d shows the electric field distribution ( ), magnetic field distribution ( , ) and electric current distribution on the SIW, respectively. In addition, Figure 9d shows the electric current distribution on the SIW. The electric currents are uniformly distributed on the surface of the SIW conductor, and they are at their maximum at the side because of the shorted via. Since these field distributions of SIW are similar to the microstrip line, the fields can be matched, and the device reflection response is improved. Simulation results are discussed and compared with the measurement results in the following section. To measure the SIW, a microstrip-fed SIW was designed with a tapered transition, as shown in Figure 8. We designed a TE 10 mode SIW that has E z , H x , H y field components. Since surface currents in transverse magnetic mode (TM) are interrupted by the via, only transverse electric mode (TE m0 ) can be supported in the SIW. Figure 9a-d shows the electric field distribution (E z ), magnetic field distribution (H x , H y ) and electric current distribution on the SIW, respectively. In addition, Figure 9d shows the electric current distribution on the SIW. The electric currents are uniformly distributed on the surface of the SIW conductor, and they are at their maximum at the side because of the shorted via. Since these field distributions of SIW are similar to the microstrip line, the fields can be matched, and the device reflection response is improved. Simulation results are discussed and compared with the measurement results in the following section. To measure the SIW, a microstrip-fed SIW was designed with a tapered transition, as shown in Figure 8. We designed a mode SIW that has , , field components. Since surface currents in transverse magnetic mode (TM) are interrupted by the via, only transverse electric mode ( ) can be supported in the SIW. Figure 9a-d shows the electric field distribution ( ), magnetic field distribution ( , ) and electric current distribution on the SIW, respectively. In addition, Figure 9d shows the electric current distribution on the SIW. The electric currents are uniformly distributed on the surface of the SIW conductor, and they are at their maximum at the side because of the shorted via. Since these field distributions of SIW are similar to the microstrip line, the fields can be matched, and the device reflection response is improved. Simulation results are discussed and compared with the measurement results in the following section. To measure the SIW, a microstrip-fed SIW was designed with a tapered transition, as shown in Figure 8. We designed a mode SIW that has , , field components. Since surface currents in transverse magnetic mode (TM) are interrupted by the via, only transverse electric mode ( ) can be supported in the SIW. Figure 9a-d shows the electric field distribution ( ), magnetic field distribution ( , ) and electric current distribution on the SIW, respectively. In addition, Figure 9d shows the electric current distribution on the SIW. The electric currents are uniformly distributed on the surface of the SIW conductor, and they are at their maximum at the side because of the shorted via. Since these field distributions of SIW are similar to the microstrip line, the fields can be matched, and the device reflection response is improved. Simulation results are discussed and compared with the measurement results in the following section.

Microstrip-Fed SIW Fabrication and Measurement
To demonstrate the proposed SIW performance, we fabricated two samples of the microstripfed SIW with solid PLA and honeycomb substrate, as shown in Figure 10. The overall substrate length and SIW length of the two samples were the same, at 75 mm and 25 mm, respectively. To have the same cutoff frequency of transverse electric TE10 mode at 2.53 GHz, the SIW width of the two samples must be different, because the effective dielectric constants of the two substrates are different. Therefore, the SIW widths of the solid PLA and honeycomb substrates were 37.2 mm and 47.3 mm, respectively. It took 30 min to 3D-print the overall structure. Figure 8 shows the geometry of the microstrip-fed SIW with solid PLA and honeycomb substrates. The honeycomb geometry was designed in consideration of the minimum 3D-printing resolution, which is 0.1 mm. Conductive patterns are realized using copper tape, and Sub-Miniature version A (SMA) connectors are mounted using silver epoxy.
The simulation and measurement results for the two prototypes are shown in Figure 11. The measured average insertion loss with the fabricated honeycomb substrate is 1.38 dB from 3.4-5.5 GHz, while that with the fabricated solid PLA is 3.15 dB for the same frequency range. The simulated and measured insertion losses of SIW fabricated on the solid PLA substrate are 2.7 dB and 3.15 dB within the frequency range from 3.4-5.5 GHz, respectively; whereas those of the SIW fabricated on the honeycomb substrate are 1.81 dB and 1.38 dB from 3.4-5.5 GHz, respectively. The simulated and measured 10-dB bandwidth of the SIW fabricated on the solid PLA substrate are 4.65 GHz and 3.14 GHz, respectively; whereas those of the SIW fabricated on the honeycomb substrate are 4.56 GHz and 4.57 GHz. The simulation and measurement results show good agreement despite fabrication tolerance. Table 2 shows a performance comparison, in which the weight of the SIW with the honeycomb substrate is 1.72 g, while that of SIW with the solid PLA is 3.0 g. Therefore, the insertion loss and weight of the proposed SIW with the honeycomb substrate are reduced by 56% and 43%, respectively. In addition, the 10-dB impedance bandwidth is increased from 70% to 102% compared to the SIW with the solid PLA material. We proposed a 3D-printed SIW with honeycomb geometry which shows low insertion loss, although cheap plastic material is used. The 3D-printed SIW has low cost, light weight, and low loss compared to the PCB-based SIW. SIWs have been applied in antennas [28][29][30], circuits [31][32][33][34][35], and sensors [36][37][38]. The proposed work could be also used to various RF applications.

Microstrip-Fed SIW Fabrication and Measurement
To demonstrate the proposed SIW performance, we fabricated two samples of the microstrip-fed SIW with solid PLA and honeycomb substrate, as shown in Figure 10. The overall substrate length and SIW length of the two samples were the same, at 75 mm and 25 mm, respectively. To have the same cutoff frequency of transverse electric TE 10 mode at 2.53 GHz, the SIW width of the two samples must be different, because the effective dielectric constants of the two substrates are different. Therefore, the SIW widths of the solid PLA and honeycomb substrates were 37.2 mm and 47.3 mm, respectively. It took 30 min to 3D-print the overall structure. Figure 8 shows the geometry of the microstrip-fed SIW with solid PLA and honeycomb substrates. The honeycomb geometry was designed in consideration of the minimum 3D-printing resolution, which is 0.1 mm. Conductive patterns are realized using copper tape, and Sub-Miniature version A (SMA) connectors are mounted using silver epoxy.
The simulation and measurement results for the two prototypes are shown in Figure 11. The measured average insertion loss with the fabricated honeycomb substrate is 1.38 dB from 3.4-5.5 GHz, while that with the fabricated solid PLA is 3.15 dB for the same frequency range. The simulated and measured insertion losses of SIW fabricated on the solid PLA substrate are 2.7 dB and 3.15 dB within the frequency range from 3.4-5.5 GHz, respectively; whereas those of the SIW fabricated on the honeycomb substrate are 1.81 dB and 1.38 dB from 3.4-5.5 GHz, respectively. The simulated and measured 10-dB bandwidth of the SIW fabricated on the solid PLA substrate are 4.65 GHz and 3.14 GHz, respectively; whereas those of the SIW fabricated on the honeycomb substrate are 4.56 GHz and 4.57 GHz. The simulation and measurement results show good agreement despite fabrication tolerance. Table 2 shows a performance comparison, in which the weight of the SIW with the honeycomb substrate is 1.72 g, while that of SIW with the solid PLA is 3.0 g. Therefore, the insertion loss and weight of the proposed SIW with the honeycomb substrate are reduced by 56% and 43%, respectively. In addition, the 10-dB impedance bandwidth is increased from 70% to 102% compared to the SIW with the solid PLA material. We proposed a 3D-printed SIW with honeycomb geometry which shows low insertion loss, although cheap plastic material is used. The 3D-printed SIW has low cost, light weight, and low loss compared to the PCB-based SIW. SIWs have been applied in antennas [28][29][30], circuits [31][32][33][34][35], and sensors [36][37][38]. The proposed work could be also used to various RF applications.

Conclusions
A low-loss and lightweight SIW is proposed using a 3D-printed honeycomb substrate. The proposed microstrip-fed SIW is compared to microstrip-fed SIW with solid PLA. The insertion loss of the SIW with a honeycomb substrate is reduced from 3.15 dB to 1.38 dB and the weight is reduced from 3 g to 1.7 g. Additionally, a wider fractional bandwidth (FBW) of 102% is achieved with the proposed structure. In addition, the advantages mentioned above of the proposed honeycomb SIW will be useful for space applications requiring light weight. However, the minimum resolution of 3D printing technology is higher than the conventional lithography fabrication process, and the postprocessing for the conductive pattern is required in this work. In addition, the maximum frequency is limited because the dielectric loss is higher in high frequencies. Nevertheless, it is acceptable in the sub-6 GHz spectrum. With the advance of 3D printing technology, if a high-performance 3D printer is used, the resolution can be minimized, and metallic patterns can also be 3D printed. In addition, if low-loss filaments are developed in the future, the operation frequency can be increased.

Conclusions
A low-loss and lightweight SIW is proposed using a 3D-printed honeycomb substrate. The proposed microstrip-fed SIW is compared to microstrip-fed SIW with solid PLA. The insertion loss of the SIW with a honeycomb substrate is reduced from 3.15 dB to 1.38 dB and the weight is reduced from 3 g to 1.7 g. Additionally, a wider fractional bandwidth (FBW) of 102% is achieved with the proposed structure. In addition, the advantages mentioned above of the proposed honeycomb SIW will be useful for space applications requiring light weight. However, the minimum resolution of 3D printing technology is higher than the conventional lithography fabrication process, and the postprocessing for the conductive pattern is required in this work. In addition, the maximum frequency is limited because the dielectric loss is higher in high frequencies. Nevertheless, it is acceptable in the sub-6 GHz spectrum. With the advance of 3D printing technology, if a high-performance 3D printer is used, the resolution can be minimized, and metallic patterns can also be 3D printed. In addition, if low-loss filaments are developed in the future, the operation frequency can be increased.

Conclusions
A low-loss and lightweight SIW is proposed using a 3D-printed honeycomb substrate. The proposed microstrip-fed SIW is compared to microstrip-fed SIW with solid PLA. The insertion loss of the SIW with a honeycomb substrate is reduced from 3.15 dB to 1.38 dB and the weight is reduced from 3 g to 1.7 g. Additionally, a wider fractional bandwidth (FBW) of 102% is achieved with the proposed structure. In addition, the advantages mentioned above of the proposed honeycomb SIW will be useful for space applications requiring light weight. However, the minimum resolution of 3D printing technology is higher than the conventional lithography fabrication process, and the post-processing for the conductive pattern is required in this work. In addition, the maximum frequency is limited because the dielectric loss is higher in high frequencies.
Nevertheless, it is acceptable in the sub-6 GHz spectrum. With the advance of 3D printing technology, if a high-performance 3D printer is used, the resolution can be minimized, and metallic patterns can also be 3D printed. In addition, if low-loss filaments are developed in the future, the operation frequency can be increased.