# Low Cost AIP Design in 5G Flexible Antenna Phase Array System Application
## 4 Antenna Manufacturing and Experimental Measurement  
Progressive and lower loss materials were used to design a millimeter-wave antenna, such as Rogers (RO 4003C or RO 4350B), low temperature co-fired ceramics (LTCC), PTFE, and liquid crystal polymer (LCP). The manufacturing process of these novel kinds of materials is complex and their manufacturing costs are very expensive. The FR4 substrate has a lower cost compared to the other kinds of materials. The material cost of a Rogers material is three to five times more expensive than that of an FR4 material. Furthermore, the choice of the manufacturer, manufacturing quantity, design metal layers, and ordering options also affect the overall cost of the whole process. On the other hand, a low loss material process is 100 times more expensive than the manufacturing cost of a traditional FR4 PCB. However, the loss tangent of the low cost FR4 material is 0.01-0.04 at a frequency band of 26.5-29.5 GHz, which restricts the performance of the antenna. The gain of the antenna that is designed on an FR4 substrate is approximately 4.5 dBi. The performance of the antenna that is designed on an FR4 substrate can be enhanced by using the air-filled cavity structure. The antenna module proposed in this study was designed with a stack of three substrates and four metal layers (M1, M2, M3, and M4) as illustrated in Figure 13. The production process started by completing the circuit etching of the middle layer (M2 and M3) followed by the addition of two layers of PP (PP_1 and PP_2) on top and below the middle layer. During this step, the upper and lower materials of M2 (PP_1 and FR4_2) were laser precut as shown in the figure. The purpose of the laser precut is to leave a cutting path that will be used as a guide for the removal of the center substrate area later in the process. The next step was the lamination of M1 and M4, and the circuit etching for both metal layers. This was followed by creating laser holes from M1 to M2 and M3 to M4, and finally from M1 to M4. The final step involved mechanical drilling at the M4 surface towards the laser precut. Once the holes from the M4 surface to the laser precut were properly drilled and aligned, the center substrate could be removed therefore exposing the area of the entire air cavity. The key point of the process technology is on the air-filled cavity structure. The tolerance of each air-filled cavity must be made as small as possible. If the tolerance turned out to be significantly large, it will lead to a significant difference in the gain of each patch antenna. In turn, the performance of the array will be affected. In addition, the reserved M1 layer and its supporting material FR4_1 must be designed to be thin in order to have a lossless air-filled cavity. Moreover, if the air-filled cavity is too large in terms of area, it will have an impact on the antenna gain due to the changed distance of the patch relative to the ground. Figure 12: Simulation results of the beam steering pattern at 28 GHz (Y-cut). The proposed array antenna was manufactured on an FR4 substrate. Figure 14 shows a photograph of the array antenna assembly. The measured results of the return loss for each port were better than 10 dB at an operating frequency band of 26.5-29.5 GHz. The comparison of the simulation and empirical results are presented in Figure 15. The empirical results are shown to satisfy the requirement of a 5G system millimeter wave band. Figure 14: Photograph of the array antenna (\(2\times 2\)). Figure 13: Manufacturing process of the antenna with an air-filled cavity. Figure 16 shows an NSI-700S-360 antenna chamber [26]. Its measurement coordinates are shown in Figure 17. The gain measurement results of each patch antenna are shown in Figure 18 (X-cut) and Figure 19 (Y-cut). The maximum gains were 8.58 dBi for Patch 1, 8.47 dBi for Patch 2, 8.49 dBi for Patch 3, and 8.64 dBi for Patch 4 in the X-cut. The maximum gain was 8.5 dBi for each Patch antenna in the Y-cut.