## 2 Proposed Antenna Design
### Fabrication and Measurement  
Figure 3 shows photographs of the fabricated 5G monopole antenna with the parasitic elements and the lattice-structure reflector on vehicle window glass. In general, glass antennas are fabricated or printed on vehicle glass using a silk-screen method [17] or an epoxy resin adhesive. However, such methods have low conductivity in the electrical connection, poor manufacturing tolerance, and low thermal durability in the fabrication of the glass antenna. To solve these problems, the proposed antenna is fabricated by etching the copper layer and employing a thermosetting adhesive to strongly stick the copper patterns to vehicle window glass. The proposed antenna is directly fed by the K-type (2.92 mm) connector, and the inner and outer conductors of the feeder are electrically connected to CPW by soldering. Note that the actual K-type connector is modeled and excited by using a waveguide port at the starting points of the inner and outer conductors to feed the CPW in the simulation conditions. On the bottom of the vehicle window glass, which is electrically isolated from the antenna feeder, the lattice-structure reflector patches are printed. To confirm feasibility, the fabricated antenna is measured in a full anechoic chamber to obtain the antenna characteristics, such as gains, reflection coefficients, and radiation patterns. To minimize the cable effects, a jig with a low dielectric constant is used to stand the proposed antenna in the air of the anechoic chamber, where the cable is connected to the K-type connector with the jig. In addition, the thick vehicle window glass substrate is measured to obtain the specific EM characteristics according to the frequency of the measurement setup, as shown in Figure 4. The measured dielectric constant (\(\varepsilon_{r}\)) and loss tangent (\(tan\delta\)) are 6.9 and 0.017 at 28 GHz, respectively, and then the measured  
Figure 2: The actual connector model with a waveguide port in the CST EM simulation software: (**a**) a feeding connection part; (**b**) a waveguide port excitation part. results are employed as input parameters to simulate the proposed antenna. Figure 5 presents the measured (solid line) and simulated (dashed line) reflection coefficients of the proposed antenna. The measured reflection coefficients agree well with the simulated results, and the values at 28 GHz are \(-\)20 dB and \(-\)15.6 dB, respectively. Figure 6 presents the bore-sight gain of the proposed antenna with and without parasitic elements. The proposed antenna with the parasitic elements has measured and simulated bore-sight gains of 2 dBi and 1.6 dBi at 28 GHz, while the simulation result of the vehicle window glass antenna without the parasitic elements is 0.3 dBi. In addition, the gain bandwidth over 0 dBi increases from 1.7 GHz to 3 GHz, according to the existence and absence of the proposed parasitic elements. The gain of the proposed antenna seems to be slightly lower than that of conventional monopole antennas due to the high dielectric loss of the thick vehicle glass. It results in the radiation efficiency of 33.68% at 28 GHz, but this radiation efficiency can be enhanced when the dielectric loss of the antenna substrate becomes lower. Figure 7 illustrates the measured and simulated radiation patterns in _zx_- and _zy_-planes at 28 GHz and 29 GHz, where the solid line and dashed line indicate the measured and simulated results, respectively. The measured half power beam widths are 42\({}^{\circ}\) and 64\({}^{\circ}\) in the _zx_- and _zy_-planes at 28 GHz. The measured maximum gains are 2.3 dBi and 2.1 dBi at angles of \(-\)25\({}^{\circ}\) and \(-\)1\({}^{\circ}\) in the _zx_- and _zy_-planes at 28 GHz, respectively. The measured radiation patterns are in good agreement with the simulated radiation patterns at 29 GHz. Figure 8 illustrates the 3D radiation patterns of the proposed antenna according to the frequency to observe the maximum gains.