## 2 Antenna Design and Performance
### Single P-Band Dipole Antenna Design and Performance  
We designed a dual-band dipole antenna for wide-bandwidth operation in the P-band, as shown in Figure 7a. Two dipole arms are printed on the opposite sides of a 1.6 mm thick FR4 substrate. As shown in Figure 7b,c, the surface currents are concentrated on the long and short dipole arms at lower and higher frequency bands, respectively. As shown in Figure 7a, there is a gap (\(g\)) between the dipole arms; optimizing this gap is important for impedance matching for dual-band operation [10]. Figure 7d shows the simulated reflection coefficient performances for various \(g\) values, and clear dual-band operation was achieved when \(g\) was 0.4 mm. When \(g\) is 0.4 mm, the simulated 10 dB \(|\)S\({}_{11}\)\(|\) covers from 513.09 to 778.51 MHz. Figure 7e shows the simulated radiation patterns at 580 MHz when \(g\) is 0.4 mm. The simulated radiation gain is 3.29 dBi at \(\theta\) = 0\({}^{\circ}\) and FBR is 3.94 dB. Figure 6: Assembled 2 \(\times\) 2 patch antenna array: (**a**) geometry of antenna array, (**b**) perspective view, (**c**) simulated reflection coefficient performances, (**d**) simulated radiation patterns at 35 GHz when Port 1 is excited, and (**e**) simulated radiation patterns at 35 GHz when Port 2 is excited. As shown in Figure 8a, we changed the feeding structure from planar to orthogonal to minimize the antenna size (a planar feeding structure utilizes the antenna shown in Figure 7a). The dipole arm on the back side is connected to the microstrip line by a metal via-pin. As shown in Figure 8b, even if the feeding structure is changed, the dual-band characteristics are still maintained. Additionally, the radiation performance of the dipole antenna remains almost the same, as shown in Figure 8c. Figure 7: Single dipole antenna: (**a**) geometry of antenna, (**b**) simulated surface current density at 528 MHz, (**c**) simulated surface current density at 667 MHz, (**d**) simulated reflection coefficient performances for various \(g\) values, and (**e**) simulated radiation patterns at 580 MHz (\(g=0.4\) mm). Although the dual-band characteristics were achieved, the 10 dB \(|\)S\({}_{11}\)\(|\) performance was not sufficient to cover the target band (450-730 MHz) of the P-band. Therefore, additional bandwidth enhancement was required. Bandwidth enhancement to the lower frequency band of the dipole antenna can be easily achieved by changing the structure of the dipole arm to a half bow-tie structure without changing the length of the dipole arms [11]. Since the path length of the surface current will be extended inside the conductor, the operating band moves toward the lower frequency band. As shown in Figure 9a, the shape of the long dipole arms was changed from flat to half bow-tie. The slope of the half bow-tie shape was determined by the value of \(d_{1}\), and Figure 9b shows the simulated \(|\)S\({}_{11}\)\(|\) performances for various \(d_{1}\) values. As \(d_{1}\) is increased, the resonance frequency around 528 MHz moves toward the lower frequency band. As shown in Figure 9c, additional bandwidth enhancement can be achieved by changing the shape of the short dipole arms from flat to half bow-tie with various \(d_{2}\) values. In this design procedure, the \(d_{1}\) value is fixed at 20 mm. As shown in Figure 9d, optimized dual-band operation is achieved when \(d_{2}\) is 6 mm. When \(d_{2}\) is 6 mm, the simulated \(|\)S\({}_{11}\)\(|\) band covers from 440.92 to 787.96 MHz, and it fully covers the target band of the P-band. The reason that there is a remarkable change in the lower frequency band compared to the higher frequency band, despite changing the short dipole structure, is that proper electromagnetic coupling occurs as the spacing between the upper and lower half bow-tie dipole arms changes with various \(d_{2}\) values. As a current is induced in the long half bow-tie dipole arms by the generated electromagnetic coupling, the operating frequency in the lower band is shifted.