## 4 Design of a Single Dipole Antenna  
In this section, we present the design of a single dipole antenna with parasitic monopoles. This step is most important since the bandwidth and beamwidth characteristics are largely determined by the dipole antenna and parasitic monopoles. Figure 5 shows the structure used in the design of a single dipole. Dimensions of Dipole \(A\) are determined by iterative parametric analysis. Dipole \(B\)'s dimensions are the same as those of Dipole \(A\) in Figure 5a except for the slot-microstrip coupling region. Dipole \(B\) in Figure 5b is presented for the purpose of comparison with Dipole \(A\). Using only a single element in the crossed dipole design is valid since the presence of the other orthogonally placed dipole has only a small effect due to a large isolation between the two dipoles. In the simulation, an infinitesimal or delta-gap current source ('Discrete Port' in CST Studio) is used between the input end of the microstrip line and the ground plane. Figure 6 shows the dimensional parameters in the dipole design. Figure 6a shows a plan view of two crossed dipoles installed on a circular ground plane along with \(x\) and \(y\) axes of the rectangular coordinate system for far-field pattern angle reference. Figure 6b-d shows the dimensional parameters of the dipole and the feed line. Dimensional parameters of Dipole \(A\) without Dipole \(B\) are iteratively adjusted using the structure of Figure 5a for broadband impedance matching and rotationally symmetric gain patterns over large elevation angles. When Dipole \(B\) is added to form crossed dipoles, its dimensions are set identical to those of Dipole \(A\) except for \(C_{1B}\), the position of the slot-strip coupling. Figure 4: Quadrature hybrid coupler (QHC): (**a**) back-side view; (**b**) front-side view. Figure 5: Structure for the design of a single dipole antenna. (**a**) Dipole \(A\) and (**b**) Dipole \(B\). The dipole length \(L\) determines the center frequency of operation, while the dipole width \(A_{1}\) determines the bandwidth. The value of \(A_{1}\) should be large enough for broadband operation. The gain pattern in the elevation plane is sensitive to the dipole height from the ground plane, whose value is to be iteratively adjusted while monitoring the elevation gain pattern over the frequency range of operation. In the initial stage, the diameter of the ground plane is made the same as the length of Dipole \(A\) and is later adjusted together with crossed Dipole \(A\) and Dipole \(B\) for the wide axial ratio beamwidth. The use of the monopole is critical to achieving a rotationally symmetric gain pattern since with a dipole alone the gain pattern will be highly asymmetric in \(E\)- and \(H\)-planes, resulting in a large axial ratio. The dipole height (\(A_{2}+A_{3}\)) determines the height \(A_{3}\) of the monopole. The current in the parasitic monopole is induced by capacitive coupling via the gap \(A_{2}\). For optimum coupling, the gap \(A_{2}\) and the width \(A_{4}\) are iteratively adjusted. The base width \(B_{3}\) of the monopole needs to be large enough for broadband operation. The position of the monopole's vertical edge is relatively insensitive to the impedance matching, gain pattern, and axial ratio as far as it is near the end of the dipole arm. Therefore, the monopole is placed such that the monopole's vertical edge is aligned with the dipole tip. The dipole is fed by using a 50-ohm microstrip line with an integrated balun. The slot width \(G\) determines the characteristic impedance of the slotline and is not sensitive to the impedance matching as far as it is not too large, so it is set to 3.0 mm, three times the  
Figure 6: Dimensional parameters of the dipole and the feed line: (**a**) ground plane and dipole placement; (**b**) dipole and monopole; (**c**) feed line for Dipole \(A\); (**d**) feed line for Dipole \(B\). substrate thickness. With this value, the characteristic impedance of the uniform slotline, i.e., without the other dipole's substrate, is 132 \(\Omega\).