## 3 Antenna Structure
For an amplitude ratio of less than 2 dB, a deviation of 15 degrees from the 90\({}^{\circ}\) phase difference can be tolerated for an axial ratio of less than 3 dB. The proposed antenna is designed using an FR-4 substrate with a dielectric thickness of 1.0 mm, a relative permittivity of 4.3, a loss tangent of 0.025, and a conductor thickness of 0.034 mm. Two crossed dipoles are excited in phase quadrature for circular polarization. The output terminals of the QHC are connected to two coaxial cables, one for RHCP and the other for LHCP (left-hand circular polarization) operation. The dipole arms are printed on two orthogonally oriented vertical circuit boards, which host a microstrip feed line with an integrated balun as well. Two parasitic monopoles of trapezoidal shape are printed under  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Axial Ratio (dB)**} \\ \cline{2-5}
**Arg(\(E_{\theta}\)) \(-\) Arg(\(E_{\varphi}\))** & \multicolumn{4}{c}{\(\left\lvert\,E_{\theta}\,\right\rvert\) 1 2 1 2 3 3 0.00 1.52 1.83 2.52 3.38 2.51 3.06 3.81 2.53 3.92 4.05 4.77 4.88 5.21 5.71 5.  
the dipole arm to improve the symmetry of the dipole's \(E\)- and \(H\)-plane gain patterns, thus improving the axial ratio, and to increase the antenna gain at low elevation angles in the dipole's \(E\)-plane. The short-circuited part of the slotline and the monopole are electrically connected at the base to the ground plane of a circular printed circuit board (PCB). In a real implementation, to accurately position the dipole PCBs, small rectangular via holes can be constructed on the ground plane PCB such that the via hole width is slightly larger than the thickness of the dipole PCB. Metal-clad legs with dimensions corresponding to the rectangular via hole are added to the bottom of the dipole PCB. Mechanical fastening of the ground plane PCB and the dipole PCB can be performed by inserting the metal-clad legs in the rectangular via holes and soldering them at suitable points. These structures are not shown in Figure 1 for clarity. In feeding a dipole, it is important to make currents on two arms of the dipole be balanced (the same magnitude and phase). This is performed by a microstrip-to-slotline transition with an integrated balun. The quadrature-phase feeding is enabled by using a broadband QHC, whose output arms are connected to the input of the dipole feed line. The QHC enables the feeding of the orthogonal dipole arms with equal magnitude and a 90-degree phase difference. A three-branch-line type is employed to obtain a bandwidth large enough for the proposed antenna. A metal coating on the top side of the circular PCB provides a ground plane (GP) for the dipole/monopole operation as well as that for the QHC. A circular-shaped GP is chosen for the angular symmetry of the gain and axial ratio pattern. The diameter of the GP is critical in the realization of the wide axial ratio beamwidth since a large-sized GP will short-circuit the tangential component of the dipole's electric field while doubling the vertical component, resulting in the deterioration of the axial ratio. A self-assembled PCB-based structure shown in Figure 1 of the dipole, monopole, feed line, and QHC results in a compact low-cost antenna. Compared with some of the existing designs, the following are among the novelties of the proposed antenna:  
1. The antenna provides decent gain and axial ratio (AR) performance over the entire angle in the upper hemisphere at frequencies covering all of the worldwide GNSS services currently in operation. 2. No additional structure for the assembly of the dipole PCB and the feed network PCB is required. 3. The design employs a simple structure amenable for low-cost fabrication. 4. It offers the ability of dual-purpose use for worldwide GNSS (RHCP) and \(L\)-band satellite phone (LHCP; 1.525-1.661 GHz; Iridium, Inmarsat, and Thuraya). Existing works do not present gain and AR characteristics at all angles in the upper hemisphere.