# Study of Global Navigation Satellite System Receivers' Accuracy for Unmanned Vehicles
## 2 Materials and Methods
### Helix Antenna Design  
For the purpose of this study, a passive helix antenna has been selected, due to its good polarization properties and simplicity of construction. A similar approach in an identical study is considered by [28]. The helix antenna consists of a spiral of wire fed by a coaxial line. The inner conductor of this line is connected to the wound conductor (through a matching section) and the shield to a metallic disk (reflector). The purpose of the reflector is to direct the radiation into one hemisphere and increase the gain, to reduce currents on the outer conductor of the coaxial line and to reduce input impedance fluctuations. The diameter of the disk is typically chosen in the range of order of (0.7 to 2) times diameter of the helix. Some design highlights and relations can be found in [29]. Helix antenna can operate in axial or normal mode of radiation. The former is present when the length of the wire loops is approximately similar to the wavelength and the pitch angle is between 10 and 15 deg. This creates the phase condition for the summation of radiated waves in a forward direction. The later mode is present when the loop length is much less than the wavelength. Typical applications of this type of helix antenna are handheld devices, wireless routers, etc. The design considerations for axial-mode, uniform-diameter antenna are led by the requirements of reasonable gain (at least several decibels) and minimal side lobes, while keeping the return loss within reasonable limits. Since only a few parameters can be modified--the helix diameter, pitch and overall length--it is possible to achieve a limited control over these antenna parameters. By changing the pitch and diameter, the propagation constant of the propagating wave, the phase angle of the current at each turn can be controlled. Since the helical antenna can be regarded as an end-fire antenna array, the same design and pattern synthesis principles are applicable, but with a much lesser degree of freedom. The optimization is performed with a 4 NEC2 Method of Moments simulator (open source) and confirmed with Ansys HFSS (FEM). An analytical solution is proposed in [30]; however, due to the complexity of the problem, the use of electromagnetic simulators is highly recommended. In general, there are multiple requirements for GNSS UAV antennas, divided into three major groups: mechanical, electrical and procurement. In relation to the first group: low profile (good aerodynamics), low mass, predictable center of gravity, low complexity of  
Figure 3: Schematic of the system design. the design and rigid construction. Related to the second one: broad main lobe, phase center position stability, minimal side lobes (to reduce noise and interference), positive realized gain, and well-defined and stable Rx frequency bands in terms of return loss. Related to the third one: low cost of production and capability for mass production. Since the popular ceramic patch antennas often deviate in their electrical performance, an additional helix antenna, as a reference, is considered in this study. Assuming the above, an axial mode helix antenna was designed and fabricated. Design properties and dimensions are as follows (in mm) (Figure 4): ground plate disk diameter: 300 mm, number of turns: 6, helix diameter: 65 mm, helix spacing (pitch): 47 mm, wire diameter: 2.3 mm (4 mm\({}^{2}\) wire gage). Figures 5 and 6 represent the antenna gain of the antenna for 1.2 GHz and 1.575 GHz, respectively. The achieved gain is 11.3 dBic and 12.5 dBic. The cross-polarization properties are presented in Figure 7. It is evident that within the main beam, the XPD varies between \(-20\) dB for the boresight to \(-15\) dB in a cone of \(\pm 30\) deg. Although cross-polarization discrimination is not critical for GNSS applications, for the purpose of the study and methodology purity, such a high value is desirable. The matching of the helix antenna is performed by bending the conductor that is closest to the connector toward the ground plane.