Polarization Design of GPS

Polarization is an important characteristic for a successful antenna design. For space applications, circular polarization (CP), such as right-hand circular polarization (RHCP) or left-hand circular polarization (LHCP), is often used for transmission, reception, and multiplexing within the same spectrum to increase system capacity. Although most WLAN systems require linear polarization, the use of circular polarization will eventually become an advantage for mobile systems.

Certain theoretical limits determine how small an antenna can be while still providing the required gain and bandwidth. For space-based (satellite) applications, an antenna with a certain form factor is required, with circular polarization, operating at 1.8 GHz uplink (the satellite's receive frequency) and 2.25 GHz downlink (the satellite's transmit frequency).

Beamforming capability is also a key requirement, allowing the satellite to maintain communications at different locations and angles. The antenna must be rugged enough to withstand shock and vibration, temperature environments (temperature variations typically range from -40°C to +70°C), and power flicker shocks.

Several options were considered, including a helical antenna, a quadrilobate helical antenna (QFHA), and various microstrip patch structures. Initial analysis and electromagnetic (EM) software simulation results demonstrated the difficulty of achieving the required performance in a small physical size.

After considering several non-traditional approaches, the ring radiator technology was chosen as a possible solution. This solution uses a resonant structure to effectively increase the path length of the radiating current (achieving high gain) while reducing the antenna size by 25% to 35% compared to other solutions.

This technology can meet the form factor requirements and achieve higher gain than larger microstrip patch antennas or resonant cavity helical antennas.

Compared to the more well-understood design and analysis methods for microstrip patch antennas, the design and analysis of loop antennas requires considerable empirical design (and empirical guesswork). Fortunately, by performing a detailed initial design and analysis process, and carefully studying the EM simulation results, the design risk of loop antennas, regardless of their complexity, can be reduced.

In a simple rectangular patch antenna, the two slots at each end of the patch can be used as radiating sources, separated by about half a wavelength. If each of the slots is about half a wavelength long, 2.1dBi gain can be obtained. Any two such antennas operating as a two-element array can theoretically provide an additional 3dB of gain.

So a simple patch antenna should be able to achieve 5.1dBi gain. With some refinement, even better gain or waveforms may be possible, depending on the ground plane type or resonant mode.

For the loop antenna, a multi-harmonic structure can be designed, and these resonators can be separated or coupled to be suitable for multi-frequency or broadband applications.

By adjusting the phase of each mode so that they work in a predetermined manner, high gain and beamforming can be achieved through phase superposition and decomposition in the far field in the appropriate direction. In most cases, these structures can achieve a gain of 9dBic (theoretical value) and a bandwidth of 17%.

Theoretically, bandwidths of 15%, 20%, and 30% can be achieved for voltage standing wave ratios (VSWR) of 1.50:1, 2.0:1, and 3.0:1, respectively. Unfortunately, it is not possible to find a system design approach that will meet the required physical and electrical performance at all frequencies.

However, with some effort, it is possible to find a design approach that meets the technical requirements of certain specific working modes.