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

　　Some theoretical limitations determine how small the antenna can be when providing the required gain and bandwidth. For space (satellite) based applications, the antenna is required to adapt to a certain waveform coefficient. The polarization direction of the antenna is circular polarization, and it operates on the 1.8GHz uplink (satellite receiving frequency) and 2.25GHz downlink (satellite transmitting frequency).

　　Beam forming capability is also a key requirement, which allows satellites to maintain communications at different locations and angles. The antenna must be strong enough to withstand shock and vibration, temperature environment (temperature variation range is usually between - 40 ℃ and+70 ℃) and power flicker impact.

　　Several options are considered in the design, including spiral antenna, four leaf spiral antenna (QFHA) and various microstrip patch structures. The initial analysis and electromagnetic (EM) software simulation results show how difficult it is to achieve the required performance on a smaller physical size.

　　After considering several non-traditional methods, the annular radiator technology was selected as a possible solution. Compared with other schemes, this scheme uses a resonant structure to effectively extend the path length of the radiation current (to achieve high gain), while the antenna is reduced by 25% to 35%.

　　This technology can meet the requirements of waveform coefficient, and can achieve higher gain than larger microstrip patch antenna or resonant cavity spiral antenna.

　　Compared with the more understandable design and analysis methods for microstrip patch antennas, the design and analysis of ring antennas require very empirical design (and empirical speculation). Fortunately, by performing a detailed initial design and analysis process and carefully studying the EM simulation results, the design risk of the ring antenna can be reduced, regardless of its complexity.

　　In a simple rectangular patch antenna, the two notches at both ends of the patch can be used as radiation sources, with a spacing of about half the wavelength. If the length of each notch is about half of the wavelength, 2.1dBi gain can be obtained. Any such two antennas working as a binary array can theoretically provide an additional 3dB of gain.

　　Therefore, a simple patch antenna should be able to achieve 5.1 dBi gain. After some improvements, even better gain or waveform may be obtained, depending on the type of ground plane or resonant mode.

　　The ring antenna can be designed as a multi harmonic structure, and these resonators can be separated or coupled to be suitable for multi frequency or broadband applications.

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