Although the design of high-gain, multi-band antenna discussed in this paper is small in size and light in weight, it can receive and transmit GPS and WLAN signals, and can cover the three frequency bands of WLAN. 

For small antennas, it is usually impossible to obtain high gain. 

However, in satellite communication applications, antennas must be designed to be small and light, and can provide beamforming, broadband and polarization purity. 

In the antenna design for multi-band global positioning system (GPS) and wireless local area network (WLAN), it is possible to design a small, lightweight antenna with polarization diversity and high gain. 

For example, for GPS applications, an antenna may be required to handle both the low-frequency band of 1.226GHz and the high-frequency band of 1.575GHz. 

For IEEE 802.11a/b/g WLAN applications, the antenna must operate in both 2.4GHz and 5GHz bands, and the bandwidth must support data rates of 11 Mbps and 54 Mbps. 

Other applications include planned Air Force satellite systems in the 1.8GHz and 2.25GHz bands. 

For a single antenna covering multiple wireless bands, the coverage from 1.8GHz to 2.1GHz should also be considered for third-generation (3G) cellular systems. 

Polarization is an important feature for a successful antenna design. 

For space applications, circular polarization (CP) is usually used, such as right-handed circular polarization (RHCP) or left-handed circular polarization (LHCP), for transmitting, receiving and multiplexing in the same spectrum range to increase system capacity. 

Although most WLAN systems require linear polarization, the use of circular polarization will eventually become an advantage of mobile systems. 

Some theoretical limitations determine how small the antenna can be when providing the required gain and bandwidth. 

For space-based (satellite) applications, the antenna is required to adapt to a certain waveform coefficient. The polarization direction of the antenna is circular polarization and works on the uplink of 1.8GHz (the receiving frequency of the satellite) and the downlink of the 2.25GHz (the transmitting frequency of the satellite). 

Beamforming capability is also a key requirement, which allows satellites to maintain communication at different positions and angles. 

The antenna must be strong enough to withstand shock and vibration, a temperature environment (which usually ranges from-40 ℃ to + 70 ℃), and power flicker shocks. 

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 the difficulty of achieving the required performance on a smaller physical size. 

After considering several unconventional methods, annular radiator technology was selected as a possible solution. 

Compared with other schemes, this scheme uses a resonant structure to effectively lengthen 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 loop antenna can be reduced, regardless of its complexity. 

In a simple rectangular patch antenna, the two slots at both ends of the patch can be used as radiation sources with an interval of about 1/2 wavelengths. 

If the length of each notch is about 1/2 wavelengths, the 2.1dBi gain can be obtained. 

Any such two antennas operating as binary arrays can theoretically provide additional 3dB gain. 

Therefore, a simple patch antenna should be able to achieve 5.1dBi gain. 

After some improvements, a better gain or waveform may even be obtained, depending on the type of grounding plane or resonant mode. 

For ring antennas, they can be designed as multi-harmonic structures, and these resonators can be separated or coupled for multi-band or broadband applications. 

By adjusting the phase of each mode, make them work in a predetermined way, so that in the far field in the appropriate direction, high gain and beamforming can be achieved through phase superposition and cancellation. 

In most cases, these structures may achieve 9dBic gain (theoretical value) and 17% bandwidth. 

In theory, 15%, 20% and 30% bandwidth can be achieved corresponding to the voltage standing wave ratio (VSWR) of 1.50, 2.0 and 3.0, respectively. 

Unfortunately, it is impossible to find a system design method that can meet the required physical and electrical performance on all frequencies.