High gain is not usually associated with a compact antenna. For satellite-communications applications, however, an antenna design must be small and light, yet provide pattern shaping, wide bandwidth, and polarization purity. In developing an antenna for multiband Global Positioning System (GPS) and wireless-local-area-network (WLAN) use, it was possible to create a small, lightweight design with polarization diversity and high gain.

For GPS applications, for example, an antenna may be required to handle both the low band at 1.226 GHz and the high band at 1.575 GHz. For IEEE 802.11a/b/g WLANs, an antenna must operate in both the 2.4- and 5-GHz bands with bandwidth to support data rates at 11 and 54 Mb/s. Additional applications include the planned Air Force satellite systems at 1.8 and 2.25 GHz. For a single antenna to cover multiple wireless applications, coverage at 1.8 through 2.1 GHz should also be considered for third-generation (3G) cellular systems.

Polarization is an important characteristic in a good antenna design. For space applications, it is customary to use circular polarization (CP), such as the right-handed circular polarization (RHCP) or left-handed circular polarization (LHCP), either for transmit and receive or reuse of the same spectrum band for added capacity. Although most WLAN systems require linear polarization, the use of CP will eventually becomes advantageous for mobile systems.

Certain theoretical limits dictate how small an antenna can be made while still providing required gain and bandwidth. For the space-based (satellite) application, the antenna design was required to fit a certain form factor while operating with CP at 1.8 GHz for the uplink (the satellite's receive frequency) and 2.25 GHz for the downlink (the satellite's transmit frequency). Pattern-shaping capability was also a key requirement, to allow the satellite to maintain communication while in different positions and angles. The antenna must be rugged enough to withstand high shock and vibration, environmental/thermal extremes (temperature swings of typically −40 to +70°C), and power cycling. Several options were considered for the design, including a helical antenna, quadrifilar helical antennas (QFHAs), and various microstrip patch configurations. Initial analysis and electromagnetic (EM) software simulations indicated the difficulty of achieving the required performance levels in the small physical size.

After considering several unconventional approaches, a ring radiator technology was selected as the potential solution. This approach uses the resonant structures to effectively create long paths (and high gain) for the radiation currents while reducing antenna size by 25 to 35 percent compared to other approaches. The technique made it possible to meet the form-factor requirements with higher gain than possible with an even larger microstrip patch antenna or a cavity-backed helical antenna.

Design and analysis of ring antennas requires very intuitive engineering (and educated guessing) compared to the better-understood design and analysis approaches used for microstrip patch antennas. Fortunately, by performing a detailed initial design and analysis procedure, and carefully reviewing the EM simulation results, it was possible to reduce the design risk for the ring antenna in spite of its complexity.

In a simple rectangular patch antenna, the source of radiation can be thought of as two slots at the two ends of the patch, approximately one-half wavelength apart. If each of these slots is about one-half wavelength in length, 2.1 dBi gain should result. Any two such antennas working as a two element array should theoretically provide an additional 3 dB gain. Therefore, it should be possible to achieve about 5.1-dBi gain from a simple patch antenna. With some refinement, it may be possible to get even better gain or pattern shaping depending on the ground plane type or resonance mode.

For a ring antenna, it may be possible to design a structure with multiple resonances that could be spaced and coupled for multiple-frequency or broadband-frequency use.

High gain and pattern shaping can be achieved by properly phasing the various modes to operate in a predetermined manner so as to combine constructively or destructively in the far field and in appropriate directions. In most cases, it was possible to achieve as much as 9 dBic gain (theoretically) from these structures with as much as 17 percent bandwidth. In theory, it was possible to achieve bandwidths of 15, 20, and 30 percent for VSWRs of 1.50:1, 2.0:1, and 3.0:1, respectively. Unfortunately, it was not possible to develop a systematic design approach that would meet all required physical and electrical parameters at all frequencies. With some effort, it may be possible to develop an approach that would meet the requirements for certain modes of operation, however.

Figure 1 shows a swept-frequency plot as it was predicted by EM simulation for an optimized antenna design. The plot indicates multiple resonances, although not all would be used for the satellite antenna The lowest resonance at 1.8 GHz provides better than 13-dB return loss, while at the high end, at 2.25 GHz, the return loss can be better than 17 dB. For a certain combination of parameters, it is possible to achieve a 10-dB return-loss bandwidth of about 15 percent. This would be an excellent broadband antenna for many applications. The return loss for the resonance at 2.1 GHz is even better, at nearly 20 dB. The antenna design's multiple resonances make it suitable for use as a single wideband antenna or for three discrete-frequency applications.

The predicted radiation pattern for the RHCP antenna at the lower resonance of 1.8 GHz indicates nearly 5.5 dBic gain (Fig. 2, top left). The axial ratio is nearly 13 dB at zenith (Fig. 2, bottom left). At the upper resonance of 2.25 GHz, the predicted radiation pattern indicates nearly 8-dBic gain (Fig. 2, top right). At this frequency, the axial ratio appears to be nearly 12 dB (Fig. 2, bottom right).