Electromagnetic Wave Group - Advanced Microwave and RF-SOP Technology

 To increase the system capacity and the transmission data rate of wireless communication systems, the smart antenna technologies [1], including the adaptive and the switched beamforming, offer significantly improved solutions. Regarding the circuitry complexity and production cost, the switched beamforming would be currently a better candidate though it can form beams only in the specific predetermined directions based on the received signal strength measurements. In addition, using circularly-polarized (CP) antennas can provide further enhancement due to its capability of reducing multipath reflections and interferences. Therefore, a CP antenna module with the switched beamforming capability is developed [2] [3]. The design specifications are as follows: the operating frequency band from 59 to 62 GHz, the input return loss > 10 dB within the band, the in-band axial ratio < 3 dB, and the LHCP peak gain > 10 dBic. The 60-GHz antenna module is composed of four width-reduced CP patch array antennas and a 4´4 microstrip Butler matrix. For broadband CP radiation, the sequential rotation technique has been applied to the patch array antennas. The total width of the antenna has also been taken into account for module integration and minimized during the design process. The performances of the width-reduced antenna have been verified experimentally. The realized 60-GHz CP switched-beam antenna modules with different element spacing both exhibit satisfactorily high gains and tilted main beams as expected. The presented antenna module may find applications in the emerging WPANs.

Also, we proposed a novel self-structuring electromagnetic scatterer (SSES) [4]. The SSES can alter its electrical shape to fulfill various operational objectives, such as radar cross section (RCS) reduction or enhancement. The SSES template comprises segments of metallic thin strips interconnected via voltage-controlled switches. By opening or closing the switches, the phase of the field scattered by the strips changes, results in destructive or constructive interference in the total scattered field. The RCS of the SSES can thus be controlled. An efficient search algorithm based on the fractional factorial designs of experiments (FFD) is adopted to find a suitable switch configuration for the SSES. The FFD estimates the effects of the switches on the scattering properties, and identifies the significant effects. For a given operational objective, a combinatorial optimization problem can be formulated in terms of these effects and solved for a suitable switch configuration. A SSES prototype was built and a series of RCS measurements were performed to demonstrate its capability to adaptively control the RCS. It is shown that the bistatic RCS can be significantly reduced in any specified direction and that the main beam maximum of the RCS pattern can be enhanced and steered within an angular range of 30 degrees. Importantly, the proposed method requires only one sixth of the number of experiments needed by a genetic algorithm to locate a comparable solution.

Planar antenna design has several advantages and has gained increasing interests for the past decade. The proven PCB antenna is low-profile, reliable and cost effective very suitable for the mass production. However, the traditional microstrip antenna [1] is of narrow-band nature and has limited applications. This research group has been dedicated to the bandwidth enhancement study of the planar antenna design. A compact aperture antenna is successfully designed for UWB applications covering 3.1 to 10.6 GHz with optional notched-band of 5~6 GHz [2]. A dual-band design is also presented for this aperture antenna using the double T-matching stubs for 2.4/5.5 GHz WLAN operations [3].

Additionally, the research interests further extend to the circularly polarized (CP) antenna design. The conventional CP patch antenna has narrow bandwidth (3~5%) in the axial ratio performance. The research team has successfully designed and developed a series of broadband CP aperture antennas. The innovative cavity-backed CP travelling wave antenna [4] shows that the 3-dB axial ratio bandwidth may be improved to 24% for the single antenna and up to 50% for the 2x2 array. The broadband and high-gain features of the presented antenna may find applications for the high speed wireless transmission at millimeter wave frequencies [5] [6].