Design of wideband reconfigurable microstrip patch antennas


Student thesis: Doctoral Thesis

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  • Shing Lung Steven YANG

Related Research Unit(s)


Awarding Institution
Award date16 Jul 2007


This thesis presents the design of wideband radiation pattern reconfigurable microstrip patch antennas. Different resonant modes with different polarizations or patterns could be excited within a microstrip patch with almost identical operating frequency range. The impedance bandwidth of the proposed antennas could be broadened by incorporating wideband feeding technique, the L-shaped probe feeding technique. The proposed antennas are radiation pattern reconfigurable when an electronic switch is installed, the radiation patterns could be switched by exciting different feeding ports which would excite different resonant modes. It is demonstrated that the TM01 and TM11 mode of an L-shaped probe coupled circular microstrip patch could be operated at almost identical operating frequency range when metallic posts are placed at appropriate positions of the patch. These two resonant modes are separately excited by two different sets of L-shaped probes. The radiation patterns of these two modes have different beam forming; the TM01 mode radiates conical-shaped radiation pattern, and the TM11 mode radiates broadside radiation pattern. Extensive studies on this design were performed, and an antenna prototype was constructed to demonstrate the proposed idea. The prototype operates at 1.85 GHz, and an impedance matching bandwidth (VSWR<2) of 24% is achieved for the two resonant modes. The antenna prototype has stable radiation patterns across the operating bandwidth, and it attains a gain of 4 dBi and 8.5 dBi for the TM01 and TM11 modes, respectively. A single pole dual throw (SPDT) switch was designed, which could be integrated with the antenna prototype to demonstrate the capability of radiation pattern reconfiguration. The SPDT switch was constructed by series connected PIN diodes and it achieves an isolation of over 20 dB, and an insertion loss of about 1dB. Moreover, a wideband amplifier was designed to compensate the insertion loss introduced by the switch. The bandwidth of the amplifier is enhanced by inserting a compact microstrip resonant cell to the emitter pin of the bipolar junction transistor. The amplifier attains an impedance matching bandwidth of about 95% (when both S11 and S22 < -10 dB) with the center frequency at 2.75 GHz, and an average gain of about 15 dB. At 2.45 GHz, the noise figure of the amplifier is about 1.3 dB and the 1-dB gain compression point is 8.5 dBm. The switch and amplifier could be implemented on the back side of the ground plane of the proposed antennas for pattern reconfigurable applications. By integrating the proposed antenna with the switches and the amplifier, the performance of the wideband radiation pattern reconfigurable antenna is demonstrated. In addition to this design, a tri-polarization design was developed by adding one more pair of L-shaped probes. The channel performance is expected to be enhanced when the conical radiation is added to the dual orthogonal broadside radiations, which would also be suitable for adaptive beam scanning applications. Apart from linear polarization, wideband reconfigurable antennas which could excite circularly polarized radiation patterns are also focused and studied. Two CP antennas for different applications were designed. The first CP reconfigurable antenna is electrically reconfigurable between circularly polarized radiation pattern and conical-shaped radiation pattern. Sequential excitation within an antenna element was used to excite CP radiation across a wide bandwidth. This antenna attains a bandwidth of about 21% (S11<-10dB and AR<3dB) when CP mode is switched on, which would be suitable for ground wave and satellite signal receptions. The second CP reconfigurable antenna is circular polarization reconfigurable. CP radiation could be obtained by exciting a perturbed square patch with L-shaped probes. It achieves an impedance bandwidth of about 25%, and an axial ratio bandwidth of about 18%. It would be suitable for satellite transmission and reception or mobile/video network link applications. The proposed antennas besides having the reconfiguration ability across a wide bandwidth; they are simple in structure and easy in fabrication, which would be favorable in manufacturing process.

    Research areas

  • Microstrip antennas, Design and construction, Wireless communication systems