Reconfigurable magneto-electric dipole antennas for wireless communications
Student thesis: Doctoral Thesis
Related Research Unit(s)
This thesis presents the design of reconfigurable magneto-electric (ME) dipole antennas for wireless communications. Reconfigurable antennas can significantly improve the efficiency of spectrum usage by adapting their operating frequency or their radiation characteristics. Therefore, reconfigurable antennas have become a promising method for meeting the increasing demand for larger wireless capacity. This research investigates several ME dipole antenna designs with low profiles and wide bandwidths. Among these designs, an ultra-wideband (UWB) ME dipole design is developed for the frequency-reconfigurable antenna used for cognitive radio (CA) applications. Then, a three-element linear ME dipole array with beamwidth reconfiguration in the H-plane is presented. Furthermore, based on the ME dipole concept, linearly polarized and dual-polarized ME dipole antennas with a dynamic beamwidth control in the H-plane are proposed. First, a low-profile ME dipole antenna with a thickness of 0.17 λ is presented. This antenna, which consists of a horizontal electric dipole, a vertically oriented folded shorted patch antenna, and a rectangular cavity-shaped reflector, exhibits an impedance bandwidth (SWR ≤ 1.5) of 54.8% and a boresight gain of approximately 8.6 dBi. Based on a similar structure, another low-profile ME dipole antenna (height = 0.169 λ) is proposed utilizing a simple coaxial feed located in the center of the structure. An impedance bandwidth (SWR ≤ 1.5) of 45.6%, an antenna gain of approximately 8.1 dBi, a stable radiation pattern with low cross-polarization and back radiation levels are achieved. Then, by combining a bowtie electric dipole and vertically oriented folded shorted patches, an ME dipole antenna excited by a stair-shaped probe feed is proposed, which can achieve an impedance bandwidth (SWR ≤ 2) of 95.2% and a stable antenna gain of approximately 7.9 dBi. Furthermore, a UWB ME dipole antenna composed of a bowtie electric dipole, a center-fed loop antenna functioning as a magnetic dipole and a rectangular cavity is designed using a microstrip-to-stripline transition as a balun to feed the antenna. An impedance bandwidth (SWR ≤ 2) of 110% and a stable gain of 8.7 dBi with a variation of 1.9 dB are realized using the rectangular cavity. Stable radiation patterns with low cross polarization (≤ -20 dB), low back radiation (≤ -13 dB) and symmetrical E- and H-plane patterns are obtained at different frequencies over the operating band. After reviewing the ME dipole designs, a frequency-reconfigurable antenna based on directed dipoles is presented to operate in either a wideband mode or four narrowband modes for cognitive radio applications. The wideband mode (0.83-2.5 GHz) is based on a folded bowtie dipole, whereas the four narrowband modes are based on a length-reconfigurable thin dipole. PIN diodes are used as switches at specific locations for choosing different modes. Unidirectional radiation can always be obtained for all the modes, but high loss is attained due to the vertically oriented DC lines for biasing purposes. Then, a frequency-reconfigurable ME dipole antenna is presented for cognitive radio applications. One wideband mode based on a wideband ME dipole and four narrowband modes based on a length-reconfigurable thin dipole are achieved for operation between 0.83 to 2.16 GHz. The ME dipole technology makes the wideband mode with a flat gain (8.7 dBi with a variation of 1 dB) and well-controlled radiation patterns (cross polarization ≤ -25 dB, back radiation ≤ -20 dB). Moreover, as parts of the ME dipole, four hollow metal posts are used to hide the DC bias lines from the RF radiation, which provides high efficiency and simple structure. A comparison between the two designs demonstrates that the ME dipole antennas possess significant advantage over the conventional dipole antennas when used for designing reconfigurable antennas. Finally, a three-element linear ME dipole array is proposed to realize beamwidth reconfiguration for base stations. This antenna array, which consists of three ME dipole elements and a switchable feeding network, demonstrates an impedance bandwidth (|S11| < -10 dB) of 15%, unidirectional radiation patterns with low cross-polarization and back radiation levels. The half-power beamwidth in the E-plane is maintained at 72°, whereas the beamwidth in the H-plane can be switched between 37° and 136°. Based on the similar concept, linearly polarized and dual-polarized ME dipole antennas with dynamic beamwidths in the H-plane are presented. The design methodology uses tunable parasitic dipoles loaded with varactor diodes placed on the sides of a driven ME dipole along its H-plane. The H-plane beamwidth with a continuous tuning range from 80° to 160° for the linearly polarized antenna and a range from 72° to 133° for both ports of the dual-polarized antenna can be measured. In addition, unidirectional radiation patterns with low cross-polarization levels (< -15 dB) and low back radiation levels (< -22 dB) for both antennas are achieved. Owing to the ME dipole concept, the radiation efficiency is higher than 80% in all states of operation for both designs.
- Electromagnetic theory, Design and construction, Antennas, Dipole, Adaptive antennas