| Summary: | 碩士 === 國立中央大學 === 電機工程學系 === 104 === In this thesis, by loading the antennas with either tuning elements or switches, the operating frequency is made reconfigurable. Two antennas are presented: a frequency-reconfigurable slot-loop antenna with ferroelectric varactors and a switchable patch antenna for 2.45-GHz and 5.8-GHz ISM bands.
First, a frequency-reconfigurable slot-loop antenna with a tunable matching network is designed, fabricated, and measured. The frequency-reconfigurable antenna is implemented by loading ferroelectric varactors along a slot loop. CPW (coplanar waveguide) feed is used to preserve the advantage of being uniplanar. However, it is usually difficult to provide a good matching over wide frequency by a direct CPW feed. To solve this problem, a tunable matching network is added. The tunable matching network is implemented using a varactor in series, which does not increase the area of the antenna. Measurement results show that, when the bias voltage of the ferroelectric varactors is increased from 0 V to 15 V, the operating frequency of the antenna can be tuned from 6.71 GHz to 9.14 GHz, equivalent to a 30.6% bandwidth. When biased at 15 V, the antenna gain is −3.1 dBi. Over the entire frequency tuning range, the return loss is greater than 20 dB, validating the effectiveness of the tunable matching network.
Second, a patch antenna that can be switched between 2.45-GHz ad 5.8-GHz ISM bands is designed, fabricated, and measured. The antenna is designed by connecting two patches with five equally spaced PIN diodes, which act as switches. When the switches are off, only one patch is connected to the feed and the operating frequency is designed to be 5.8 GHz. On the other hand, when the switches are turned on, the two patches are electrically connected, resulting in an increase in electrical length and lowering the operating frequency to 2.45 GHz. The switchable patch antenna is fed using a coaxial probe feed. The position of the feed is adjusted so that good matching is observed for both frequencies. Measurement results show that, when the bias voltage of the PIN diodes is −10 V, i.e., when operated in the off state, the resonant frequency of the patch antenna is 5.75 GHz and its gain is 6 dBi. On the other hand, when the overall bias current is 250 mA, when operated in the on state, the resonant frequency of the patch antenna is 2.53 GHz with a gain of −0.8 dBi. At both frequency bands, the return loss is greater than 15 dB.
In this thesis, two frequency-reconfigurable antennas using either ferroelectric varactors or PIN diodes are successfully designed and implemented. The tunable slot-loop antenna achieves the widest effective bandwidth when compared with other ferroelectric-based tunable antennas in the literature.
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