Research of Slotted-Ground-Plane Filters andCircularly Polarized Quasi-Loop Antennas

• Main Authors: Chia-hsien Lin, 林家賢
• Other Authors: Chien - Jen Wang
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/759ag8

碩士 === 國立臺南大學 === 通訊工程研究所碩士班 === 97 === In this thesis, several types of a slotted-ground-plane filter (also callingthe defected ground structure resonator, DGS) and a circularly polarizedquasi-loop antenna are designed and analyzed by fabricated on the FR4 substrates. First, for the slotted-ground-plane resonators, three techniques of frequency reduction and size miniaturization are proposed. In addition, due to enhancement of the stored power in the resonator, the quality factor increases, and the bandstop bandwidth decreases. Second, by breaking the resonator path of the slot etched in the ground plane, a transmission zero at a low frequency band is excited. The bandstop haracteristics at high frequencies can be improved by adding several stepped-impedance open stubs connected to the microstrip line. The compact low-pass filter with an elliptic-function frequency response is demonstrated without utilizing the cascade topology, compared to traditional slotted-ground-plane low-pass filters. Finally, a microstrip-fed circularly polarized loop-like antenna for applications of the global positioning system (GPS) and the digital communication system (DCS) is proposed. The proposed antenna comprised of a quasi-C antenna, an inverted-L sleeve strip connected with the ground plane and an L-shaped slit embedded in the ground plane. The first part presents simple designs of compactness of a bandstop resonator with a slotted ground plane. By adding two parasitically coupled patches and embedding shorting pins, a resonant frequency of the slotted-ground-plane resonator is significantly shifted down and an improvement of harmonic suppression is easily achieved. The two identical patches are symmetrically placed beside a transmission line on the top of the slotted-ground-plane. The patches acting a parasitic capacitance increase equivalent capacitance of the resonator so that the resonant frequency is reduced, a size reduction is achieved, and the quality factor is increased. Due to increase of the quality factor, a narrow rejection bandwidth is provided. Furthermore, the technique of loading the patches with two shorting pins significantly miniaturizes resonator size. The harmonics at high frequencies are suppressed by varying location of the shorting pins. A corresponding equivalent-circuit model for each topology is developed and lumped-circuit parameters are extracted using a simple circuit analysis method. Compared with a conventional slotted-ground-plane resonator (the quality factor =7.628), measured results show that the resonator with the shorted patches provides the quality factor larger than 23. In the second part, new techniques of designing a compact coplanar-waveguide (CPW) harmonic-suppressed resonator using the defected ground structure (DGS) resonator with floating patches or shorting patches are proposed. Two T-shaped slots as a bandstop resonator are etched in two ground planes of a CPW 50�� transmission line. Two rectangular patches are symmetrically placed on the bottom layer of the substrate. The patches modeled as an additional capacitance increase the effective capacitance of the resonator. By tuning the geometrical dimension of the patches, the resonance frequency is lowered. Furthermore, frequency reduction and harmonic suppression are improved by embedding two shorting pins or adding two shorting walls inside the patches. The rejection bandwidth decreases and the quality factor (Q) increases due to enhancement of stored energy in the resonator. In third part, designs of a compact elliptic-function lowpass filter using a slotted-ground-plane resonator are presented. Unlike conventional design of cascading bandstop slotted-ground-plane resonators in the literature, the introduced filter is presented, which consists of a quasi-π slot in the ground plane, and a microstrip line with several stepped-impedance open stubs. The quasi-π slot provides a very sharp cutoff frequency response with low insertion loss. Furthermore, to increase rejection bandwidth, additional attenuation poles are added into frequency characteristics by connecting the stepped-impedance open stubs at the microstrip line. An equivalent circuit of the proposed lowpass filter is simply represented by RLC lumped elements. According to measured results, compared with a conventional H-slot resonator, the resonance frequency of the resonator is reduced from 3.5 GHz to 2.0 GHz and the ratio of frequency reduction is about 43 %. The measured insertion loss at a passband is below 0.3 dB, and the rejection band over 25 dB is from 1.95 to 9.0 GHz. The final part presents a circularly polarized (CP) GPS/DCS loop-like antenna with a microstrip feed. The proposed antenna comprised of a quasi-C antenna, an inverted-L sleeve strip connected with the ground plane and an L-shaped slit embedded in the ground plane. The C-like antenna generates a resonant mode with a poor impedance matching condition. The inverted-L grounded strip and the embedded L-slit are not only capable of modifying two orthogonal electric fields with equal amplitude and phase difference of 90 degree for radiating circular polarization at 1.575 GHz, but the impedance characteristics is also improved and the operating frequency is reduced. Both simulated and measured results are provided to validate the impedance and CP performance of the proposed antenna. For the optimized antenna case, the measured bandwidth with an axial ratio (AR) of less than 3 dB is larger than 19 % and the measured impedance bandwidth of reflection coefficient S11 < -10 dB is about 30.1 %.