Pei-Ling Chi

High Isolation and Compact Diplexer Using the Hybrid Resonators

In this letter, a compact microstrip diplexer with very high output isolation is proposed. This diplexer consists of compact hybrid resonators, which are capable of introducing transmission zeros at desired frequencies and meanwhile suppressing signals below the resonant frequencies through tapping the feeding line close to the shorted vias. By designing the transmission zero of the lower-frequency channel in the higher passband, and utilizing the high lower-stopband suppression property of the higher-frequency channel, very high diplexer output isolation can be achieved. The proposed diplexer with center frequencies at 1.8 and 2.45 GHz, respectively, has demonstrated better than 55 dB output isolation and more than 55 dB suppressions in the stopbands. Furthermore, the diplexer occupies only a small area of 0.13λ0 × 0.2λ0.

Compact Quarter-Wave Resonator and Its Applications to Miniaturized Diplexer and Triplexer

Novel and compact composite right/left-handed (CRLH) quarter-wave type resonators are proposed in this paper. The resonator can resonate at the frequency where the electrical length is phase-leading or negative, which results in a smaller size as compared to the conventional phase-delayed microstrip-line resonator. Furthermore, it is only half the size of the CRLH half-wave resonator resonating at the same frequency. In addition, the proposed resonator is capable of engineering the multiresonances very close to each other, which makes it suitable to implement the miniaturized multiband microwave components such as diplexers and triplexers. A very compact diplexer and a very compact triplexer are proposed based on the proposed CRLH quarter-wave resonators in this paper and both of them have demonstrated very good performance. Specifically, compared to the referenced works based on the conventional microstrip resonators, the proposed diplexer and triplexer are 50% and 76% smaller than their microstrip counterparts, respectively.

Miniaturized Dual-Band Directional Couplers Using Composite Right/Left-Handed Transmission Structures and Their Applications in Beam Pattern Diversity Systems

This paper demonstrates the feasibility of dual-band compact and bandwidth-enhanced directional couplers, including both 90deg and 180deg hybrids, using composite right/left-handed (CRLH) transmission structures. Novel and systematic design methodology leading to size miniaturization is proposed. By considering all possible dual-band phase transitions of the constituent transmission lines of the directional couplers, this design approach ensures the compactness of the hybrids and reduces footprint size significantly compared to the previously exhibited standard transmission-line- and CRLH-based hybrids. The proposed dual-band 90deg and 180deg hybrids achieve 10% and 43.7% size reduction, respectively, compared to conventional microstrip couplers in the lower band of the dual-frequency operation. In addition, the fabricated 180deg hybrid is shown to have enhanced bandwidth performance in both isolation and 3-dB magnitude balance. The combination of either of the proposed couplers with the proposed dual-band CRLH-based delay line enables antenna pattern diversity with distinct beam patterns in dual bands. This serves as an effective dual-band beam pattern diversity system when connected to the antenna array. Moreover, the isolation improvement by cascading the antenna array with this assembly was measured experimentally at two operational bands: 22.7 dB at 1 GHz and 10 dB at 2 GHz.

Antenna Miniaturization Using Slow Wave Enhancement Factor from Loaded Transmission Line Models

Miniaturization of slow wave antennas exploiting the slow wave enhancement factor is presented. The printed antennas are periodically loaded with shunt capacitors to slow down the guided wave in the structures. In this paper, the loaded unit cell of the equivalent transmission line model is utilized to extract the slow wave enhancement factor, the ratio of the loaded to the unloaded propagation constants of the wave in the antennas. From this model, the slow wave enhancement factor of a loaded antenna agrees very well with the miniaturization factor, and therefore load parameters in the circuit model can be readily obtained when a specific size reduction is attempted. This claim was substantiated by demonstrating two small radiators, a high-frequency (HF) slot-loop antenna and a planar inverted F antenna (PIFA), to achieve the desired size reductions. Experimental results show that both of the antennas demonstrate greater than ten-times size reduction from their unloaded counterparts at the expense of the degraded gains and impedance bandwidths. Specifically, the loaded slot loop presents the predicted gain and measured bandwidth on the order of -34.9 dBi and 0.38% for VSWR ≤ 2, respectively. Therefore, a matching network derived from filter design techniques is proposed to increase the antenna bandwidth so that a measured fractional bandwidth of 1.78% is achieved. The slot loop combined with the impedance matching circuit occupies a footprint size of 0.031λ0 × 0.017 λ0 at the operating frequency. On the other hand, the measured radiation gain and bandwidth of the loaded PIFA are reduced to -22.6 dBi and 0.15% for VSWR ≤ 2, respectively, with a footprint of 0.013 λ0 × 0.018 λ0 at the operating frequency.

Compact and Tunable Slot-Loop Antenna

A compact and tunable slot-loop antenna is studied in this communication. By periodically loading the slot line with varactor diodes, the slow-wave loop is capable of operating at frequencies from 2.34 GHz to 4 GHz with the measured input reflection coefficient better than -7.5 dB while occupying a small footprint of 0.072 λg × 0.072 λg (8.1 mm × 8.1 mm), where λg is the guided wavelength of a slot line at 2.34 GHz. The slot-loop in the ground plane is excited by a simple microstrip line, which enables the impedance matching from 3.2 GHz to 4 GHz for VSWR ≤ 2. A LC low-pass circuit is configured into the bias network to isolate the RF signal from the DC power path and consistent radiation patterns are experimentally obtained across the entire tunable frequencies of interest. The slot loop combined with the feed line and the bias circuitry is only 40 mm × 38.97 mm. Experimental results validate the feasibility of the presented antenna and agree well with simulation data.

A Miniaturized CPW-Fed Capacitor-Loaded Slot-Loop Antenna

A simple and efficient approach for antenna miniaturization is proposed in this paper. A slow wave structure, which applies to the concept of increasing the propagation constant in the resonant path and so results in the decreased resonant frequencies, is used for implementation. In this paper, we present a CPW-fed slot-loop antenna periodically loaded with capacitors to effectively increase the propagation constant for the purpose of antenna miniaturization. The equivalent transmission line circuit model for the established antenna was utilized as an approach for analysis and it predicted the slow-wave behavior well when compared to the full-wave simulation. Both simulation and measurement demonstrate a physical size reduction of about six times with respect to an unloaded slot-loop antenna.

A compact dual-mand metamaterial-based rat-race coupler for a MIMO system application

A compact dual-band rat-race coupler is proposed in this paper. By using composite right/left-handed (CRLH) transmission lines (TLs) and investigating all possible combinations of phase responses of the individual TLs in the coupler, a dual-band and miniaturized rat-race coupler was implemented. The dual-band rat-race coupler shows a 55% size reduction. This CRLH-based coupler is used as a mode-decoupling network in a dual-band front-end MIMO system, along with a planar antenna array, to split two orthogonal radiation modes from the connected array. A pair of compact dual-band antennas were built and closely spaced to demonstrate pattern diversity by in-phase or out-of-phase excitations from the coupler. Good experimental isolation of the system, −29 dB at 2.4 GHz and −34.2 dB at 5.2 GHz, is exhibited verifying the desired decoupling property. Furthermore, pattern diversity in MIMO communications was observed by two measured orthogonal radiation patterns.

Dispersion engineering with CRLH metamaterials

Composite right/left-handed (CRLH) metamaterials have demonstrated unique and advantageous features when employed in microwave and millimeter-wave circuits and systems. In this paper, an overview of the CRLH metamaterials is addressed by introducing the dispersion-engineerable property from the equivalent unit-cell circuit model. Thereafter, several examples based on the CRLH structures are examined to validate the feasibility and superiority of the proposed works in terms of the compactness and functionality which are found constrained using conventional materials.

Novel diplexer synthesis using the composite right/left-handed phase-advance/delay lines

A novel and simple approach for diplexer synthesis using the composite right/left-handed (CRLH) phase-advance/delay lines combined with a coupler is proposed. By engineering CRLH-based transmission lines with desired phase responses at two arbitrary frequencies of interest, the connected CRLH 90° or 180° coupler is excited in a manner such that signals at designated frequencies are separated to the corresponding output ports of coupler. Based on the present configuration, design complexities such as optimization of the interconnection junctions and the harmonic spurious suppression involved in conventional filter-based diplexers can be avoided. In addition, channel isolation is beneficially achieved from the isolation property of directional couplers. The measured insertion loss and isolation are less than 1 dB and better than 20 dB in dual bands respectively. Good agreement was observed between simulation and measurement.

A new way of bandpass filter design based on zeroth-order and negative-order resonance modes

A new method of bandpass filter design is proposed by using the CRLH transmission line theory. The method starts from the dispersion diagram of CRLH transmission line and is based on the negative first order and zero order resonance modes of an unbalanced CRLH transmission line. The relationship between the new method and conventional coupling, method of filter design is discussed. By using the new method, a two-order mushroom filter is designed, fabricated and tested in this paper. The filter achieves a small size and a wide-stopband.