Moon-Que Lee

A Passive Circulator with High Isolation using a Directional Coupler for RFID

In radio-frequency identification (RFID) system, a directional coupler has been used to isolate RX from TX due to its simplicity and low cost compared with a circulator. Nevertheless, a conventional microstrip directional coupler has inherent drawback of poor isolation due to unequal phase velocity between even and odd mode. In this paper, a newly proposed circulator using a microstrip directional coupler is proposed to achieve good isolation between TX and RX. The proposed circulator is used for UHF band RFID reader with a single antenna, of which the operating frequency band is 860 MHz-960 MHz. The measurement of the fabricated circulator using a modified directional coupler exhibits excellent isolation of 64 dB and directivity of 49 dB in its frequency band. The validity of the proposed circulator is demonstrated by comparing the TX leakage at the RX port of RFID reader system with that of a conventional directional coupler

High-efficiency harmonic loaded oscillator with low bias using a nonlinear design approach

We present a design method for an optimized high-efficiency harmonic loaded oscillator. The proposed approach predicts the performance of oscillators including output power, dc-RF conversion efficiency, and dc-bias current shift during start-up transition. In this method, the performance of the oscillator can he optimized based on the performance analysis of the active device under the assumed operation conditions. The effects of fundamental and harmonic loading on output power and efficiency are investigated by the proposed approach. Two kinds of stability conditions are addressed for an oscillator initially biased at a low gate voltage. Using the proposed approach, we design an oscillator that has a high efficiency of 61% at 1.86 GHz with a very low bias voltage of 2.0 V.

RFID Reader Front-End Having Robust Tx Leakage Canceller for Load Variation

A novel front-end structure for ultra-high-frequency RF identification reader is proposed. The proposed front-end consists of a balanced coupler and four-port feed antenna system. The proposed balanced coupler structure has high transmit/receiver isolation feature and good insensitivity to load variations. The proposed four-port feed antenna system is applied for removing the coupling among antennas, which deteriorates the isolation. The proposed system is numerically analyzed and verified experimentally as well.

Design of Compact Quadruple Inverted-F Antenna With Circular Polarization for GPS Receiver

A compact quadruple inverted-F antenna (QIFA) with right-hand circular polarization for global positioning system (GPS) receiver is presented. The proposed QIFA consists of four spiral-shaped inverted-F antennas fed by a compact multilayered feed network. In order to improve the radiation efficiency of four-port feed antenna like QIFA, we analyze an optimum matching method considering both reflection coefficient at each port and the mutual coupling between ports simultaneously. Experimental results show that QIFA of size 10 x 10 x 12 mm3 has a 3-dB beamwidth of more than 130°, the peak gain of -1 dBic and the axial ratio under 2 dB.

Low Power OOK Transmitter for Wireless Capsule Endoscope

A new structure of on-off keying(OOK) transmitter with low power consumption and high data rate is proposed using a current-reused CMOS differential LC-VCO for wireless capsule endoscope application. The carrier frequency (440 MHz) of the transmitter is determined as the minimum loss frequency point considering the antenna loss and the loss property of human body, etc. The transmitter is able to output 0 dBm of power at the single-ended output to a monopole 50-Ohm antenna to meet the demands of high-data rate speed (40 Mb/s) and short range (15-cm) wireless communication. This transmitter, has been fabricated using 0.18-mum CMOS technology, consumes only 860-muA of current from a 3 V supply.

A Passive Circulator for RFID Application with High Isolation using a Directional Coupler

In this paper, a newly invented circulator for T/R switch of UHF radio-frequency identification (RFID) application is proposed to overcome TX-to-RX leakage problem. A microstrip coupled-line directional coupler is used for this passive circulator and the isolation characteristic is drastically improved not by complex method such as compensating phase velocity, but by considering both directional coupler itself and imperfect input impedance of the employed antenna. The measurement result of the proposed circulator using modified microstrip coupled-line directional coupler with employed antenna shows excellent TX-to-RX leakage suppression in the 860 MHz - 960 MHz, more than 45 dB enhancement at the center frequency compared to the conventional coupler. Experimental verification using RFID system is also performed to prove the validity of the proposed circulator

A small and slim printed Yagi antenna for mobile applications

In this paper, we propose a slim printed Yagi antenna. The length of the proposed antenna is reduced by 43% compared with a typical dipole antenna applying the meander structure. In addition, the spacing between the antenna and the reflector is designed as 0.01lambda. A folded dipole and a lumped balun transformer were adopted to compensate the low radiation resistance caused by the reduced spacing. Finally, the performance of the proposed antenna was verified through application on vehicle GPS systems (1.575GHz).

Printed square quadrifilar spiral antenna for UHF RFID reader

This paper proposes the printed square quadrifilar spiral antenna (QSA) for mobile RFID reader. A folded inverted-F antenna structure is used as a spiral to increase the impedance of QSA. Results show that the array antenna has obtained a sharp 3 dB beamwidth, high gain and good axial ratio.

Compact Dual-Band Printed Quadrifilar Antennas for UHF RFID/GPS Operations

This letter presents a simple and innovative method for designing and optimizing two circularly polarized printed quadrifilar antennas (PQAs). The PQAs are operated at UHF RFID (902-908 MHz) and GPS (1575 MHz) and intended for use at a portable terminal. This new antenna system comprises two PQAs concentrically arranged, but electrically isolated. Each PQA comprises four inverted-F monopoles arranged orthogonally and excited with a phase difference of 90°.

Printed Square Quadrifilar Helix Antenna (QHA) for GPS Receiver

A new printed square quadrifilar helix antenna (QHA) for circular polarization (CP) is proposed and experimentally investigated for the application as a GPS receiving antenna in frequency range of about 1.575 GHz. To increase the input impedance of QHA, we use the folded inverted-F antenna as a helix. Experiment results show that the proposed antenna has a 3-dB beamwidth of more than 120deg and a front-to-back ratio of more than 15 dB. Also, the proposed antenna shows the peak gain of -2.5 dBic and the axial ratio under 0.5 dB in wanted frequency band.