Boon-Kuan Chung

A WIDEBAND E-SHAPED MICROSTRIP PATCH ANTENNA FOR 5-6 GHZ WIRELESS COMMUNICATIONS

A wideband E-shaped microstrip patch antenna has been designed for high-speed wireless local area networks (IEEE 802.11a standard) and other wireless communication systems covering the 5.15-5.825 GHz frequency band. Two parallel slots are incorporated to perturb the surface current path, introducing local inductive effect that is responsible for the excitation of the second resonant mode. The length of the center arm can be trimmed to tune the frequency of the second resonant mode without affecting the fundamental resonant mode. A comprehensive parametric study has been carried out to understand the effects of various dimensional parameters and to optimize the performance of the antenna. A substrate of low dielectric constant is selected to obtain a compact radiating structure that meets the demanding bandwidth specification. The reflection coefficient at the input of the optimized E-shaped microstrip patch antenna is below -10 dB over the entire frequency band. The measurement results are in excellent agreement with the HFSS simulation results.

Dielectric Constant Measurement for Thin Material at Microwave Frequencies

Apractical problem in the reflection method for dielectric constant measurement is the difficulty to ensure the sample is placed exactly at the waveguide flange. Asmall position offset of the dielectric sample will give rise to some errors in calculating the dielectric constant, especially when a thin sample is used. To circumvent this problem, a method to determine the dielectric constant by measuring the transmission coefficient of the thin slab placed in a waveguide has been developed. Slab position offset from the measurement reference plane has no effect on the measurement accuracy. An explicit expression for the dielectric constant is obtained in terms of the transmission coefficient by simplifying the exact solution for transmission through a thin dielectric slab. The method is verified with measurement on Teflon of 0.5-mm thickness. The measured dielectric constant of Teflon shows excellent agreement of both e � and e �� with published data. Subsequently, the dielectric constant of a vegetation

ULTRA WIDEBAND POWER DIVIDER USING TAPERED LINE

A power divider with ultra-wideband (UWB) performance has been designed. The quarter-wave transformer in the conventional Wilkinson power divider is replaced by an exponentially tapered microstrip line. Since the tapered line provides a consistent impedance transformation across all frequencies, very low amplitude ripple of 0.2dB peak-to-peak in the transmission coe-cient and superior input return loss better than 15dB are achieved over an ultra-wide bandwidth. Two additional resistors are added along the tapered line to improve the output return loss and isolation. Simulation performed using CST Microwave Studio and measured results conflrm the good performance of the proposed circuit. The return loss and the isolation between the output ports are better than 15dB across the band 2{ 10.2GHz. Standard ofi-the-shelf resistance values can be selected by optimizing the physical locations to mount the resistors. Better performance can be achieved with more isolation resistors added. Hence, the number of isolation resistors to be used may be selected based on the desired bandwidth and level of isolation and return loss speciflcations.

TRANSMITTER AND RECEIVER DESIGN OF AN EXPERIMENTAL AIRBORNE SYNTHETIC APERTURE RADAR SENSOR

An Experimental Airborne Synthetic Aperture Radar (SAR) Sensor has been designed and,developed at Multimedia University, Malaysia. The airborne system is an inexpensive C-band, single polarization, linear-FM airborne radar sensor. An innovative cancellation network is implemented to overcome the poor isolation of the circulator thus allow a single antenna to be used for transmitting and receiving the radar signal. The system will be used for monitoring and management of earth resources such as paddy fields, oil palm plantation and soil surface. This paper highlights the design and development of the SAR transmitter and receiver, as well as the evaluation result of the sensor. Calibration has been performed in the laboratory to verify the performance of the radar sensor. External calibration is accomplished by using three artificial point targets, i.e., 12” conducting sphere, 4”×8” dihedral corner reflector and 8” trihedral corner reflector. The field measurements are conducted in an empty car park, which is a low reflection outdoor environment. Both range detection and radar cross section (RCS) measurement capability are verified in the field experiments.

Design and construction of a multipurpose wideband anechoic chamber

An electromagnetic anechoic chamber has been constructed at the Multimedia University. It is designed to operate over a very wide frequency range from 30 MHz through 18 GHz. It can be used for electromagnetic compatibility (EMC) tests, antenna measurements, radar cross section (RCS) measurements, testing RF transceivers, calibration of scatterometers, and other electromagnetic research experiments. The geometry of the chamber is asymmetrical, consisting of a combination of rectangular and tapered volumes. The size of the chamber is about 64 ft /spl times/ 32 ft /spl times/ 24 ft high. Ease of construction and ease of lining of the absorbers were preserved so that the actual quietness is comparable to the theoretical simulation result. The transmitting source is placed at one end of the chamber, and the receiving antenna or object under test is placed in a designated quiet zone at the other end. The walls and ceilings are configured such that no first-order and or second-order reflections (except those reflected from the floor) propagate into the quiet zone. Hence, less-expensive absorbers can be used to achieve the required wideband performance. Reflection from the floor is required by the international EMC standards for radiated electromagnetic interference (EMI) emission tests; therefore, the chamber is designed for this capability. However, absorbing material can be placed on the floor to convert a semi-anechoic chamber into a fully anechoic chamber for radar and antenna measurements.

Design of a Microstrip Patch Antenna Array for Airborne SAR Applications

A microstrip patch antenna array has been developed for a C-band airborne Synthetic Aperture Radar. The antenna operates at 6 GHz with a bandwidth of 168 MHz (SWR<1.5). It is a rectangular patch fed by a probe from a feed network placed behind the ground plane. A cost effective substrate with low dielectric constant and moderate loss is used for the patch and feed network. The antenna has an optimally shaped beam pattern (with a beamwidth of 24°) in the E-plane with the sidelobe suppressed by 15 dB. In the H-plane, a narrow beamwidth of 3° is designed with the sidelobes suppressed by 30 dB. The antenna achieved a gain of 21.2 dBi. A practical method to compensate for the losses in the feed distribution network is introduced.

A convenient method for complex permittivity measurement of thin materials at microwave frequencies

A practical problem in the reflection method for measuring permittivity of thin materials is the difficulty in ensuring the sample is placed exactly at the waveguide flange. A small position offset of the dielectric slab will give rise to significant errors in calculating the permittivity. To circumvent this problem, a measurement method using a waveguide partially filled with a thin material slab has been developed. The material sample can be easily prepared and inserted into the guide through a longitudinal slot on the broad wall of the waveguide. Multiple material slabs can be measured rapidly because one does not have to disconnect the waveguide system for sample placement. The method is verified with measurement of Teflon, glass and FR4 fibreglass. The measured permittivity show good agreement with published data. Subsequently, the permittivity of a vegetation leaf was measured. The method presented in this paper is particularly useful in measuring the permittivity of a thin and narrow slab of natural materials such as a paddy leaf.

A microwave anechoic chamber for radar-cross section measurement

A microwave anechoic chamber has been developed at the Department of Electrical Engineering, University of Malaya, for monostatic and bistatic radar-cross-section measurements. The structure of the chamber is a quarter-section geodesic dome, with a 12 foot radius, and raised three feet above the floor. An antenna railing system is installed inside the chamber. The antennas can be moved along the rails in the elevation direction, with the microwave beam pointing at the center of the dome where the target is located. This design enables a very large combination of incidence and scattering angles in bistatic measurements. Four transmitting antennas are fixed at different elevation angles next to one of the antenna rails. By using an azimuth-over-elevation positioner as the pedestal for the target, and by positioning the movable antenna along that rail beside those fixed transmitting antennas, monostatic measurements with incidence angles ranging from 0 to 90 degrees can be accomplished. A vector network analyzer is utilized to measure the amplitude and phase of the radar returns. An IEEE-488.2 interface bus is used to control various hardware components, as well as to perform data acquisition. A computer program was written to automate the measurement system. Data are stored in raw format, and processed later with dedicated software, so that different processing methods and parameters can be applied. This paper highlights the design and construction of the microwave anechoic chamber, as well as the measurement-system configuration for scattering-cross-section measurements.

THE MASAR PROJECT: DESIGN AND DEVELOPMENT

In 2002, the MASAR (Malaysian Airborne Synthetic Aperture Radar) project was initiated at Multimedia University (MMU), in collaboration with the Malaysian Centre for Remote Sensing (MACRES). The main objective of this project is to construct an instrument for earth resource monitoring in Malaysia. The proposed SAR system is a C-band, single polarization, linear FM radar. This paper outlines the major design issues and considerations for MASAR. In particular, the design and construction of the microwave system, microstrip antenna, and a high-speed data recording system are described. The SAR processing algorithm which incorporates motion compensation capability for high resolution image generation is also outlined.

Variability analysis of impedance matching network

This paper investigates the L network impedance matching analytically. The equation describing the effect of load impedance variation on the impedance matching performance is derived. The deviation from perfect match is proportional to the variation in load reflection coefficient. However, if the load impedance is too large or too small, resulting in a large reflection coefficient, the solution to the impedance matching problem will have poorer quality in term of variability. The variability factor increases rapidly when the load reflection coefficient is larger than 0.7. A small variation in the load impedance will cause a large deviation from perfect match.