Umair Rafique

Transmission of RF signals through energy efficient window using FSS

Focusing on the radio frequency (RF) transmission through energy saving window, this paper presents the transmission characteristics of a dual-bandpass frequency selective surface (FSS) for GSM 900 MHz and 1800 MHz frequency bands used in GSM system. Through simulation, better transmission and less heat loss has been analyzed and investigated in this paper. Better stability has been achieved for both perpendicular (TE) and parallel (TM) polarizations at 0° incident angle.

A tri-band H-shaped microstrip patch antenna for DCS and WLAN applications

In this paper, a tri-band H-shaped microstrip patch antenna is presented for DCS and WLAN (IEEE 802.11a/b/g) applications. The proposed antenna has symmetrical properties and has been designed by etching H-shape structure on FR-4 substrate with coaxially fed input. It radiates for DCS (1.79-1.81 GHz) and WLAN (2.43-2.47 GHz, 5.11-5.27 GHz) frequency bands. It is demonstrated that the fabricated antenna offers improved values of S11 and VSWR. The noted gain at 1.8 GHz, 2.45 GHz and 5.2 GHz are 1.6 dBi, 1.9 dBi and 2.1 dBi, respectively. The return loss S11 of the fabricated antenna was simulated using Ansoft HFSS which was in good agreement with experimental data.

Multiband Microstrip Patch Antenna for Microwave Applications

In this paper, we present a fractal shape slot based patch antenna for multiband operations. The fractal shape slot is designed on the rectangular patch and the antenna is fed through a microstrip line. The proposed design and feeding technique allows the antenna to operate at multiple frequencies in the range of 120 GHz. It is demonstrated through simulation that the Return Loss (RL) values occurred at 1.33 GHz, 4.16 GHz, 7 GHz, 9.8 GHz, 12.7 GHz, 15.5 GHz and 18.3 GHz, respectively, having VSWR ≤ 2. It is also observed that the gain of proposed design is higher than the conventional patch antenna. Parametric study is also included to give an overview for the performance of proposed design.

An efficient mechanism to simulate DC characteristics of GaAs MESFETs using swarm optimization

A five parameters I-V model has been presented to predict I-V characteristics of submicron GaAs MESFETs. The proposed model guaranteed convergence in an optimization process and does not trap in a local minimum which is frequently observed in Dobes model. Comparison between the Dobes and the proposed model has been carried out by developing an optimization code based on particle swarm optimization. By evaluating RMS errors as a function of Vgs and Vds it has been shown that the proposed model is efficient both in time and accuracy compared to the Dobes model and it should be a useful tool for the designing of future integrated circuits with sub-micron gate length GaAs MESFETs.

A wideband slot-coupled inverted microstrip patch antenna for wireless communications

A wideband slot-coupled superstrate based microstrip patch antenna is presented for wireless communications. The antenna is designed by employing a stacked geometry and inverted configuration of patch antenna. The circular patch antenna design is used for the particular design. The proposed antenna is able to operate for a wide range of frequencies, i.e., 19.5-22.6 GHz. Through simulation, the impedance bandwidth is achieved up to 15% having VSWR less than 2. By employing a technique of thicker substrate and stacked geometry, maximum peak gain of 8.5 dBi of the antenna is achieved. The paper also presents a parametric study showing that the impedance of patch decreases with the change in patch dimension which also effects VSWR and gain of the antenna.

Design of a Ku-band high gain low noise amplifier

A single stage low noise amplifier is developed for Ku-band applications. The design is aimed to provide low noise figure and high associated gain. Submicron gate length GaAs MESFET is used as an active device because of its superior high frequency properties. Device equivalent circuit parameters are first evaluated with and without pad capacitors using measured S-parameters. Low noise amplifier is then designed by employing common source configuration. Additional circuit components are employed to achieve unconditional stability. Whereas, matching networks of different types are evaluated to achieve VSWR closer to unity. Simulated data showed a bandwidth of 1.63 GHz with minimum noise figure 1.02 dB and 14.1 dB associated gain.

A wideband power divider for microwave applications

A wideband power divider on single layer microstrip technology is proposed, analyzed and fabricated. The proposed divider is designed by using two-section impedance transformers with an input and two symmetrical output extension lines. The analysis of the power divider is performed by applying transmission line theory. Better return loss and isolation factor is demonstrated over 50% impedance bandwidth by applying -15 dB bandwidth criterion. The prototype of the proposed power divider is fabricated and tested to verify the design and it has been noticed that the simulated results are in agreement with the measured data.

Optimization of linear antenna array for low SLL and high directivity

This work focuses on optimal synthesis of a linear array to achieve narrower half-power beam width, reduced side-lobe levels, and nulls control as per the need of the design. To accomplish these goals we considered the optimization of excitation amplitude, phase and inter-element spacing of the array. Since, the optimization problem involved is not convex in nature, so a global optimization algorithm is employed. An effort has been made to optimize the design variables by using Particle Swarm Optimization (PSO), which employed a fitness function defined using null direction and side lobe level (SLL) constraints. It has been shown that for improved array characteristics an appropriate weighted input should be employed and number of elements defining the array should be at least 16.

Design of an Efficient High Power Microwave antenna

Efficient High Power Microwave radiating systems are required to extract high power microwaves from HPM devices. Efficiency of an HPM antenna can be improved by varying its physical shape and dielectric used in it. The most critical location in an HPM antenna is its interface from where microwave leaves and enters into the air. In this paper, the performance of an HPM antenna has been investigated as a function of its profile. It has been demonstrated that by adopting Gaussian beam distribution at the aperture of an horn antenna with an appropriate beam waist, the E-field intensity at the output window can significantly be reduced resulting into 29 % improvement in its gain. Thus, in general, improved characteristics of an HPM antenna can be achieved by adopting an appropriate horn profile, beam waist and dielectric.

Breakdown in a high power microwave antenna

In this work, a high power microwave (HPM) antenna is designed and its breakdown limits are investigated. It has been shown that the designed antenna can handle Giga Watt (GW) power without attaining breakdown. The flash-over which usually occurs at the interface of HPM antennas, where microwave leaves and enters into the air, was controlled by adjusting the field intensity at the output window of the antenna. It has been shown that by adopting Gaussian beam distribution at the aperture of horn with appropriate beam waist, the E-field intensity at the output window can be reduced significantly. It has also been demonstrated that an HPM antenna with 0.4 m aperture radius can sustain >30 GW power for a pulse duration of 1ns and the same is reduced to <;5 GW for the pulse having duration > 1μs.