A microstrip bandpass filter is presented based on Complementary Split Ring Resonators (CSRRs) and a pair of openloop resonators that has a single pair of transmission zeros at finite frequencies that causes an improvement at skirt response. An equivalent circuit is introduced to make analysis and optimization faster. Finally a filter is designed using the proposed cell and the simulation results with both equivalent model and full wave analysis are in very good agreement. The filter was fabricated and the measurement result was also in good agreement with simulation results. Besides, the size of the designed filter is very small and it occupies an area less than 0.23λ g × 0.16λ g , where λ g is the guided wavelength at the midband frequency.

Figure 8 shows the measured radiation patterns including the copolarization and crosspolarization in the H-plane (y-z plane) and E-plane (x-y plane). It can be seen that the radiation patterns in both of y-z and x-y planes are nearly omnidirectional for the two frequencies. Figure 9 shows the measured and simulated maximum gain of the proposed antenna and demonstrates a variation similar to other PIFA antennas. As shown in Figure 9 for the WLAN/ WiMAX frequencies, the measured antenna gain agrees very well with the simulated results. CONCLUSIONSIn this article, a novel compact printed inverted-F antenna has been proposed for simultaneously satisfying WLAN and WiMAX applications. The fabricated antenna has the frequency band of 4.95 to over 5.91. The desired resonant frequencies are obtained by adjusting the dimension of T-shaped notch. Also, to enhance the impedance bandwidth characteristic, a rectangular slot is inserted in the ground plane of the proposed antenna. Prototypes of the proposed antenna have been constructed and studied experimentally. The measured results showed good agreement with the numerical prediction and good multiband operation. ACKNOWLEDGMENTThe authors are thankful to Iran Telecommunication Research Center (ITRC) for its financial support of the work. ABSTRACT: Gallium nitride has attracted a great deal of interest in recent years due to its power handling ability. In addition, its noise performance is known to be good. In this letter, we present a method for determining the bias current density needed to obtain optimal noise figure for gallium nitride high-electron mobility transistors (HEMTs). Particle swarm optimization is used to fit transistor S parameters to a model, enabling the calculation of the transistor's two-port noise parameters. This process is performed for different bias points for different-sized transistors, leading to the conclusion that a current density of 0.3 mA/lm yields the best minimum noise figure. Key words: gallium nitride; noise figure; noise modeling; MMIC; low noise amplifiers INTRODUCTIONIn the past few years, gallium nitride has attracted the interest of microwave circuit designers. This is been principally due to its high suitability for power amplifiers, including its performance at high temperatures, its good power handling capacity and its high breakdown voltage [1, 2]. In addition to its suitability for high heat and high power applications, previous studies have noted its high resistance to ionizing radiation, which is much higher than the radiation resistance of gallium arsenide circuits [3]. If a designer would like to benefit from the advantages that gallium nitride offers, then one obvious route is to consider its use in a complete integrated circuit.In this case, a proper understanding of the noise behavior of the transistors in the gallium nitride process being considered for use is required. Different authors have reported on the noise of gallium nitride circuits and these types of transistors are reported to have good noise performance (see...

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