is obtained for minimum thickness while least agility 36.76% (0.918 GHz) is achieved for highest insulator thickness (500 Å). It is also observed from Figures 5 and six that the variation in resonance frequency with bias voltage is faster near zero bias voltage that becomes steady towards higher values of bias voltage. This is because of the fact that change in frequency is solely governed by the MOS capacitance [Eq. (4)]. It is evidently clear that the proposed frequency agile active microstrip antenna can be operated with varying tuning capabilities at microwave range by controlling the bias voltage of the device integrated with the patch.The theoretical and simulation results for return loss of proposed frequency agile active microstrip antenna as a function bias voltages are shown in Figure 7. It is observed that the return loss remains well below the Ϫ10 dB range (varies from Ϫ19.85 to Ϫ14.3 dB). The radiation pattern of the antenna is shown in Figure 8. That is invariant with the bias voltage of MOS capacitor.
CONCLUSIONA new technique to enhance the frequency agility has been proposed. The frequency agile active microstrip antenna can be operated with varying tuning capabilities at microwave range by controlling the bias voltage of MOS capacitor integrated with patch. It found the frequency agility (1.527 GHz) for proposed antenna is the maximum for the minimum thickness of insulator layer of typical MIS diode structure (Au-Si 3 N 4 -Si).
INTRODUCTIONThree-terminal transistor can work in a reflection mode instead of the common transmission configuration, to present high reflection gain and low noise features in reflection-type amplifiers [1-4], or provide negative resistance for loss compensation in active filters [5, 6]. Such circuits should properly choose the feedback and terminating impedance on one port to give negative-resistance and optimum noise measured at the other port. When this topology is much adopted in active filters and one-port reflection-type amplifier [4], it is not suitable for two-port reflection-type amplifiers [1][2][3], because of the necessary presence of the bulky circulator for input and output ports (I/O) fundamental frequency isolation.On the other hand, a frequency doubler has inherent I/O frequency separation and it is easy to adopt this topology. Desirably, the circulator can be replaced by filters or ring resonator. The reconfigured structure utilizes feedback to introduce negativeresistance at second harmonic to ensure the circuit has more than unity conversion gain. To prove the design methodology, two frequency doublers with filters or ring resonator are designed at S-band and both have more than 3 dB conversion gain.
DESIGN PROCEDURE AND THEORYFigures 1 and 2 show the two new frequency doubler circuits, one with filters and the other incorporating a ring resonator. Both circuits can be divided into two parts, which are designed sepa- rately, the I/O network and the one-port reflection-type doubler, which is depicted in Figure 3. There are two steps in the design pro...
