Design and Study the Effect of Switching in Microstrip Patch Antenna for Cognitive Radio Applications

Design of microstrip antenna having switches for accomplishing frequency reconfigurability is proposed. The proposed design is having six resonating frequency bands between 3.34GHz to 3.67GHz which were achieved by using different switching configurations. Switches were assumed as ideal so as to study the simulated results of the antenna .The antenna design was made on the FR4 substrate having height of 1.6mm.The overall size of the antenna is 39×40mm


Introduction
Recently the use of RF switches in microsrtip patch antenna for achieving reconfigurability has been a great area of research. The parameters, for example, f radiation pattern, frequency and polarization are changed to satisfy the application requirements. These antennas can be used in application like cognitive radio which requires the process of spectrum sensing to use the spectrum wisely 1,8,9 . There are other techniques that have been applied as well to achieve a desired operation in an antenna such as, physical size changing, addition of multiple layers and gaps 14 , using stacked patches 12 , inserting slits and changing the substrate material 13 . However these tequniques can increase the size of the antenna significantly which will make it difficult for these antennas to fit into small devices [2][3][4] .Further design optimization techniques needs to be employed on these antennas to decrease the effective length and width of antenna [3][4][5][6] .
There have been multiple innovative techniques that have been used to achieve the reconfigurable operation in an antenna such as use of PIN diodes, varactor diodes to act as switches 10,11 . To change the current distribution in the slot FET components have also been used which actually alters the effective length of the antenna. The use of micro-electro-mechanical system to improve the accuracy in switching has also been widely employed in the designs, but this increases the cost and circuit complexity in the design [5][6][7] .
This paper aims to design and analyze microstrip antenna and study the effect of switching on the parameters of the antenna for example, gain, return loss, Keywords: Cognitive, PIN Diodes, Reconfigurable, Switching directivity and VSWR of the antenna. The different switching configurations are presented in this paper. The switches are acting as ideal so small conducting slits were used to show the ON position of the switch while doing the simulation on CST Microwave Studio.

Antenna Configuration and Design
We will be discussing about the design structure of the antenna in this section. Figure 1

Simulated Results of Initial Design with No Slot
The Figure 4 is showing return loss plot of the initial design structure having no slot cut in the ground plane. The antenna was resonating at 3.63GHz and was having a return loss of -21.85dB. The VSWR of the design was also simulated its value was 1.17. The radiation pattern of the initial design was omni-directional having a gain of 3.31dB. The design was then modified and a slot was cut in the bottom ground plane of the antenna to introduce multiple resonating modes in the antenna by varying the overall effective size of the antenna.

Simulated Results by Cutting Slot in Bottom Ground Plane and by Introducing Switches
By inserting the slot and introducing four small switches in bottom ground plane of designed antenna, multiple resonant frequency bands were observed. Four switches were positioned optimally in the slot. The different switch configurations are shown on the Table 1. When S1 is on the antenna is resonating at 3.34GHz having a return loss of -39.72dB. Similarly by switching on switches S2, S3 and S4 antenna was resonating at 3.45GHz, 3.60GHz and 3.52GHz having return loss of -44.27dB, -39.19dB,-37.31 dB respectively. The radiation pattern at all frequencies as having same orientation with different gains. Similarly the VSWR of the design was having value below 1.5 at all resonating frequencies. The percentage bandwidth at all resonating frequencies was lying in the range of 3.11% to 4.31% and overall bandwidth ratio of 1.1:1 was measured from the simulated results. Radiation patterns at various resonating frequencies can be seen in fig.7. All the radiation patterns are having similar radiation pattern orientation except the gain at each resonating frequency is different. The maximum gain of 4.04dB was measured at resonating frequency of 3.45GHz and minimum gain of 3.31dB was measured at resonating frequency of 3.63GHz. At all other frequencies the gain of the antenna lies in this range.

Conclusion
Effect of switching in microstrip patch antenna has been presented in this paper. Multiple resonant frequency bands within bandwidth ratio of 1.1:1 are shown. It has been observed that by changing the switching configuration in the design the effective size of the radiating patch changes and antenna resonates at different frequency. RF components and MEMs can be used to fabricate the design's switching configuration.