A narrowband frequency-tunable antenna for cognitive radio applications

In this paper, a frequency-tunable microstrip antenna, for cognitive radio applications, is proposed. The approach, presented herein, is based on electronically adjusting the resonant frequency of a band-pass filter, which is integrated into a wideband antenna, to achieve a tunable narrowband frequency operation for the antenna. For this sake, the design of open loop resonator (OLR) based band-stop filter, and its transformation to a band-pass filter, is investigated. A prototype of each filter is fabricated and measured. A credible agreement between Ansoft HFSS-simulated and measured patterns is obtained. The incorporation of an OLR based band-pass filter, into a wideband antenna, is then looked into. The assembled structure is found to resonate at the same operating frequency of the contained bandpass filter. The proposed filter-antenna (filtenna) is designed and simulated using Ansoft HFSS. An SMV1405 varactor is used to adjust the electrical length of the OLR contained in the bandpass filter, and thus, attaining frequency tunability for the filtenna. The computed reflection coefficient, for each case, is less than -10 dB over the operating frequency. A prototype of the designed filtenna is fabricated and measured. A good analogy between simulated and measured S₁₁ plots is witnessed. Besides the higher gain figure at 4.18 GHz, a minimally distracted omnidirectional radiation pattern is also revealed at the same frequency.