A novel multi-mode frequency reconfigurable MIMO and an ultrawideband (UWB) MIMO antenna design for cognitive radio applications is proposed in this paper. The antenna structure contains two modified triangular-shaped printed monopole antenna elements and a combination of varactor and PIN diodes. The proposed antenna has three operating modes: UWB mode for spectrum sensing, a frequency reconfigurable MIMO mode for communication over different frequencies, and UWB antenna operating in MIMO configuration. The UWB mode and reconfigurable communication mode are obtained by utilizing different switching ON/OFF states of PIN and varactor diodes. In the UWB mode, the antenna can cover the spectrum from 1 to 4.5 GHz, while it achieves the frequency reconfigurability over a wide range from 0.9 to 2.6 GHz using varactor diode tuning in the reconfigurable communication mode. The proposed antenna has a compact and planar structure with overall dimensions of 120 × 60 × 1.5 mm 3 . The proposed design also exhibits good MIMO performance; the minimum isolation measured is 12.5 dB, while the envelope correlation coefficient (ECC) of less than 0.19 is achieved for the whole operating band. To validate the proposed concept, the prototype of the antenna system is fabricated and measured. The simulation results are in good agreement with the measurement results.INDEX TERMS Cognitive radio (CR), frequency reconfigurable antenna, MIMO, PIN diodes, ultrawideband (UWB), varactor diode. I. INTRODUCTIONCognitive radio (CR) has appeared as a revolutionary technology for dynamic, efficient and flexible utilization of the scarce frequency spectrum [1]. A CR system architecture includes two antennas: ultrawideband (UWB) sensing antenna for constant monitoring of the unoccupied frequency channels and a reconfigurable communication antenna for communication within those unoccupied channels [2]. Majority of designs reported in literature for CR applications employ separate antennas for spectrum sensing and communication purposes. However, the use of two different antennas results in large physical size, more space requirement and complex structure, which is not suitable for today's communication. Thus, to deal with these limitations, a single antenna capable of both communicating and sensing is very desirable for cognitive radio communication.The associate editor coordinating the review of this manuscript and approving it for publication was Chan Hwang See.

No abstract

No abstract

Recent research results on negative-ion-rich plasmas in a large negative ion source have been reviewed. Spatial density and flow distributions of negative hydrogen ions (H(-)) and positive hydrogen ions together with those of electrons are investigated with a 4-pin probe and a photodetachment (PD) signal of a Langmuir probe. The PD signal is converted to local H(-) density from signal calibration to a scanning cavity ring down PD measurement. Introduction of Cs changes the slope of plasma potential local distribution depending upon the plasma grid bias. A higher electron density H2 plasma locally shields the bias potential and behaves like a metallic free electron gas. On the other hand, the bias and extraction electric fields penetrate in a Cs-seeded electronegative plasma even when the electron density is similar. Electrons are transported by the penetrated electric fields from the driver region along and across the filter and electron deflection magnetic fields. Plasma ions exhibited a completely different response against the penetration of electric fields.

No abstract

No abstract

Experiments by a four-pin probe and photodetachment technique were carried out to investigate the charged particle flows in the beam extraction region of a negative hydrogen ion source for neutral beam injector. Electron and positive ion flows were obtained from the polar distribution of the probe saturation current. Negative hydrogen ion flow velocity and temperature were obtained by comparing the recovery times of the photodetachment signals at opposite probe tips. Electron and positive ions flows are dominated by crossed field drift and ambipolar diffusion. Negative hydrogen ion temperature is evaluated to be 0.12 eV.

No abstract