A printed reconfigurable ultra-wideband (UWB) monopole antenna with triple narrow band-notched characteristics is proposed for cognitive radio applications in this paper. The triple narrow band-notched frequencies are obtained using a defected microstrip structure (DMS) band stop filter (BSF) embedded in the microstrip feed line and an invertedπ-shaped slot etched in the rectangular radiation patch, respectively. Reconfigurable characteristics of the proposed cognitive radio antenna (CRA) are achieved by means of four ideal switches integrated on the DMS-BSF and the invertedπ-shaped slot. The proposed UWB CRA can work at eight modes by controlling switches ON and OFF. Moreover, impedance bandwidth, design procedures, and radiation patterns are presented for analysis and explanation of this antenna. The designed antenna operates over the frequency band between 3.1 GHz and 14 GHz (bandwidth of 127.5%), with three notched bands from 4.2 GHz to 6.2 GHz (38.5%), 6.6 GHz to 7.0 GHz (6%), and 12.2 GHz to 14 GHz (13.7%). The antenna is successfully simulated, fabricated, and measured. The results show that it has wide impedance bandwidth, multimodes characteristics, stable gain, and omnidirectional radiation patterns.

In this paper, a ε-negative metasurface superstrate is proposed for mutual coupling reduction of large antenna arrays. Unlike the previous decoupling metasurface works that are mostly confined to two-ports antennas, the proposed decoupling superstrate can be extended to massive multiple-input multiple-output (MIMO) systems. A 4×4 antenna array is used as an example to illustrate the decoupling performance of the proposed metasurface. With the decoupling metasurface, the worst mutual coupling of the antenna array is improved by 8 dB over the operation bandwidth with a maximum mutual coupling reduction of 25 dB. Moreover, the decoupling metasurface also help restore the radiation patterns, bring down the active voltage sanding wave ratio, and broaden the bandwidth of the array.

In this paper, we present the first demonstration of distributed and symmetrical all-band quasi-absorptive filters that can be designed to arbitrarily high orders. The proposed quasi-absorptive filter consists of a bandpass section (reflective-type coupled-line filter) and absorptive sections (a matched resistor in series with a shorted quarter-wavelength transmission line). Through detailed analysis, we show that the absorptive sections not only eliminate out-of-band reflections but also determine the passband bandwidth. As such, the bandpass section mainly determines the out-of-band roll-off and the order of the filter can be arbitrarily increased without affecting the filter bandwidth by cascading more bandpass sections. A set of 2.45-GHz 1-, 2-, and 3-pole quasi-absorptive microstrip bandpass filters are designed and measured. The filters show simultaneous input and output absorption across both the passband and the stopband. Measurement results agree very well with the simulation and validate the proposed design concept.

handling is enough for some applications with a following power amplifier used to amplify the RF pulses.The switching time of the RF-pulse former was measured based on the RF pulse's envelope observed on the digital oscilloscope HP 54124T. The 35-GHz CW signal is gated by the control impulses generated from the impulse generator at the pulse repetition frequency of 100 KHz. The inverters integrated in the RF-pulse former sharpen the rising and falling edges of control impulses, hence eliminating external effects, such as measurement cables and bonding wires, to the switching speed of the RF-pulse former which is mainly determined by the small gate resistor and gate-source capacitor of the MOSFET transistors. The envelope of the 35-GHz RF pulses at the output of the RF-pulse former can be clearly seen on the digital oscilloscope triggered by the trigger signal produced from the impulse generator. Figure 6 shows the measured 0.8-ns RF pulse and its spectrum. There is no RF leakage seen from the measured spectrums; the leakage, if any, is smaller than the magnitude of the actual RF pulse, which demonstrates the ultra-high isolation of the RF-pulse former. From Figure 6(a), the 10-90% rising time and 90-10% falling time of the RF-pulse former are determined to be 136 and 70 ps, respectively. The total measured switching time of 206 ps is 26 ps larger than the simulated result due to the delay in the inverters. The small switching time of 206 ps allows the RF-pulse former to produce very narrow RF pulses for high resolution radar and high-data-rate communication systems. Figure 7 shows the measured 200-ps RF pulse, demonstrating the possibility of generating very narrow pulses.ABSTRACT: In this article, we present a compact wide-slot antenna with reconfigurable notch bands, for ultra-wideband (UWB) and multiband communication applications. The dual band-notch characteristics are realized by using three open-ended stubs inserted into the fork-like radiating element, and the reconfigurable characteristics are achieved by integrating ideal switches in the stubs. By controlling the ON/OFF states of the ideal switches, the proposed antenna can function either as a UWB antenna with two notch-bands, or a single notch-band UWB antenna, or even a multiband antenna. The simulation and measurement results show that the proposed antenna is suitable for UWB as well as multiband communication systems.