In this letter, an ultrawideband (UWB) multiple-input-multiple-output (MIMO) antenna using a novel fence-type decoupling structure, which has high isolation in the UWB, is presented. The MIMO design consists of two half-cutting UWB antenna units that have good low-frequency impedance-matching performance because of the use of a rectangular slot on the radiation patch. Meanwhile, the isolation in the operating band is enhanced by introducing the fence-type decoupling structure at the ground of the antenna. Furthermore, the L-shaped parasitic branches are designed to heighten the impedance bandwidth and isolation in the low-frequency band, i.e., 3-3.4 GHz. The simulation and experiments show that the antenna has low mutual coupling (S21 <; -25 dB) and a low envelope correlation coefficient (ECC <; 0.004) in the UWB.

A small ultrawideband (UWB) slot antenna is proposed using a stepped impedance resonator feeding line, aiming at improving band-edge selectivity. The proposed antenna mainly comprises a feeding line with low-pass characteristics at the top of the dielectric substrate and two slots at the bottom. The stepped impedance resonator is used to improve the upper band-edge selectivity, and the narrow slot near the stepped slot improves the lower band-edge selectivity. The frequency selection range, the frequency selective characteristic, and the reflected zeros of the antenna upper band-edge are analyzed and studied. The measured results show that the antenna covers the frequency range of UWB systems from 3.1 to 11 GHz (114%) with the reflection coefficient less than -10 dB. Compared with the traditional UWB antenna, the proposed compact antenna has advantages in size and band-edge selectivity and is more suitable for UWB applications. The ratio of the -3 dB bandwidth to the -10 dB bandwidth is 1.038, which is very close to 1.

In this letter, a continuously tunable cutoff frequency filter is proposed, which is composed of tunable spoof surface plasmon polaritons (SSPPs) loaded with p-i-n diodes and varactor diodes. The dispersion behavior of the unit structure can be controlled by switching the SSPP unit with a p-i-n diode, accomplishing reconfigurability at two cutoff frequencies of 6.51 and 7.68 GHz. In order to realize more tunable cutoff frequencies, a new unit structure that can obtain a cascaded combination of varactor diodes and p-i-n diodes is presented. The simulated and measured results verify that the cutoff frequency of the filter can be tuned continuously from 5.34 to 7.23 GHz by controlling the magnitude of the biasing voltage, and the relative tunable bandwidth of the SSPP filter is up to 70.8%.

In this article, an amplitude equalizing spoof surface plasmon polaritons (AESSPPs) filter deposited on flexible substrate is proposed for the first time. This new hybrid device can realize both the characteristics of amplitude equalization and interference filtering when embedding into a wideband communication system, which brings the compensation process of gain fluctuation back to the microwave front end instead of traditional complex algorithm in digital chip. It is mainly composed of equalizing thin layer and spoof surface plasmon polaritons (SSPPs) waveguide. The equalizing thin layer is realized by ITO (Indium Tin Oxide) and periodically sputtered along the propagation path of SSPPs waveguide to dissipate EM energy at different frequencies. In order to better reveal the nature property of equalization and filtering, an equivalent circuit of AESSPPs unit cell is proposed and analyzed theoretically. The equalizing value and bandwidth can be regulated independently by surface impedance of ITO thin layer and the structural parameter of SSPPs, respectively. Finally, we fabricated and measured three AESSPPs filter prototypes with different critical parameters. All the theory, simulation and measurement results are proved to have achieved the artificial dominated equalizing curve with different equalizing value and bandwidth, which validate that the proposed filter demonstrates the desired flexibility, equalization and filtering capabilities simultaneously.

Abstact-A compact spoof surface plasmon polaritons (SSPPs) bandpass filter (BPF) with improved out-of-band rejection is proposed in this letter. By employing a novel folded slotline cell, the filter achieves a 67% reduction in longitudinal size compared to traditional rectangular slotline cells at the same cut-off frequency, and the filter measures 0.88λg× 0.16λg, where λg is the guided wavelength at the center frequency, which significantly shortens its electrical length. The lower cut-off frequency is adjusted by the microstrip-slot transition structure, and the higher cut-off frequency is determined by the depth of the SSPPs unit cell groove, which forms a BPF with controllable frequency band. In addition, the performance of the filter with and without unit symmetry is also analyzed, indicating that the filter of making symmetry structure can obtain better performance with the same size. The simulation results demonstrate that the proposed filter has a passband of 1.3-4.1GHz, and superior out-of-band suppression. To validate the accuracy of the simulation results, the filter proposed in this study is fabricated and subjected to empirical measurement, and the experimental outcomes confirm the practical feasibility of the proposed design.

A novel compact microstrip ultra-wideband (UWB) bandpass filter with three sharply notched bands and good selectivity is presented in this letter. Triple-notched bands filtering effects are achieved by adding open-circuited stubs and introducing asymmetric dual-line coupling structure to the feed lines, providing a high degree of adjusting freedom and not increasing the filter size. The frequencies of the notched bands and the upper stopband transmission zero can be independently controlled by adjusting the structural parameters of feed lines. Finally, a microstrip UWB filter with triple-notched bands is simulated and fabricated with good agreement.