The efficacy of a simple, electrically small, low-profile, Huygens source antenna that radiates in its broadside direction is demonstrated numerically and experimentally. First, two types of electrically small, near field resonant parasitic (NFRP) antennas are introduced and their individual radiation performance characteristics are discussed. The electric one is based on a modified Egyptian axe dipole NFRP element; the magnetic one is based on a capacitively loaded loop (CLL) NFRP element. In both cases the driven element is a simple coax-fed dipole antenna, and there is no ground plane. By organically combining these two elements, Huygens source antennas are obtained. A forward propagating demonstrator version was fabricated and tested. The experimental results are in good agreement with their analytical and simulated values. This low profile, ~ 0.05 λ 0 , and electrically small, ka = 0.645, prototype yielded a peak realized gain of 2.03 dBi in the broadside direction with a front-to-back ratio (FTBR) of 16.92 dB. A backward radiating version is also obtained; its simulated current distribution behavior is compared to that of the forward version to illustrate the design principles.Index Terms -Broadside directivity, electrically small antennas, Huygens source, low profile, metamaterial-inspired structures, near field resonant parasitic antennas.

A compact planar ultra-wideband (UWB) antenna with continuously tunable, independent band-notches for cognitive radio applications is presented. The antenna is fabricated using a copper cladded substrate. A radiating patch with an inverted rectangular T-slot is etched on the top side of the substrate. A straight rectangular strip with a complete gap is embedded into the T-slot. By placing a single varactor diode across this gap, a frequency-agile band-notch function below 5 GHz is realized. On the bottom side of substrate, a U-shaped parasitic element having an interdigitated-structure is placed beneath the radiating patch. The second narrow band-notch is created by inserting a second varactor diode into the gap on one leg of the parasitic element. It has a frequency agile performance above 5 GHz. The presence of the interdigitated structure suppresses higher-order resonant modes and enhances the tunability of the notched bandwidth. Because these antenna structures naturally block DC, a very small number of lumped elements are required. The experimental results, which are in good agreement with their simulated values, demonstrate that both band-notches can be independently controlled and the entire frequency-agile fractional bandwidth is as high as 74.5 %, demonstrating a very wide notched frequency-agile coverage. Index Terms-Band-notch filters, frequency agile, frequency tunable, planar antennas, UWB antennas I. INTRODUCTION ltra-wideband (UWB) technology has been widely applied in wireless sensor networks, biomedical and healthcare wireless systems, and some other in-house devices in radar detecting, locating, and communications [1]. These applications benefit from the unique features of low-power spectral density and consumption associated with UWB

The morphological information on the very front of a spreading liquid is fundamental to our understanding of dynamic wetting. Debate has lasted for years concerning the nanoscopic local angles and the transition from them to the macroscopic counterpart, θ(D). This study of nonvolatile liquids analyzes the interface profile near the advancing contact line using an advanced atomic force microscopy. The interface is found following the macroscopic profile until bending in a convex profile around 20 nm from the substrate. This shoe-tip-like feature is common in partially wetting while absent for completely wetting, and its curvature varies with advancing speed. The observation ends the long-standing debate about the nanoscopic contact angles and their speed dependency. The convex nanobending provides a mesoscopic link and effectively complicates the dynamic wetting behaviors.

-A mutual coupling reduction strategy that employs meta-structures is introduced for wideband, dual-polarized, high-density, planar, patch antenna arrays. The meta-structures consist of two types of resonators: grounded capacitively loaded loops (GCLLs) and π-shaped elements. By incorporating the meta-structures into the array configuration, the isolation levels between adjacent radiating elements in both the E-and H-plane orientations are improved by as much as 7.15 dB. The surface current distribution behaviors of the array with only the GCLLs and with only the π-shaped elements are investigated thoroughly to explain the mutual coupling reduction mechanisms. A proof-of-concept array was constructed and tests were performed that validate the reported design principles and simulation results. M. (5G) communication systems [1,2]. It has several unique advantages that arise from its ability to facilitate the presence of additional signal channels instead of requiring the use of extra frequency spectrum or power. Benefits include being able to increase the data capacity and to make the system more adaptable. Index TermsMIMO approaches are based on antenna arrays. With the ever increasing demand for more capacity, it is anticipated that massive MIMO will be central to 5G systems; and it will be facilitated by compact dense arrays. However, strong inter-element coupling occurs when the element spacing is small; it dramatically increases the spatial correlations and seriously deteriorates the signal-to-interference-plus-noise -ratio (SINR) [3]. Consequently, mutual coupling effects between array elements have attracted intense attention.There exist a plethora of reported approaches that are capable of reducing the mutual coupling in the physical layer. However, because of known fundamental physical limitations [4], it cannot be eliminated completely. Nevertheless, mutual coupling reduction can significantly improve the performance of MIMO systems.In general, the existing, most widely used mutual coupling reduction strategies can be classified into the following three categories. One approach focuses on introducing a variety of compact isolation elements between the radiators. The choices have been rather diverse. They include parasitic elements, such as monopoles [5], scatterers [6], and radiation patches [7]. Also, they include electromagnetic band-gap (EBG) structures, such as mushroom-like EBG structures [8], F-shaped EBG structures [9], and uniplanar compact EBG structures [10,11]. Defected ground structures (DGSs), such as periodic rectangular slits [12], back-to-back U-shaped slots [13], T-shaped slots [14], and loop slots [15] have also been used. More recently, numerous metamaterial-inspired structures have been considered, such as folded single split ring resonators [16], grounded split-ring resonators (GSRRs) [17], multiple split-ring resonators (MSRRs) [18] While the above strategies have reduced mutual coupling, one witnesses certain drawbacks accompanying each of them. As a result, their widespre...