Lianhong Zhang

Transformation optics for antennas: why limit the bandwidth with metamaterials?

In the last decade, a technique termed transformation optics has been developed for the design of novel electromagnetic devices. This method defines the exact modification of magnetic and dielectric constants required, so that the electromagnetic behaviour remains invariant after a transformation to a new coordinate system. Despite the apparently infinite possibilities that this mathematical tool introduces, one restriction has repeatedly recurred since its conception: limited frequency bands of operation. Here we circumvent this problem with the proposal of a full dielectric implementation of a transformed planar hyperbolic lens which retains the same focusing properties of an original curved lens. The redesigned lens demonstrates operation with high directivity and low side lobe levels for an ultra-wide band of frequencies, spanning over three octaves. The methodology proposed in this paper can be applied to revolutionise the design of many electromagnetic devices overcoming bandwidth limitations.

Surface Wave Cloak from Graded Refractive Index Nanocomposites

Recently, a great deal of interest has been re-emerged on the possibility to manipulate surface waves, in particular, towards the THz and optical regime. Both concepts of Transformation Optics (TO) and metamaterials have been regarded as one of key enablers for such applications in applied electromagnetics. In this paper, we experimentally demonstrate for the first time a dielectric surface wave cloak from engineered gradient index materials to illustrate the possibility of using nanocomposites to control surface wave propagation through advanced additive manufacturing. The device is designed analytically and validated through numerical simulations and measurements, showing good agreement and performance as an effective surface wave cloak. The underlying design approach has much wider applications, which span from microwave to optics for the control of surface plasmon polaritons (SPPs) and radiation of nanoantennas.

Analysis of on-body propagation at W band by using ray tracing model and measurements

The propagation between a transmitter (TX) and a receiver (RX) operating at 94 GHz, located in proximity of a human body, is investigated in this paper. The TX is positioned near the head and a line of RXs is located on the left shoulder. The high frequency method of ray tracing has been used to simulate these links. The simulated Path Loss (PL) has been compared to measured data. The agreement between simulations and measurements supports the possibility of using ray-based method to correctly predict the PL at millimeter wavelengths.

A Wide-angle Multi-Octave Broadband Waveplate Based on Field Transformation Approach.

Transformation optics (TO) offers a geometrical approach in designing optical components of any shapes. Although it has been proven to be a versatile and robust mathematical tool, TO has, however, limited control over electromagnetic (EM) field polarization in the process of coordinate transformation. Such a technique can be extended to a so-called “Field transformation (FT)” which provides direct control over the impedance and polarization signature of an arbitrary object. In this work, we demonstrate a FT application by designing and manufacturing a novel waveplate, which defies the fundamental limit of bandwidth and incident angles and has the ability of converting between TE (transverse electric) and TM (transverse magnetic) as well as LCP (left-handed circular polarization) and RCP (right-handed circular polarization). Such a waveplate can also be applied to different operating modes for both transmitted and reflected waves by adjusting its thickness and adding an optional metallic ground plane. The proposed design approach presents a remarkable degree of advance for designing future devices with arbitrary polarization controls, artificial waveguides or antenna substrates and polarization-enabled resonators with angle-insensitive functionalities. Our approach has far reaching implications applicable from radio to optical frequencies.

Statistical Path-Loss Model for On-Body Communications at 94 GHz

A great amount of work on antennas and propagation for body-centric wireless communication has been studied at frequencies up to X band; however, on-body radio propagation at millimeter/sub-millimeter wave frequencies still remains largely unexplored. This paper presents a study of on-body radio propagation at 94 GHz, particularly focusing on the analysis of specific channels such as waist-to-torso and head-to-shoulder links. Measured data are compared with results obtained with numerical simulations emphasizing the sensitivity of the simulated path loss to the positioning of the receivers with respect to the human body.

An experimental millimetre wave imaging system

MMW (millimetre wave) imaging is considered as an effective approach to CWD (concealed weapon detection). In this paper, an experimental millimetre wave imaging system based on an aperture synthesis approach is designed. The system is used to scan a metallic object and the resulting field distribution has been recorded. To reconstruct the image, a holographic processing algorithm is applied and a focused image is obtained. The system is active and works at a single frequency, 92.5 GHz. It can be used to investigate the optimized system parameters such as antenna beam width, operating range from the target and aperture array element spacing, etc.

A broadband wide-angle polarizer through field transformation

This paper presents a broadband wide-angle polarizer based on a field transformation (FT) method. By adjusting the thickness of such a polarizer, we achieve polarization conversions between TE and TM, LCP and RCP, linear and circular polarizations respectively for both transmission and reflection cases. Such a polarizer can be realized through sub-wavelength dielectric gratings based on the effect media theory, their performance have been verified through full-wave numerical simulations and experiments in microwave regime.

Accuracy of asymptotic techniques for on-body channel characterization at W band

Wireless Body Area Networks (WBANs) are an established filed of research and continue to attract interest from both the scientific community and the industrial world. They have been extensively studied at frequencies up to X Band, including Ultra-Wide Band (UWB) applications. However, the interest in higher frequencies has been recently increasing, in particular for the realization of on-body systems at V and W band. The research on the on-body propagation channel at such frequencies is its infancy, and presents several challenges, such as the significant computational effort required by full-wave simulations. Given the large electrical dimensions of the human body at millimeter wave frequencies, asymptotic methods offer a promising alternative: however, their accuracy should be carefully evaluated. This paper presents a comparison between measured and simulated results for the path loss characterization of the belt-to-chest link at 94GHz.

Woodpile EBG-based antennas for Body Area Networks at 60GHz

Two metamaterial Electromagnetic Band Gap (EBG) concept-based antennas are presented for applications in Body Area Network (BAN). The antennas work in V band at 60 GHz and are realized by means of a woodpile planar and cylindrical structure respectively. These structures are made of low loss alumina and can be manufactured through extrusion free-forming techniques. Both the here presented antennas take advantage from the directivity enhancement due to the resonant effect created by the partially reflecting woodpile structures. The reduced dimensions associated with the obtained directivity and pattern shape allow these antennas to be suitable for BAN applications. Specifically, the cylindrical antenna could be used for on-body applications, while the flat antenna is suitable for BAN-to-BAN or off-body communications.