Mirko Barbuto

Circular Polarized Patch Antenna Generating Orbital Angular Momentum

The recent extension of the orbital angular momentum (OAM) concept from optical to microwave frequencies has led some researchers to explore how well established antenna techniques can be used to radiate a non-zero OAM electromagnetic fleld. In this frame, the aim of the present paper is to propose a new approach to generate a non-zero OAM fleld through a single patch antenna. Using the cavity model, we flrst analyze the radiated fleld by a standard circular patch and show that a circular polarized (CP) TMnm mode excited by using two coaxial cables generates an electromagnetic fleld with an OAM of order §(ni1). Then, in order to obtain a simpler structure with a single feed, we design an elliptical patch antenna working on the right-handed (RH) CP TM21 mode. Using full-wave simulations and experiments on a fabricated prototype, we show that the proposed antenna efiectively radiates an electromagnetic fleld with a flrst order OAM. Such results prove that properly designed patch antennas can be used as compact and low-cost generators of electromagnetic flelds carrying OAM.

Design of a Non-Foster Actively Loaded SRR and Application in Metamaterial-Inspired Components

In this paper, we investigate on the use of non-Foster active elements to increase the operation bandwidth of a split-ring resonator (SRR) for possible application in metamaterial-inspired components. First, we design the circuit topology of the active load required to compensate the intrinsic reactance of the SRR and get a broadband response. Then, we show that the same procedure can be successfully applied to the case of a SRR-based monopole antenna and, in principle, to any metamaterial-inspired device employing SRRs. Finally, integrating an electromagnetic and a circuit simulator, we propose a possible realistic implementation of the active load, based on the employment of commercially available circuit elements. The obtained results (seven times improvement of the impedance bandwidth of the SRR-based monopole antenna) prove that non-Foster active loads can be successfully used to overcome the inherent narrow-band operation of SRR-based passive metamaterials and metamaterial-inspired components. The implementation issues related to circuit element dispersion, parasitic effects, and stability of the active circuit are fully considered in the proposed design.

A Combined Bandpass Filter and Polarization Transformer for Horn Antennas

Recently, our group has proposed a self-filtering linearly polarized horn antenna that can be used to reduce the noise captured by regular horn feeds. However, the approach used in that design is inherently limited to linear polarization, while possible interesting applications are likely in satellite receiving systems, which are typically based on circularly polarized signals. In this letter, we propose a new approach to obtain a filtering horn antenna working in circular polarization. The proposed solution is based on the design of a linear-to-circular polarization transformer that consists of a complementary electrically small resonator etched on a metallic screen. We first show that this component is able to transform the linear polarization of a regular rectangular waveguide working on the fundamental mode into a circular one. Then, integrating this polarization transformer in a conical horn, we show how it is possible to obtain a circularly polarized filtering horn antenna. The numerical simulations and the measurements performed on a prototype prove that the proposed structure can be effectively used to design a bandpass filtering horn antenna for circularly polarized signals.

Horn Antennas With Integrated Notch Filters

In this communication, we present the design of filtering horn antennas with band-stop characteristics obtained through the use of electrically small magnetic resonators. In particular, a split-ring resonator (SRR) etched on a Rogers Duroid RT5870 dielectric substrate is inserted within the metallic flare of the horn at a proper distance from the throat. At around the resonant frequency of the SRR transmission is highly reduced and a single notched-band is obtained. In order to extend the result to dual-band operation, we also present the design of the filtering module made by two SRRs with different dimensions. The validity of the proposed approach is verified through proper sets of full-wave simulations and experiments on fabricated prototypes. The proposed solution is economical, light, electrically small, easily implementable on already existing radiators, and can find application in wideband communication systems affected by narrowband interfering signals.

Mantle cloaking for co-site radio-frequency antennas

We show that properly designed mantle cloaks, consisting of patterned metallic sheets placed around cylindrical monopoles, allow tightly packing the same antennas together in a highly dense telecommunication platform. Our experimental demonstration is applied to the relevant example of two cylindrical monopole radiators operating for 3G and 4G mobile communications. The two antennas are placed in close proximity, separated by 1/10 of the shorter operational wavelength, and, after cloaking, are shown to remarkably operate as if isolated in free-space. This result paves the way to unprecedented co-siting strategies for multiple antennas handling different services and installed in overcrowded platforms, such as communication towers, satellite payloads, aircrafts, or ship trees. More broadly, this work presents a significant application of cloaking technology to improve the efficiency of modern communication systems.

Characteristic impedance of a microstrip line with a dielectric overlay

Purpose – The purpose of this paper is to present an analytical expression for the characteristic impedance of a microstrip line in presence of a dielectric cover. Design/methodology/approach – Assuming a quasi-TEM propagation mode, a rigorous conformal mapping based on the Schwarz-Christoffel transformation is employed to derive the equivalent capacitance model, which can then be applied to derive a closed analytical expression for the effective permittivity and the characteristic impedance of the line. Findings – Such a formulation is not limited to the case of a single cover layer, but an arbitrary number of electric overlays can be considered as well. Comparisons with published numerical results and full-wave simulations in the case of a single cover layer have been also performed to test the validity of the proposed approach. Originality/value – The new analytical formula for the characteristic impedance of a microstrip line with a single dielectric cover shows better performances compared to the one...

Exploiting Intrinsic Dispersion of Metamaterials for Designing Broadband Aperture Antennas: Theory and Experimental Verification

The intrinsic frequency dispersion has usually been considered as a relevant drawback of metamaterials, limiting their application to narrowband microwave components. However, we have recently demonstrated that a particular metamaterial-based lens, consisting of a combination of an epsilon-near-zero (ENZ) material and air, is able to enhance the gain of short horn antennas over a broad frequency range. This result was achieved by exploiting the dispersive nature of ENZ metamaterials. In this communication, we further discuss the latter aspect, giving more details about the physical mechanism enabling broadband operation of shortened horns loaded with ENZ-air lenses. The discussion is supported by a set of experimental results obtained on a fabricated prototype.

Linear-to-circular polarization transformer using electrically small antennas

In this contribution, a linear-to-circular polarization transformer based on the employment of electrically small antennas is presented. The proposed setup consists of a circular metallic disk with a sub-wavelength aperture through which two sets of connected circularly polarized electrically small antennas are placed. We assume to use such a structure to close an open-ended linearly polarized circular waveguide in order to change the polarization from linear to circular. The proposed setup works around 1.627 GHz and exhibits 8 MHz of 3-dB axial ratio bandwidth.

Single patch antenna generating electromagnetic field with orbital angular momentum

We investigate on the possibility to generate an electromagnetic field with a non-zero orbital angular momentum (OAM) using a single patch antenna. First, we report an analytical study of a circular patch antenna in order to show that a circular polarized TMnm mode generates an OAM of order n - 1. Then, we design an elliptical patch antenna that radiates a circular polarized electromagnetic field with OAM of the first order. The analysis of the proposed antenna is supported by a proper set of full-wave numerical simulations.

Filtering Chiral Particle for Rotating the Polarization State of Antennas and Waveguides Components

Here, we present a compact component for waveguide systems and horn antennas, enabling both filtering and polarization rotation capabilities. In particular, the proposed module consists of an electrically thin-chiral particle, which is first used to design a waveguide twist to connect two waveguide sections mutually rotated by 90°. We show that around the resonant frequency of the particle, good transmission efficiency between the two mutually orthogonal waveguides is achieved. Then, the same module is inserted at the throat of a vertically polarized horn antenna to rotate its polarization plane and narrow its matching bandwidth. The result is a filtering horn antenna that, within the operating bandwidth of the particle, efficiently radiates a horizontally polarized field. The effectiveness of the proposed module is verified through numerical and experimental results.