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The intrinsic frequency dispersion has usually been considered as a relevant drawback of metamaterials, limiting their appli- cation to narrowband microwave components. However, we have recently demonstrated that a particular metam aterial-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 physic al 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

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.

A simple and cheap solution is proposed to transform a wideband and linearly polarised horn antenna in a dual-band circularly polarised horn filtenna. The filtering and polarisation transformation capabilities are achieved by inserting inside the throat of the horn a complementary electrically small resonator etched on a thin metallic sheet. In this way, the overall structure is well matched in two frequency ranges (13-14 and 16.4-17 GHz) and radiates left-handed and right-handed circularly polarised fields in the lower and upper frequency bands, respectively. The measurements performed on a realised prototype confirm the good performance of the proposed structure, which may be employed in satellite communication systems requiring different circular polarisation for the up- and down-link frequency ranges.

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 field. In this frame, the aim of the present paper is to propose a new approach to generate a non-zero OAM field through a single patch antenna. Using the cavity model, we first analyze the radiated field by a standard circular patch and show that a circular polarized (CP) TMnm mode excited by using two coaxial cables generates an electromagnetic field with an OAM of order ±(n−1). 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 effectively radiates an electromagnetic field with a first order OAM. Such results prove that properly designed patch antennas can be used as compact and low-cost generators of electromagnetic fields carrying OAM.

In this contribution, we review our latest results about the unprecedented possibilities enabled by electromagnetic cloaking in antenna theory and applications. In particular, we show that, in the receiving regime, cloaking allows designing antennas exhibiting a minimum level of scattering for a given level of absorbed power, whereas, in the transmitting one, cloaking enables the co-sitting of different antennas in a confined region. Both full-wave and experimental results are presented to support the theoretical considerations.

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.

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.

The intrinsic frequency dispersion has been usually considered as a relevant drawback of metamaterials, limiting their application to narrowband microwave components. However, it has been 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 interesting result has been achieved by exploiting the dispersive nature of ENZ metamaterials. In this contribution, 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.

Recently, we have presented a novel approach to design metamaterial-inspired notch filters that can be integrated within horn antennas of receiving systems to mitigate the effects of narrowband interfering signals. The filter module consists of a single Split Ring Resonator (SRR), whose rejection band needs to be matched to the bandwidth of the particular interfering signal we want to suppress. Extending our previous work, we show here how it is possible to control the bandwidth of such a filtering module by using different metamaterial-inspired resonators. In particular, we show that, while a reduction of the rejection band can be easily obtained by increasing the miniaturization rate of the resonator, the enlargement of the rejection band cannot be obtained in the same way by simply reducing the resonator quality factor. We show that a solution of the latter problem can be worked out by applying the ``critical coupling'' concept and considering the filtering module to be made of two equal SRRs with a proper optimal separation. The effectiveness of the approach is demonstrated trough proper full-wave simulations and experiments on a fabricated prototype. The proposed technique, used here to design a filtering module for a specific radiating system, has a more general relevance and can be applied to all cases where the operation bandwidth of a component is limited by the resonant nature of a single metamaterial-inspired particle.