Robert K. Shaw

The Characterization of Conductive Textile Materials Intended for Radio Frequency Applications

Antennas constructed in part from conductive textile materials (also known as e-textiles) by means of standard textile manufacturing techniques are currently receiving increasing attention from antenna theorists and antenna manufacturers alike. However, due mostly to the unique fabrication methods employed, these novel materials cannot be treated as simple, equivalent substitutes for the more-conventional metallic antennas. Conductive yarns can have considerably less-than-ideal conductivity, and their inhomogeneous internal structure, with features small with respect to the skin depth, can be difficult to analyze directly in terms of conductive-material bulk resistivity. Furthermore, the undulating and sometimes non-planar nature of stitched or woven conductive textile yarns introduces a significant phase delay that must be properly taken into account. This article describes a method to determine the conductivity, sigma , which accurately represents a lossy inhomogeneous textile conductor for a MoM segment having the same radius as the actual conductive yarn. This method has three steps. First, the resistance per unit length of the textile conductor is determined experimentally, in a transmission-line test cell. Next, this measured resistance per unit length is adjusted to account for the nonuniform current distribution across the multiple yarn conductors. Finally, a surface-impedance formulation is employed to derive an equivalent MoM-segment bulk conductivity that accurately represents the measured conductors performance. Excess phase delay, inherent in textile conductors, is determined by examination of the phase component of the test cell scattering parameter, S21.

A

Metamaterials with properly engineered surface properties have been recently proposed for application in the design of broadband hybrid-mode horn antennas, such as soft and hard horns. In this paper, we present the design, fabrication, and measured results of a square dual-polarization horn antenna with thin metasurfaces lining the four walls, demonstrating broadband, negligible-loss hybrid-mode operation. By employing a powerful genetic-algorithm (GA) design optimization technique, we have dispersion-engineered low-index metaliners whose surface impedances satisfy the balanced hybrid condition across the Ku-band. The optimized metaliners were synthesized based on conventional printed-circuit board technology, leading to a lightweight and low-cost construction. To improve the cross-polarization response, a simple dielectric plug was placed in the throat of the horn to perform effective mode conversion. Measurements showed that the fabricated horn antenna prototype provided low sidelobes, low cross-polarization levels, and radiation patterns that are approximately independent of polarization. Excellent agreement was found between measured and simulated results across the entire band of operation. Both the far-field radiation patterns and the aperture field distributions confirm the hybrid-mode operation of the horn, validating the balanced metasurface design. This metamaterial-enabled antenna represents a low-cost alternative to other types of soft feed horns, such as corrugated horns.

K_{u}

Metamaterial and metasurface structures, which satisfy the balanced hybrid conditions, have been recently proposed for designing hybrid-mode soft and hard horn antennas. In this paper, we present the design of broadband soft metasurfaces and their applications in hybrid-mode conical horns. These designs exhibit surface characteristics that can be controlled by adjusting the constituent structural elements. The analysis and design of metasurfaces were performed on both a planar platform and in a cylindrical waveguide structure. Such dispersion-engineered metasurfaces with customized surface properties can support the desired hybrid modes in a cylindrical waveguide, facilitating their usage in horn antennas. By engineering the metasurfaces with regard to their surface impedance as well as tailoring their spatial distributions along the wall of the horn, we demonstrate an inhomogeneous metahorn with superior performance and broader bandwidth compared to those with uniform liners. The symmetric far-field radiation patterns with low sidelobe and cross-polarization levels, together with the corresponding aperture field distributions, signifies the hybrid-mode operation of the horn, validating the efficacy and broadband performance of the metasurfaces. Such metasurface-lined horns can be employed as soft feeds in dual polarization antenna systems.

-Band Dual Polarization Hybrid-Mode Horn Antenna Enabled by Printed-Circuit-Board Metasurfaces

This paper reports on the detailed design and experimental demonstration of a metamaterial-enabled low-sidelobe horn antenna (metahorn) based on principles similar to those of earlier soft horn antennas. The target application is a linearly polarized feed horn in the super-extended C-band (3.4-6.725 GHz) for communication satellite reflector antennas. The paper describes the detailed design and manufacturing of the -plane metamaterial liner (metaliner) based on a freestanding wire grid without the need for a dielectric substrate material. The measured copolarized and cross-polarized antenna patterns from the feed horn demonstrate over an octave pattern bandwidth with negligible loss. The results show similar bandwidth with lower sidelobes and backlobes than those of the trifurcated horn that is currently used as the standard C-band feed for single linear polarization. This demonstration shows promise for lightweight metamaterial horns to replace heavy and expensive C-band corrugated horns.

Inhomogeneous Metasurfaces With Engineered Dispersion for Broadband Hybrid-Mode Horn Antennas

It was shown by moment method analysis that soft and hard metamaterial horns offer broadband performance if a metamaterial with the required dispersion can be implemented. It was also shown that the required dispersion curve is monotonically increasing similar to the Drude dispersion, strongly indicating that these horns can be realized. In two accompanying papers the design and analysis of a hexagonal hard metamaterial horn for a multi-beam reflector antenna is presented [8, 9].

Demonstration of an Octave-Bandwidth Negligible-Loss Metamaterial Horn Antenna for Satellite Applications

A satellite-based waveguide feed network was designed that can produce dual LP in one band and dual CP in another. This design is simpler than the alternative designs, not having a symmetric OMT and requiring fewer phase matched paths. Because phase errors only rotate the linear polarization and do not degrade it, there is no cross-polarization risk due to phase errors for applications where the ground station is free to rotate its polarization to match the satellite polarization. Additionally, the bandwidth of the network is not limited by the axial ratio bandwidth of the septum polarizer, since a good axial ratio is not required in the LP band. Measurements demonstrate cross-polarization isolation levels >48 dB.

Metamaterial hybrid mode horn antennas

Hybrid-mode (HM) horn antennas with internal metamaterial wall liners, here denoted meta-horns and meta-liners, respectively, were first proposed in [1]. A soft meta-horn comprises a low index meta-liner with a given permittivity and thickness (see Figure 1b) that supports a balanced hybrid mode at the design frequency. These soft horns offer considerably larger bandwidth than the widely used corrugated horn due to evanescent propagation of the EM fields normal to the meta-liner. A hard meta-horn has a dielectric liner on top of a low index meta-liner as shown in Figure 2b. These horns also offer considerably larger bandwidth and comparable aperture efficiency when compared with other hard horns, which also is due to evanescent EM propagation normal to the meta-liner. Modeling and analysis of these meta-horns based on a cylindrical model as well as full-wave MoM analysis (WIPL-D) were reported in [1–4]. When compared with dielcore horns (Figures 1a and 2a) [4] both the soft and hard meta-horns exhibit comparable bandwidth performance without the disadvantages of insertion loss in the dielectric core, dielectric-air mismatch and additional weight. Full-wave analysis of both horns have shown that the desired frequency dispersion matches the Drude model, indicating that these low index meta-liners can be implemented with large bandwidth.

Waveguide feed network producing dual-linear and dual-circular polarizations for satellite applications

A hexagonal metamaterial hard horn design was shown with better aperture efficiency and relative cross polarization than existing circular horns used in multibeam reflector antennas. The horn antenna achieves an aperture efficiency greater than 83% and a relative cross polarization level less than 30 dB over a 27% bandwidth. If a profiled horn were used to eliminate the phase variation, the aperture efficiency could be increased to 87%. Additionally, arrays of hexagonal apertures can be packaged without gaps, corresponding to up to a 10% increase in aperture efficiency or 0.42 dB increase in gain over circular apertures. This design is dependent on the ability to construct a metamaterial with the desired dispersion characteristics and on the ability to cope with the mutual coupling of tightly packed horn antennas. An accompanying paper [11] analyzes the hexagonal metamaterial horns performance in a multibeam reflector antenna, comparing it to one using multi-mode horns.

Status on meta-horn development – theory and experiments

Soft-surface horn antennas are known to provide low cross-polarization [1, 2, 3] and a symmetrical radiation pattern with low sidelobes. Early soft-surfaces were realized in corrugated horns [1, 2]. Although they provide good performance, corrugated horns are expensive to manufacture and relatively heavy, so several alternatives have been proposed, including horns loaded with dielectric cones [4]. Trifurcated horns, though not based on soft-surfaces, are another alternative, also providing low sidelobes with low cross-polarization [5].

Hexagonal hard metamaterial hybrid-mode horn with applications in multibeam reflector antennas

Horn antennas that support balanced hybrid-modes are desirable in satellite communication applications due to their polarization-independent patterns and low cross-polarization over a wide frequency bandwidth [1]. Besides corrugated horns which utilize an electromagnetically soft lining, dielectric loaded horns can provide an alternative realization of hybrid-mode horns with either soft or hard wall boundary conditions [2]-[4]. Recently, homogeneous metamaterial liners have been used to realize hybrid-mode horn designs [5]. With the availability of metamaterials that exhibit a dielectric constant of less than unity over a reasonably wide bandwidth, use as a liner allows the removal of the standard dielectric core. This metamaterial liner can improve the bandwidth and significantly reduce the weight of the antenna compared to its dielectric loaded counterpart, all while maintaining the performance of a hybrid-mode horn. In this paper, we show a broadband metasurface satisfying the balanced hybrid condition and its use as a liner to implement hybrid-mode horns. The design approach combines a nature-inspired optimization technique and an efficient full-wave electromagnetic solver, making it possible to realize metamaterials or metasurfaces with customized dispersive properties. With regard to surface impedances, the metasurface is found to be equivalent to a homogeneous metamaterial slab backed by a PEC ground plane. Therefore, we will consider the results of a numerical study of a conical horn antenna with a homogeneous metamaterial liner. The far-field radiation patterns and aperture field distribution confirm hybrid-mode operation over a wide bandwidth, validating the metasurface design methodology.