Darwin Blanco

Holographic Surface Leaky-Wave Lenses With Circularly-Polarized Focused Near-Fields—Part I: Concept, Design and Analysis Theory

A novel concept of planar printed-circuit microwave lens, based on the transformation of a cylindrical surface wave into a modulated leaky wave, is presented in this paper. Inspired on holographic concepts, the TM0 cylindrical leaky wave is properly tapered across the two-dimensional surface by using an Archimedean-spiral shaped slotted printed-circuit, so that it is obtained a quadratic-phase, quasi-uniform amplitude aperture distribution which creates a highly focused circularly-polarized near-field pattern. The first part of this paper describes the general analysis and synthesis methodology to obtain planar holographic lenses with high radiation and focusing efficiency. As an example, a printed-slot lens with 6λ0 diameter and focal ratio F/D = 1 operating at 10 GHz is designed and theoretically studied by simulations. The second part of the paper concentrates on the prototype and measurements.

Holographic Surface Leaky-Wave Lenses With Circularly-Polarized Focused Near-Fields—Part II: Experiments and Description of Frequency Steering of Focal Length

This paper reports experimental results which demonstrate the possibility to synthesize focused near-fields from a planar printed circuit which is fed with a simple coaxial probe. A prototype operating at 10 GHz has been manufactured and tested. As predicted by the theory described in the first part of this paper, high 75% focusing efficiency of circularly polarized fields is obtained as a result of the proper modulation of the dimensions of the Archimedean-shaped printed slot. The designed coaxial feeding provides good matching (below -10 dB) in the entire frequency band. Finally, the scanning of the focal length as a function of the frequency is described in this paper, showing measured results and accurate theoretical explanation based on the frequency dispersion of the tapered leaky mode. This phenomenon is general for this type of two dimensional leaky-wave lenses, and might find many applications in imaging, sensing and heating.

Directivity Enhancement and Spurious Radiation Suppression in Leaky-Wave Antennas Using Inductive Grid Metasurfaces

Fabry-Perot antennas (FPA) achieve high broadside directivity due to the simultaneous excitation of a pair of nearly degenerate TE/TM leaky-wave modes using a partially-reflecting surface on top of a ground plane. This partially-reflecting surface can be obtained using a dielectric superstrate or via a capacitive or inductive metasurface (MTS). By using an equivalence between the conventional dielectric superstrate and the MTS-based structures in terms of the dominant TE/TM modes, we show that the use of inductive grid MTSs leads to a directivity enhancement. A higher roll-of in the radiation patterns is achieved as a result of the intrinsic suppression of the spurious TM0 leaky wave mode. This suppression is mathematically demonstrated and validated with full-wave simulations. The achieved improvement in more than 1 dB for inductive strip grid based MTS with respect to dielectric based super-layers, for the same frequency band of 2.5%, is verified with measurements. Two prototypes, with the dielectric superlayer and inductive strip grid based MTS, have been fabricated and measured supporting the claim of this work.

On the Use of Leaky Wave Phased Arrays for the Reduction of the Grating Lobe Level

Dielectric superlayers can be used to reduce the grating lobe levels in thinned phased arrays, i.e., arrays with large periodicities. In this contribution we show how the mutual coupling impacts the active impedance and the roll-off of the embedded patterns necessary to achieve the grating lobe angular filtering in this type of arrays. The reduction of the grating lobes in the thinned array radiation pattern depends on the dielectric superlayer constant. The larger the dielectric constant the higher the attenuation of the grating lobe will be. However, this can only be obtained at the cost of an increased mutual coupling. This mutual coupling will impact on the embedded patterns reducing the actual roll-off that can be achieved. Several 11 × 11 phased arrays with different dielectric superlayers are studied in order to establish the maximum useful permittivity as a function of the mutual coupling level. We show that antenna elements based on dielectric superlayers leading to mutual coupling levels larger than -20 dB suffer from a loss of directivity in the embedded pattern and a loss of gain in the phased array because of the highly resonant active impedance. As a reference we also compare the performances of the 11 × 11 leaky wave phased array with an 11 × 11 phased array of standard conical horns. We show that an increase in the gain of more than 2.2 dB over all the frequency and scanning ranges is obtained in the leaky wave array with respect to the reference horn array. The leaky wave array leads to a reduction of the grating lobe of more than 10 dB.

Radially Polarized Annular-Slot Leaky-Wave Antenna for Three-Dimensional Near-Field Microwave Focusing

Microwave three-dimensional near-field focusing with radial polarization using a holographic annular-slot antenna is demonstrated in this letter. This radial polarization can generate axially polarized focused fields in the near-field regime, which complements the more conventional synthesis of transversally polarized near-field focusing patterns. Moreover, it is shown that more symmetrical focal region is obtained thanks to the radial polarization for similar Fresnel number N and operation frequency. The theory is demonstrated with experiments for a design operating at 10 GHz, with aperture diameter D=6λ and a Fresnel number N=4.

Leaky wave enhanced phased array for the reduction of the grating lobe level

It is possible to reduce the cost of building phase arrays by reducing the number of active transceivers at the cost of having grating lobes and a reduced gain For small coverage angle applications for applications with a limited coverage angle. Here we want to show that dielectric and FSS super-layers can be used to improve the performances of such phased arrays by achieving an angular filtering at the array element pattern level. Such angular filtering can be synthesized with leaky waves.

Near and far-field focusing with holographic two-dimensional tapered leaky-wave spiral antennas

The design of two near and far-field printed-slots spiral antennas working at 10GHz is reported in this work. By properly modulating the radial separation between the slots and its width, it is shown how the TM0 leaky-mode pointing angle and leakage rate can be adequately tapered to efficiently synthesize focused near and far-field patterns, while keeping a flat single-layer low-profile planar antenna design which is simply fed by a vertical coaxial probe. The far-field operation has been reached using the split condition in which the alpha and beta constants are equal. The two designs are based on an Archimedean-spiral shaped modulated slot to offer circular transverse polarization of the focused fields. An analysis in frequency shows that the near-field antenna has the ability to generate frequency steerable near-field focusing patterns. Simulation results are reported in this paper, showing that high radiation (more than 75%), illumination and focusing efficiency (more than 70% and 25% for near and far-field respectively) can be achieved and agree with the theoretical values.

Textile Fabry-Pérot Antenna

A wearable version of Fabry–Pérot antenna is presented. This is a simple way of designing a medium- to high-gain antenna with low back radiation. The study of the effect of antenna bending in the performances is presented. Besides, the replacement of a superstrate layer by a metallic frequency selective surface is proposed. In this way, there is no need of finding a specific material and thickness for a targeted gain and frequency. Experimental validation confirms the viability of this design.

Leaky-Wave Thinned Phased Array in PCB Technology for Telecommunication Applications

We present a practical implementation of a leaky-wave thinned phased array in printed circuit board (PCB) technology. In this paper, we demonstrate that a reduction of the grating lobes, and therefore an improved gain in a thinned phased array, with respect to standard solutions, is achieved by virtue of the angular filtering introduced by a leaky-wave cavity in the far field. The presented array is designed in PCB and integrated with an inductive partial reflective surface. A full study of the performances of the 7×7 phased array antenna for several scanning angles and frequencies is presented. This paper shows an improved gain, directivity, grating lobe level, back lobe level, beam efficiency, and active reflection coefficient with respect to a reference solution based on 2 × 2 subarrays. The results are validated via the measurements of a 3 × 3 array prototype.

Evaluation of the performance of wearable directive antennas based on Fabry-Perot type

Nowadays new communication scenarios demand new types of antennas to be integrated with garments, including directive antennas. Simplicity is a must in this kind of applications and technology, and a simple way to obtain a directive antenna is by using the known as Fabry-Perot. This work study some realistic designs based on the use of textiles and other materials that can be washed and integrated in a clothing item. It is know that these antennas are strongly resonant, so we analyze as well how the effect of bending the surface where they are deployed is affecting their performance.