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The paper describes the development of passive, chipless tags for a novel indoor self-localization system operating at high mm-wave frequencies. One tag concept is based on the low-Q fundamental mode of dielectric resonators (DR) which exhibits peak scattering at its resonance frequency. As the radar cross-section (RCS) of DRs at mm-wave frequencies is far too low for the intended application, arrays of DRs and combinations with dielectric lens and corner reflectors are investigated to boost the RCS while keeping the scattering retro-directive over wide-angle incidence. Satisfactory results are demonstrated experimentally in W-band with metal corner reflectors combined with planar arrays of DRs; the tags produce a high RCS level over a moderately broad angular range and a wide frequency range where they exhibit a notch at the resonance frequency of the dielectric resonators. These designs suffer from low coding range of 3 to 6 bit, degradations of RCS in angular range, and a difficult separation of the tag response from strong clutter. Both the suppression of large clutter interference by using time gating of the tag response and a larger coding range are promised by a chipless tag concept based on multiple high-Q resonators in photonic crystal (PhC) technology. Experimental samples are characterized as transmission resonators and as retro-directive tags at the 230 GHz band. As a concept to boost the retro-directive RCS with a truly wide-angle response, the integration of PhC resonators with a Luneburg lens is discussed.

This letter presents the integration of frequency selective surfaces (FSSs) with retroreflectors for the realization of chipless wireless indoor localization tag landmarks. As an example, the high radar cross section (RCS) of a trihedral corner reflector is signed with an FSS-based stopband filter, so that the backscattered power from several corner reflectors can be distinguished by a mobile reader according to the frequency response of the FSS. Measurement results with a 3 × 3 × 3 cm3 trihedral corner reflector and a stopband FSS in a Rogers RT/Duroid 5880 high-frequency laminate at 90 GHz shows an RCS above $-$ 25 dBsqm for 90° coverage in the TM plane. Due to the high RCS, measurements at distances up to 4 m with a standard 25 dBi gain horn antenna and a vector network analyzer as a reader are shown. These preliminary results show the potential of the concept for applications such as indoor localization with sub-mm accuracy, where high bandwidths, but only a low number of bits, are needed for the identification of the tag landmarks.

In this paper, the gap waveguide technology is examined for packaging liquid crystal (LC) in tunable microwave devices. For this purpose, a line based passive phase shifter is designed and implemented in a ridge gap waveguide (RGW) topology and filled with LC serving as functional material. The inherent direct current (DC) decoupling property of gap waveguides is used to utilize the waveguide surroundings as biasing electrodes for tuning the LC. The bed of nails structure of the RGW exhibits an E-field suppression of 76 dB in simulation, forming a completely shielded device. The phase shifter shows a maximum figure of merit (FoM) of 70 °/dB from 20 GHz to 30 GHz with a differential phase shift of 387° at 25 GHz. The insertion loss ranges from 3.5 dB to 5.5 dB depending on the applied biasing voltage of 0 V to 60 V.

This article introduces retroreflective lenses for millimeter-wave radio-frequency indoor localization. A three-dimensional (3D) gradient-index Luneburg lens is employed to increase radar cross section (RCS) of photonic-crystal high-Q resonators and its performance is compared to conventional radar retroreflectors. A classic Luneburg lens with and without a reflective layer is realized with 25 mm diameter (6.7 λ0), showing a realized gain of 24.6 dBi and a maximum RCS of -9.22 dBm2 at 80 GHz. The proposed Luneburg lens with embedded high-Q resonators as frequency-coded particles in a photonic crystal structure, operating as a reflective layer, achieved a maximum RCS of -15.84 dBm2 at the resonant frequency of 76.5 GHz and showed a repeatable response each 18° over ±36° in two perpendicular planes. With this high RCS of the Luneburg lens, a maximum readout range of 1.3 m could be achieved compared to 0.15 m without the lens at 76.5 GHz for the same transmit power, receiver sensitivity, and gain of the reader antenna.

This paper presents a novel passive frequency-coded retroreflective tag landmark (radar target) based on a flattened quasi-conformal transformation optics Luneburg lens enabling an extreme field-of-view angle. A three-dimensional photonic crystal structure with embedded high-Q resonators is exploited as a coding retroreflective surface at the bottom of the lens. The retroreflector’s performance is investigated at a frequency of 80 GHz (W-band) enabling a precise mm-Wave indoor localization.

This paper studies the suitability of frequency-coded spherical retroreflectors as passive tag landmarks in a mmwave indoor self-localization system. By designing a spherical tag as a combination of two simple structures, spherical retroreflectors with frequency selective surfaces, wide readout angles are expected, while keeping manufacturing complexity low. The fabricated tag presents successful frequency coding at 82 GHz in an angular range of 120°, with a measured average radar-cross section of −15 dBsqm in that range. Moreover, the tag works for both transverse electric and transverse magnetic plane waves and has an operating bandwidth of 6.1%, allowing for the encoding of 9 notches, or 3.17 bits, in W-band.

This paper presents a planar Luneburg lens frequency-coded retroreflector based on a high-resistive silicon (HR-Si) photonic-crystal (PhC) high-Q tag with two resonators working at 237 GHz and 243 GHz. A characterization of the retroreflector, including the readout of the resonance frequency of the resonators, is performed at 30 cm over 130-degree angular range in azimuthal plane. The measured gain of the lens antenna is 15.9 dBi at a frequency of 240 GHz and the radar-cross section (RCS) of the frequency-coded tag landmark achieves -30 dBsm at 237 GHz.

This paper studies the use of gridded square-ring FSSs to encode the backscattered response of trihedral corner reflectors, acting as passive landmarks in a mm-wave indoor self localization system. Previous tags presented low resonance stability under oblique incidence of the incoming wave, which limits the system performance under real operating conditions. The designed FSS uses a miniaturized unit cell to minimize phase differences between its elements, which are responsible for the fluctuating resonance response. The smaller these phase differences, the less varies the response. The new design presents a more stable response than a previous work at W-band (65-110 GHz), at the cost of lower frequency selectivity and range. But, the operating angular region of the tag is ± 35°, which allows to encode 45 notches, which means 5.5 bits, under transverse magnetic planar wave excitation for identification of tags in indoor self-localizing applications.

A possible passive infrastructure for mm-wave self localization systems is proposed in this paper. W-band photonic crystal based high-Q resonators coupled to free space by a dielectric rod antenna are employed. Q-factors of approximately 300 and wireless tag readout over 10 cm can be achieved using Rogers RT/duroid® 6010.2LM. The maximum readout range increases to 40 cm when lower-loss additively manufactured Alumina tags are used. Development of a 3 bit prototype tag is successful, demonstrating the possibility to increase the number of resonators per tag using the presented approach.

This paper discusses a temperature-dependent characterization of deep reactive ion-etched 10 k$\Omega$ cm high-resistive silicon (DRIE HR-Si), 3D printed alumina (A$l_{2}O_{3}$) and milled Rogers RT/Duroid® 6010.2LM. The characterization is performed by measuring high-Q photonic crystal resonator samples in W-band and the measurements are taken from 30$^{\circ}C$ to 115$^{\circ}C$. HR-Si is the material with the lowest losses at room temperature. However, its losses increase with temperature and become higher than 3D printed alumina at 75°C, reducing the radar cross-section and maximum readout range of chipless wireless RFID tags integrating several of these resonators. These results demonstrate that, while HR-Si performance is higher for the usual temperatures achieved in an indoor localization scenario, 3D printed alumina is more suitable if a temperature-stable response is needed or if the tags need to operate at high temperatures, such as in case of fire.

In this paper, a novel approach is proposed to increase the information content of RFID tags using uniform helically arranged spherical dielectric resonator arrays. The proposed tag design is scalable to operating frequency in the mm-wave/THz range and provides a larger information content due to new additional values per digits according to the excited OAM modes $(m_{n}=\ldots,-2,-1,0,1,2,\ldots)$ instead of the binary values ( $b_{n}=0$ and 1). This new values can be called OAM values. By means of the orbital angular momentum (OAM) concept three uniform helically arranged spherical dielectric resonator arrays operating at three different frequencies $f_{1}=10\ \text{GHz},\ f_{2}=10.5$ GHz, and $f_{3}=11\ \text{GHz}$ are established to provide a $m_{1}m_{2}m_{3}$ 3-digits OAM coded tag. Each array consists of 8 dielectric resonators (DRs) with three different radii in order to enhance the isolation between the different arrays. The OAM mode orders are generated by the interaction of phase delayed radiation from each DR element in the array. Each DR element is elevated in order to achieve the right phase shift, which is related to the mode order, the number of elements, and the propagation wavelength. Two examples of 3-digits OAM coded tags with the code −110 and −210 have been simulated and compared with a $b_{1}b_{2}b_{3}$ 3-digits conventional coded tag with the code 111 in order to compare the two approaches.

In this paper, the performance of multilayer Barium-Strontium-Titanate (Ba 0.6 Sr 0.4 TiO 3 ) and Magnesium Borate (Mg 3 B 2 O 6 ) cylindrical resonators for their use as chipless wireless temperature sensors is studied. It is shown in theory and measurements how the engineering of the sensor element quality factor can increase the received power at the reader. As an example, it is shown that the insertion of a 4.45 mm Magnesium Borate intermediate layer increases the backscattered power or radar cross section of the cylindrical resonator by 11.9 dB.

This paper presents the design and measurement results of a 4-bit chipless 3D printed ceramic wireless RFID tag in alumina. It is shown how the accuracy and material quality of the recently presented manufacturing method for millimeterwave components can be used to implement photonic crystal high-Q resonators. Wireless measurements of a four-resonator RFID tag show a loaded quality factor of 902 and a radar cross section equal to -43.3 dBsqm at 90.4 GHz.

In this paper, a 2D Luneburg lens is proposed to improve the Radar Cross Section (RCS) of dielectric resonator (DR) tags, which would be used for an indoor localization system at THz frequencies where the low RCS of the tag makes the detection against clutter signals difficult. A Luneburg lens made of PTFE is fabricated using milling technique based on metamaterial concept, and characterized using a Vector Network Analyzer (VNA). Three ceramic resonators with different resonance frequencies are placed at different positions in the lens focal area to test the performance of the lens. To prove the concept, the simulation and measurement have been performed at scaled frequencies in the 4.5-7 GHz range. The RCS improvement is found to depend on the lens dimensions and frequencies of operation. A Luneburg lens with a height of 35 mm and a diameter of 66 mm boosts the RCS by 18 dB at 6.2 GHz.

This paper presents investigations on a passive chipless wireless pressure sensor to enhance tool life prediction in smart milling machines. High reflective environments in terms of radar clutter are investigated and benefits of using the time gating technique are discussed. Moreover, due to the proposed highly robust design, the sensor can be applied in areas with high temperatures and strong vibrations. The working principle is based on a dielectric resonator which is metal coated on its lateral surface. Perturbing the resonance mode with a metal membrane in vicinity of the resonator leads to a pressure dependent resonance frequency. A broadband interrogation signal is used to determine the resonance frequency, which is represented by a notch in the backscattered spectrum. The pressure sensitivity and pressure range is configurable by using different membrane and spacer thicknesses, respectively. Furthermore, the sensor operates in the frequency range from 20 GHz to 23 GHz with read-out ranges up to 150 cm.

This paper presents a novel approach in passive chipless wireless pressure sensing for tool monitoring in smart factories. The working principle is based on the pressure dependent detuning of a dielectric resonator resulting in a notch in the backscattered spectrum at its resonance frequency. The read-out of the sensor is performed by a reader device that transmits a broadband signal and makes use of time gating to remove clutter from the machine or other environmental obstacles. The dielectric resonator approach is chosen since it allows for these long impulse response times and simultaneously enables high pressure sensitivity and flexibility for the measurement range. Furthermore, the proposed sensor concept is applicable in harsh environments in terms of strong vibrations and high temperatures. The functionality of the design is confirmed by simulation results and experimentally validated.

In this paper, a novel hybrid time-frequency modulation scheme for the integration of chipless wireless identifications and sensors is presented. This approach combines the wide bandwidth of localizable time domain reflectometry RFID tags and the narrow-band characteristic of high-Q resonator-based transducers for the design of a transducer-integrated TDR-RFID tag where both identification and sensing are coded within the same bandwidth. Two prototypes have been manufactured and measured to show the feasibility of the concept for chipless wireless sensing and identification.

Temperature monitoring of tools is one key method for the prediction and detection of tool failure to prevent damage of valuable workpieces, e. g. in aircraft manufacturing. Therefore, a chipless wireless dielectric temperature sensor for machine tools, with very high temperature capability and compatibility to the harsh machine tool environment, is investigated. Based on these findings, a demonstrator is introduced that is characterized in the lab and tested in a real machine tool. The paper focuses on real world challenges of the sensor system implementation.

This paper describes a simple procedure to create a compact resonance effect in the context of new contactless gap waveguide structures. The resonance is achieved shortening a given nail in the textured surface and can be used to feed slots in a slot array. In the following, the resonance effect is explained and a basic radiating cell with two slots is designed to illustrate the potential of the resonant device.