Matthias Maasch

Wave Propagation in Periodic Structures

Wave propagation in periodic structures is closely related to propagation of waves in continuous media. Thus, dispersion parameters such as wavenumber and wave impedance or permeability and permittivity, can be used to describe the propagation of waves in one-dimensional periodic structures like loaded or artificial transmission lines and waveguides, and in two- and three-dimensional structures like artificial lenses, transmit arrays and meta surfaces. In this chapter, the connection between transmission line theory and effective material parameters, that can be used to describe the propagation of waves in periodic lattices, is investigated. Furthermore, based on a general form of a transmission line model and effective material parameters, a physical limitation for dispersion characteristics and corresponding equivalent circuit are derived. Finally, the effect of discretization and finite unit cell size in periodic structures on effective material parameters is considered.

A novel passive phase modulator based on LH delay lines for chipless microwave RFID applications

This paper discusses novel techniques to overcome problems arising when porting the surface acoustic wave approach of building passive radio-frequency identification and measurement systems to the electromagnetic domain without the need of mechanical (acoustical) delay lines. These techniques include the utilization of left-handed artificial delay lines and the increase of information density by using a higher order modulation scheme. The benefit from the porting is a broader field of applications, e. g. RFID tags working at very high temperatures or in other harsh environments become possible.

Frequency Multiplexed 2-Dimensional Sensor Array Based on Split-Ring Resonators for Organic Tissue Analysis

A frequency multiplexed 2-dimensional sensor array was developed using microstrip-line-excited split-ring resonators (SRRs). With the proposed structures, it is possible to spatially resolve the relative changes of the dielectric properties of a Material Under Test (MUT) in one and two dimensions. The SRRs are designed to have different resonant frequencies and are decoupled from each other. With these design characteristics, it is possible to track changes on the resonant frequency of individual SRR that will indicate the dielectric properties of the MUT around the ring and therefore its location within the array. The number of pixels of the dielectric image corresponds to the number of SRRs within the sensor. Several prototype sensors have been realized and tested with different MUT such as dielectric bricks, animal tissue and phantoms of human tissue to prove the concept.

Phase modulation scheme for chipless RFID- and wireless sensor tags

Inspired by state of the art surface acoustic wave (SAW) technique an approach for the realization of chipless microwave RFID tags based on electrical circuits is proposed. For information coding of the retransmitted signal a passive QPSK coding scheme and for wireless monitoring of physical quantities an analog phase modulation scheme are investigated. Key elements for the realization of tags are lumped element delay lines. Therefore properties of left- and right-handed lines are discussed with the view on the special application. Finally, the proposed principles have been experimentally verified with a prototype.

Performance evaluation of left-handed delay lines for RFID backscatter applications

The operation principle of a passive RFID tag on the basis of delay lines is presented and the performance of left- and right-handed lines for this application is compared. Additionally, limitations due to the dispersion properties of left-handed lines for the maximum number of usable cells are discussed. In conclusion, an expression relating the maximum number of usable bits for the tag impulse response to system parameters like bandwidth, centre frequency etc. is given. A proof of concept is realized with a 100 cell four bit backscatter tag. The set up of the tag is described and the performance analysed.

Tunable composite right/left-handed leaky wave antenna based on a rectangular waveguide using liquid crystals

A leaky wave antenna based on a composite right/left-handed waveguide structure is presented. The antenna features beam steering capabilities for a fixed operation frequency of 7.6GHz. The frequency independent tunability is obtained by inclusion of liquid crystal (LC) material whose dielectric properties can be tuned by static external electric or magnetic fields. Simulations as well as measurements of a built prototype are shown for scattering parameters and far field patterns. The maximum obtained beam tilt amounts to ±10° around broadside.

Artificial transmission lines for high sensitive microwave sensors

Two common and two new unit cells for artificial lines made of four reactive elements are studied concerning their sensitivity to capacitive changes and their applicability as capacitive sensors. It is shown that selection of the unit cell is very important and can highly influence the sensitivity. The shown principle is very powerful and can be scaled up to THz frequencies, only limited by available fabrication technology. A built planar differential microwave sensor based on artificial lines working at 2.3 GHz is presented. It features a simple output power evaluation instead of a classic but more complicated phase difference measurement. With power variation of up to 60 dB in dependence of small environmental dielectric changes it shows a very high sensitivity and dynamic range. Functional layer coatings can be applied easily due to its planarity and open wide application areas for biomedical sensing.

Liquid crystal-based tunable CRLH-transmission line for leaky wave antenna applications at Ka-Band

In this work, a liquid crystal based tunable composite right/left-handed transmission line for future leaky wave antennas working at the Ka-band is presented. The tuning of the liquid crystal is achieved by means of magnetic and electric biasing. For this purpose, different prototypes are fabricated for each biasing technique and their dispersive characteristics compared. Electric tunability is achieved by implementing highly resistive bias lines in the unit cell layout. Both techniques yield similar tuning capabilities at the operation frequency of 27GHz whereas the electric one has the advantage of being easily integratable in the layout.

Investigation on the beam-scanning capability of a gradient index fishnet structure

In this work the beam-scanning capability of a gradient index fishnet array, which is designed to operate at 22.5 GHz, is investigated. The results are discussed based on unit cell simulations and far-field measurements. The gradient is realized by changing the permittivity of a dielectric layer in each unit cell element resulting a tuning of the magnetic resonance frequency. Hence, an effective differential refractive index of 3.8 is achieved at 22.5 GHz yielding a theoretical beam-scanning angle of ±6.6°. Farfield measurements of a 5×5 array confirm the beam-scanning capability with a beam-scanning angle of ±3°. The deviation of the measured results is discussed.

Planar Microwave Sensor for Theranostic Therapy of Organic Tissue Based on Oval Split Ring Resonators

Microwave sensors in medical environments play a significant role due to the contact-less and non-invasive sensing mechanism to determine dielectric properties of tissue. In this work, a theranostic sensor based on Split Ring Resonators (SRRs) is presented that provides two operation modes to detect and treat tumor cells, exemplary in the liver. For the detection mode, resonance frequency changes due to abnormalities are evaluated, and in the treatment mode, microwave ablation is performed. The planar sensor structure can be integrated into a needle like a surgery tool that evokes challenges concerning size limitations and biocompatibility. To meet the size requirements and provide a reasonable operating frequency, properties of oval shaped SRRs are investigated. By elongating the radius of the SRR in one direction, the resonance frequency can be decreased significantly compared to circular SRR by a factor of two below 12 GHz. In order to validate the detection and treatment characteristics of the sensor, full wave simulations and measurements are examined. Clear resonance shifts are detected for loading the sensor structures with phantoms mimicking healthy and malignant tissue. For treatment mode evaluation, ex vivo beef liver tissue was ablated leading to a lesion zone 1.2 cm × 1 cm × 0.3 cm with a three minute exposure of maximum 2.1 W.