Lukas W. Mayer

Measurements and channel modeling for short range indoor UHF applications

The recent emergence of short range applications, e.g. RFID (radio frequency identification), requires careful investigation of wave propagation and adequate description of the radio channel. In this contribution, we present indoor channel measurements at 868MHz that expose the transition between the near and far field as well as the effects of fading. Furthermore, we investigate the appliance of a simple statistic channel model, based on the Rician distribution, and show that the underlying assumptions do not hold in general.

A Dual-Band HF / UHF Antenna for RFID Tags

Conventional RFID (radio frequency identification) systems operating at one single carrier frequency can hardly handle the great variety of shapes and materials that occur in products and packaging. In this contribution a dual-band antenna is presented that allows operation in the 13.56 MHz band as well as in the 868 MHz band. This provides two options to establish a data link to each object equipped with a dual-band RFID tag. The credit-card sized antenna consists of a very thin copper- plated polyimide substrate that is flexible and can be integrated into packaging. Measurements revealed that the antenna provides a quality factor of 54 in the 13.56 MHz band and a gain of -4.69 dBi in the 868 MHz band.

Determining the Dielectric Properties of a Car Tire for an Advanced Tire Monitoring System

Conventional tire pressure monitoring systems (TPMS) measure the air pressure in each tire of a car with a sensor mounted on the rim. In future, advanced tire monitoring systems (ATMS) will measure additional data like tire tempera- ture, contact area, vertical load, and slip angle. As a consequence, ATMS modules must be mounted directly at the tire tread. Furthermore, tire identification and lifecycle management via radio frequency identification (RFID) will be enabled. To design appropriate antennas for these applications, it is mandatory to investigate the interaction of antennas with the tire. Therefore we explore the tire structure and the dielectric material properties of the tire rubber. In this contribution the characterization of the dielectric tire materials using an open-ended coaxial probe is presented. We measure the reflection coefficients for each rubber tire layer and determine the relative permittivity and loss tangent by a numerical curve fitting method. The dielectric properties of each rubber layer are measured for a standard and a run-flat tire.

2x2 MIMO at Variable Antenna Distances

Various publications have drawn a precise picture on how correlation and capacity change when decreasing the receive-antenna distance of a 2 x 2 MIMO transmission to less than 0.1 lambda; however, little is known on how todays commonly employed coding schemes actually perform under such harsh conditions. In this paper, we evaluate - by measurements with a testbed - the uncoded BER over antenna distance performance of: spatial multiplexing, a linear dispersion code, the Golden Code, as well as Alamouti coding. We show, that even at very small antenna distances, a significant gain compared to an ordinary 1 x 2 SIMO transmission can be measured.

Evaluation of passive carrier-suppression techniques for UHF RFID systems

In this paper we investigate the signal that is returned from an interrogator antenna when communicating with a radio-frequency identification (RFID) transponder at a carrier frequency of 866 MHz. This signal contains the response of the transponder and also a part of the carrier signal that is leaking into the receive path. Depending on the type of interrogator antenna configuration used, both signal components are expected to vary widely in terms of power. We perform measurements with patch antennas that radiate linearly and circularly polarized waves in various configurations. Furthermore, passive methods that minimize the leaking carrier are explored and compared. Results are obtained for transponder positions covering the transition region between near- and farfield. It is found that separated transmitter and receiver antennas offer highest carrier-suppression and thus require the least effort for the receiver.

Efficient rectangular spiral coil simulation based on Partial Element Equivalent Circuit method using quasistationary approximation

We present an efficient method for simulating the impedance of a rectangular spiral coil in the high frequency regime based on the Partial Element Equivalent Circuit (PEEC) method. We use closed form expressions of the corresponding mixed potential integral equation, considering self and mutual inductance as well as capacitance between the elements of a rectangular spiral coil geometry. From the PEEC circuit elements, we abstract an equivalent circuit model that accounts for dominant field interactions. We compare the obtained impedance as a function of frequency with the results of a commercial field solver and benchmark the simulation duration. Our Matlab simulator is by a factor of 8.2 faster, which allows for quick optimization of the spiral coil.

Circularly polarized patch antenna with high Tx / Rx-separation

In this contribution an antenna is described that allows simultaneous transmission and reception in the same frequency band. Circular polarization is employed for a most reliable communication with radio frequency identification (RFID) transponders that typically have linearly polarized antennas. To achieve high separation between transmitted and received signals, a square patch antenna originally transmitting horizontally and vertically polarized radiation is combined with a 3 dB-hybrid circuit. With this hybrid circuit the antenna can simultaneously radiate a right-hand circularly polarized wave and receive a left-hand circularly polarized wave. Furthermore, the transmit signal that is unintentionally leaking into the receive path can be compensated by tuning the hybrid circuit with two variable capacitance diodes. At 866MHz a maximum Tx / Rx-separation of 65 dB was achieved in a static scenario. In a time variant indoor scenario with a metal object moving on a conveyor belt a Tx / Rx-separation of more than 52 dB was achieved by continuously tuning the hybrid circuit with a minimum-search algorithm.

Compact low profile UHF switched-beam antenna for advanced tyre monitoring systems

Advanced tyre monitoring systems are based on small sensor units directly embedded into the tyre rubber. In contrast to conventional tyre monitoring systems, more data like vibration information and slip angle are collected at this exposed position, so vehicular safety systems can be further improved. Powering of and communication with these sensor units, could be done with UHF radio frequency identification technology. In this contribution we propose a novel compact switched-beam antenna suitable for UHF frequencies. When mounted below the body floor pan of a vehicle, its directivity of 6 dB and the possibility to switch the main beam direction in 90? steps enables selection of a specific tyre for communication. Finally, we present measurements of the assembled antenna performed in an anechoic chamber.

Low-Profile Switched-Beam Antenna Backed by an Artificial Magnetic Conductor for Efficient Close-to-Metal Operation

This paper presents a compact switched-beam antenna (SBA) mounted on the bottom of a car. The antenna and associated circuits read RFID data sensed directly from all four tires. The frequencies of interest are the European UHF RFID band (866 MHz) and the 2.45 GHz ISM-band. In both cases, the antenna is electrically close to the conductive floor of the vehicle. In order to improve performance, dual-band periodic printed surfaces are designed and the efficiency, pattern, and loss are characterized for the antenna mounted above the FSS. In the UHF-band, the size of the assembly is 0.87λ0 × 0.87λ0 × 0.11λ0 and its performance is not sensitive to the position of the conductive car bottom surface. The antenna over FSS assembly has a directivity of 5.6 dB and the efficiency is enhanced by a factor of 2.35 (3.7 dB).

Rectangular spiral coil analysis for inductively coupled RFID systems based on Partial Element Equivalent Circuit method

In order to achieve maximum power efficiency with inductively coupled RFID tags, the impedances of the spiral coil and the integrated circuit must be matched to each other. In this paper, we present a numeric approximation of the impedance of a rectangular spiral coil by means of a coupled electromagnetic and circuit analysis called Partial Element Equivalent Circuit method and verify our simulation with measurement results. We include the effective permittivity in our 3D capacitance extraction method and fit the simulated impedance to a simple RLC equivalent circuit model which is also used by our measurement device and compare the resulting element values. Our Matlab-based simulator shows good agreement over the frequency band of interest and allows for optimization for various design goals.ZusammenfassungUm maximale Wirkleistung in einem induktiv gekoppelten RFID-System zu übertragen, müssen die Impedanzen der spiralen Induktivität und des Microchips aneinander angepasst sein. Im Folgenden präsentieren wir eine numerische Approximation der Impedanz einer rechteckigen spiralen Induktivität anhand der so genannten Partial Element Equivalent Circuit-Methode, wobei die elektromagnetische Analyse mit einer Schaltungsanalyse kombiniert wird. Wir inkludieren die effektive Permittivität in unsere 3D-Kapazitätsberechnung und verifizieren unsere Approximation mit Messergebnissen. Die Impedanz der Induktivität wird auf ein einfaches RLC-Ersatzschaltbild abgebildet, welches auch von unserem Messgerät verwendet wird, so dass ein fairer Vergleich möglich ist. Unser Matlab-basierter Simulator zeigt gute Übereinstimmung mit den Messergebnissen und bildet die Basis für unterschiedliche Optimierungsziele.