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Various approaches to designing unconventional antenna‐based RF probes for obtaining homogeneous 600 MHz B1 magnetic field in 14 T MRI scanners are investigated. Proposed solutions include microstrip patch antennas with engineered multidielectric substrates, acting as surface coils, and systems of vis‐à‐vis placed patch antennas to be used as volume coils with essentially increased Q‐factors compared to those of birdcage coils. Various options of controlling magnetic near fields in antenna systems with flat, cylindrical and cosine‐shaped substrates, as well as with tapered patch electrodes, are demonstrated. Problems of fabricating unconventional microstrip patch antennas for MRI applications are also addressed. [ABSTRACT FROM AUTHOR]

This paper investigates the disagreements that may occur between on-wafer measurements and electromagnetic (EM) simulations of high-Q inductive devices. Such disagreements are highlighted on a planar spiral inductor and a 3-D solenoid which exhibit measured maximum Q-factors of 27 and 35, respectively, while 42 and 45 were expected from EM simulations. Both devices are fabricated on high-resistivity substrates. A radiative interaction is identified between RF probe and inductive device under test. By using EM simulations, extra-losses associated with this parasitic effect are fully modeled through the calculation of radiation and dissipation related Q-factors. Adjustments of on-wafer probe setup are proposed to reduce this parasitic effect. Finally, the 3-D solenoid inductor is characterized using a new experimental fixture, and the maximum Q of 45 predicted by EM simulation is retrieved. [ABSTRACT FROM AUTHOR]

It is proposed to replace RF volume coils in high-field Magnetic Resonance Imaging (MRI) scanners by a microstrip patch antenna system. The performed computational studies have demonstrated an opportunity to form extended 3D volumes of homogenous RF magnetic fields by using a system of placed vis-à-vis two patch antennas. In order to control antenna dimensions and homogeneity of antenna near fields, the substrates of antennas have been engineered by appropriate patterning of high permittivity dielectric inserts. A variety of antennas with engineered substrates responding at 600 MHz for application in 14T MRI scanners have been designed, and the homogeneity of magnetic fields achieved inside the antenna system has been demonstrated. [ABSTRACT FROM AUTHOR]

ABSTRACT For optimizing sensitivity and RF excitation, high resolution NMR probes have to be tuned at the Larmor frequency and matched to 50 Ω-transmission lines to which they are connected. For achieving this setting, one usually resorts to the wobulation function which consists in sweeping the frequency of the power source while monitoring and minimizing the reflected power taken from a directional coupler through adjusting the tuning and matching capacitors. However, the presence of a transmission-reception switch between the directional coupler and the probe leads to the appearance of extra loss and reactance, and affects the determined values of the previous mentioned capacitors. Even if this situation is usually satisfactory, the true tuning-matching condition is required for optimal NMR sensitivity and for reliable spectra analyses when nonlinear effects are present (radiation damping, spin-noise experiments, etc.). To circumvent the need of network analyzer directly attached to the probe for finding this optimal tuning-matching condition, in the current work, we report a procedure which combines wobulation and choice of a transmission-line phase shift obtained by a variable phase shifter inserted between the probe and the switch. The technique was successfully validated with help of a network analyzer. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 59-68, 2015 [ABSTRACT FROM AUTHOR]

Inductive antenna design for passive wireless implantable use presents several challenges not currently addressed. A small form factor is desired for minimally invasive implantation and monitoring, and a low frequency is necessary for effective through-body power transfer. However, a small inductor limits sensitivity to changes in capacitance at low-frequency operation. It is thus necessary to optimize the inductor for maximal sensitivity while satisfying tight area and low frequency constraints. Here, a design methodology is presented for planar circular spiral inductors used with capacitive pressure sensors to form a passive wireless implantable pressure sensor. Several analytical expressions are collected to find the optimal geometric parameters that maximize the quality factor and sensitivity of the sensor frequency response. The analysis is validated through comparison with field solvers and wireless measurements of fabricated devices. [ABSTRACT FROM PUBLISHER]

This paper presents two feeding techniques for the radiation characterization of integrated antennas at millimeter-wave frequencies. The first method involves feeding the antenna-under-test (AUT) with a radiofrequency (RF) probe. We compare the measurements obtained with a standard RF probe, an extended probe, and an extended probe with reversed connector orientation. We find that the reversed orientation probe produces the best results in terms of effective measurement angular sector and reduction of ripples due to diffraction. The second method involves feeding the AUT through a flexible transmission line. This method achieves a larger range of measurement angles, but still suffers from parasitic scattering likely due to the mounting bracket and still need further development. [ABSTRACT FROM PUBLISHER]

In this study, a new type of low temperature co-fired ceramic (LTCC) probes fabricated using the system-in-package (SiP) technology is proposed; these probes are used for high-frequency measurements. The advantage of LTCC probes is that suitable structures of the metal tips of their electrodes can be easily created by cutting LTCC sheets during the fabrication process. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 2083–2087, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25416 [ABSTRACT FROM AUTHOR]

Microwave characterization of an integrated inductor is often carried out with respect to a local ground reference. The measurement outcome, however, does not necessarily reproduce the behavior of the device in an actual circuit where a ground reference does not exist. Furthermore, inductor measurements can be affected by the unequal currents on the inner and outer conductors of the coaxial lines internal to the RF probes. Such currents tend to radiate the microwave energy into the surroundings and manifest themselves as unexpected ripple and resonance features in measurement results. In this study, a measurement model is developed including both effects and is used to analyze experimental data obtained from integrated spiral inductors. A novel deembedding technique is then proposed and applied to eliminate the local ground and unequal coaxial current effects. [ABSTRACT FROM AUTHOR]

A versatile orthogonal-coil radio frequency (RF) probe suitable for detecting the resonant frequency of miniature implantable passive sensors has been designed and tested. The probe sensitivity has been tested using printed-circuit spiral inductors of various sizes (3-15 mm) in series with discrete surface-mount capacitors designed to resonate over a range of frequencies (50-200 MHz). Close agreement between theoretical calculations and experimental results has been obtained. An equation is derived for transmit/receive (T/R) isolation that agrees with experimental measurements over the frequency range 1-500 MHz. The probe includes an additional coil to compensate for the effect of eddy currents in the human body on the probe. T/R isolation of at least 90 dB over the frequency range 1-100 MHz can be achieved when the probe is placed in close proximity to the human body. [ABSTRACT FROM AUTHOR]