Katja Poković

Design, optimization, realization, and analysis of an in vitro system for the exposure of embryonic stem cells at 1.71 GHz

The aim of this study was to develop an exposure system which enables in vitro experiments to be conducted under variously modulated radiofrequency exposures. Based on the evaluation of different possible systems, it was decided to realize a system based on rectangular waveguides. The system was optimized for the following parameters: (1) homogeneity of the cell exposure, (2) simultaneous exposure of several Petri dishes, (3) efficiency, (4) strict environmental control, (5) quick and easy access to the Petri dishes, (6) cost, and (7) simple operation by non-engineering personnel. The implemented control software enables investigation of a wide spectrum of amplitude modulation schemes between 0.1 Hz and 1 kHz, including the modulation schemes of current and future digital mobile communication systems as well as other exposure protocols. The system described has been initially utilized for a study on the differentiation and cell functions of embryonic stem cells. Detailed numerical and experimental dosimetry and environmental tests have demonstrated that it meets all target objectives. The entire system including the sham exposure system fits into a single incubator. It enables the carrying out of various experiments designed to test biological responses to RF exposures at 1.2-1.7 GHz by using various modulation schemes and long term exposure protocols as well as simultaneous data logging.

Millimeter-resolution E-field probe for isotropic measurement in lossy media between 100 MHz and 20 GHz

Currently, most advanced isotropic E-field probes for near-field measurements in lossy dielectric media have a tip diameter of typically 4-6 mm, housing three 2-3 mm long sensors. Although these probes are very well suited for many applications, their general use is constrained by several limitations: (1) upper frequency range of <4 GHz in tissue simulating liquids, (2) spatial resolution of a few millimeters, 3() unsuitable for measurements closer than 3-5 mm from any media boundary (boundary effects); and therefore (4) inapplicable for assessment of the induced field strengths in structures of a few millimeters. On the other hand, many current and future research projects require evaluation of the field distributions for frequencies larger than a few gigahertz, for measurements within small structures (e.g., in in vitro and in vivo experiments), for evaluations of special physical interactions such as strongly nonhomogeneous field distributions or characterization of larger probes, etc. A new probe has been developed which is applicable for frequencies well above 10 GHz in tissue-like media and provides a spatial resolution of at least 1 mm. Although it is a one sensor probe, it has been designed such that the isotropic measurement (spherical isotropy: <0.2 dB) can be obtained simply by taking three measurements each shifted by 1200 rotation around the probes axis. The probe has been fully characterized and tested in various applications.

Reliable prediction of mobile phone performance for realistic in-use conditions using the FDTD method

The objective of this study was to analyze whether advanced simulation platforms provide the effectiveness, accuracy, reliability, and efficiency to predict impairment of mobile-phone RF performance under various usage patterns. The investigation was based on the mechanical CAD data of a commercial phone with two alternative antennas. Three significant hand positions were modeled and evaluated with the device against the SAM head. The results demonstrated high reliability and suitability for providing decision rationale for the design of complex high-end multi-band mobile phones.

The Effect of Diode Response of Electromagnetic Field Probes for the Measurements of Complex Signals

Diode detectors present inside electromagnetic field probes are typically calibrated for linearity using continuous sinusoidal waveforms (CW). In this paper, it is shown that CW linearization is not adequate for the measurement of complex wireless communication signals with high peak-to-average power ratios. While previous analog and digital communication signals (1G and 2G) can be more easily corrected for linearity, newer 3G and 4G communication protocols employ complex modulations with stochastic signal envelopes. As a result, proper linearization depends on the diode response and signal characteristics, and large errors results if CW linearization is used. The errors introduced when measuring such signals with probes employing CW linearization are quantified in this paper. A numerical model of the diode response is provided and validated against measurements. Errors due to CW linearization can exceed 2 dB, whereas linearity errors within 0.4 dB are attainable using the proposed calibration procedures for even increased dynamic ranges.

Novel probes and evaluation procedures to assess field magnitude and polarization

The immense development rate of wireless technologies has also brought new requirements for the RF design of transmitters. The design challenge is to optimize multiband devices with minimal dimensions and weight as well as an appealing appearance, which nevertheless operate well within varyingly complex environments such as frequently changing positions within the closest vicinity of the human body. The optimization of such transceivers requires new analysis tools providing precise measurement of electric and magnetic field strength distributions, even in the closest proximity of RF transmitters. In this study, novel field probes were analyzed optimized, and constructed enabling not only the assessment of the local field strength, but also information on the polarization of the field. The ellipse parameters are reconstructed by a combination of a downhill simplex and a Givens updating algorithm, which has proven to be fast and robust. The developed probes and procedures greatly enhance the quality of the information needed for analysis and optimization of antennas and transmitters.

Novel Sensor Model Calibration Method for Resistively Loaded Diode Detectors

Miniaturized radio-frequency electromagnetic nearfield broadband probes based on resistively loaded diode detectors have a limited linear dynamic range of ≈30 dB. Generic linearization schemes have been proposed and applied for continuous wave like or pulse-modulated signals to extend the dynamic range to over 50 dB. Modulation specific linearization has been proposed to also enable precise measurements of current wireless systems with complex modulation schemes. However, calibrations that require an experimental amplitude sweep for each signal have become impractical due to the growing number of wireless communication systems. In this paper, a novel sensor model calibration method has been developed and validated. It is based on calibration of the optimized sensor equivalent circuit model that was derived from the sensor response as a function of bandwidth, duty cycle, modulation schemes, data rate, and statistical distribution. It is shown that all the elements of the equivalent circuit model can be sufficiently accurate determined by the dynamic response to a set often generic signals. This model is then used to numerically determine the linearization parameters for any digitized communication signal. The method was tested on various probes for over 200 modulations, resulting in a linearity uncertainty of less than <;0.4 dB (k = 2) for a dynamic range of >50 dB. The proposed method will improve the precision of measurements, reduce calibration costs, increase the flexibility for application of diode-loaded sensors, and enable the use of real-time information for automated probe linearization during or after measurements.

Experimental and Numerical Near-Field Evaluation of RF Transmitters

Precise evaluation of the near-field generated by transmitters operating in the radio frequency (RF) range (30 MHz to 6 GHz) is still an engineering challenge not only requiring sophisticated tools but also involving different procedures with a multitude of parameters to be considered. This paper reviews the state-of-the-art of near-field evaluation with emphasis on compliance testing of handheld RF transmitters with safety limits Similar tools and techniques can be utilized for the analysis and optimization of antennas operating in complex environments as well as for evaluation of special electromagnetic compatibility and interference problems occurring in the near-field of transmitters. Another area of applications is the analysis, evaluation and optimization of exposure setups used in RF safety research to investigate possible basic, therapeutic or adverse health effects from non-ionizing radiation.

Past, present, and future of SAR evaluations

Innovation in fast and reliable electromagnetic exposure evaluation has been driven by rapid advances in both measurement technology and wireless communication devices. Novel sensors, field reconstruction algorithms and fast processing have improved the speed of specific absorption rate (SAR) measurement by orders of magnitude. At the same time, wireless device capability, driven by the expansion in the number of signal modulation schemes, antenna configurations and usage modes on or near the body, has dramatically increased the number of tests needed to demonstrate compliance with exposure limits and has driven the need for faster and more automated SAR testing. International standards organizations and national regulatory agencies have been essential in adapting procedures for these new technologies and requiring a high level of confidence that the SAR measurements are reliable and conservative. This presentation reviews past and current SAR evaluation methods and examines the innovations driving future technologies.