Tomas J. F. Pavlasek

Preliminary survey of the electromagnetic interference environment in hospitals

Electric field levels were predicted using a free-space propagation model, and then measured in three major hospitals in downtown Montreal. The measurements were performed using industry-standard techniques and novel broadband, omnidirectional, triaxial, electrically small (BOTES) techniques. Although the absolute values of predictions were often unreliable, they acceptably predicted the type of electromagnetic environment observed at different hospitals. Results from both sets of measurements were closely related, suggesting continued future development of lower-cost BOTES measurement techniques. None of the fields encountered were above the limit prescribed by the Food and Drug Administration (FDA) susceptibility standard for medical devices. Correlations of survey results and of previous reports of device malfunctions in the rooms surveyed suggest that the FDA standard should be mandatory rather than voluntary.<<ETX>>

Design of Absorber-Lined Chambers for EMC Measurements Using a Geometrical Optics Approach

Absober-lined chambers (ALCs) have been found useful for EMC measurements over an extremely wide frequency range. However, only limited information about the field structure inside ALCs can be efficiently determined by measurements, while the characteristics of the rough absorbing surface do not render their analysis amenable to formal methods. The computation of field structure as a function of various parameters must thus resort to empirical modeling. A simple computational technique presented here predicts fields inside ALCs to a good approximation. Use is made of the Geometrical Theory of. Optics (GO). The absorber material is empirically modeled by its reflectivity as a function of frequency and angle of incidence. To establish the validity of the technique, computed results are compared to measured data. The methodology is then extended to compute fields inside a variety of ALC configurations. A study of the effect of shape, size, and absorber characteristics on these fields is presented to demonstrate the utility of the technique as a tool for ALC design purposes.

Risk of patient injury due to electromagnetic-interference malfunctions: estimation and minimization

Healthcare needs wireless informatics. To implement this need safely, hospitals require suitable wireless-EMC policies. However, most lack such policies, partly because some are unsure that patients really risk being injured by EMI malfunctions. Factors affecting this risk are critically overviewed, specifically the probabilities: (1) that RF sources operate at given locations; (2) that fields from such sources propagate to a susceptible medical device; (3) that this device malfunctions; and (4) that the malfunction causes patient injury. Only qualitative estimates of the resultant overall EMI-injury risk are possible. This risk is probably very small but must be minimized. Ways to do so are critically overviewed.

Measurement of indoor propagation at 850 MHz and 1.9 GHz in hospital corridors

The fields at 850 MHz and 1.9 GHz were measured in five hospital corridors. The field strength tended to decline with distance at a free-space rate within about 2 m of the source, but at a slower rate when further away. Fields at 1.9 GHz declined more slowly than 850-MHz fields. The implications for hospital EMC management are discussed.

Design Criteria for Cost-Effective Broad-Band Absorber-Lined Chambers for EMS Measurements

The problem considered here concerns simulating the electromagnetic environment for electromagnetic susceptibility (EMS) measurements over a wide frequency range. Means of bridging the existing gap between low-and high-frequency techniques are suggested by extending the use of anechoic-type chambers downward in frequency. Results of measurements over a wide frequency range are presented to demonstrate the behavior of such absorber-lined chambers (ALCs). Basic criteria and guidelines for the design of ALCs are proposed.

Evaluation of free-space propagation in hospital corridors

It has been proposed that increased usage of wireless communication (e.g. portable radio-frequency (RF) sources; wireless local area networks) should reduce health care costs and improve clinical-care efficiency. However, such increased usage must take place without increasing the risk of electromagnetic interference (EMI) to medical devices. Recommendations for electromagnetic compatibility (EMC) in health care environments have been described [e.g., ref 1]. Central to such recommendations is the requirement to manage RF sources and susceptible medical devices so that their interaction is minimized. It is usually proposed that this be done by specifying a Minimal Separation between RF sources of given power and medical devices of given immunity, assuming that free-space propagation is approximately valid. Alternately, it has been proposed that Zones be specified where approved RF sources and approved medical devices can operate. The latter approach would be required in areas where free-space propagation is invalid. We describe a preliminary study that assesses the validity of free-space propagation in hospital corridors, in order to evaluate whether minimal separations predicted by free-space propagation might safely permit EMC within corridors of a typical urban hospital.

Automated Computer-Based Facility for Measurement of Near-Field Structure of Microwave Radiators and Scatterers

An automatic facility for measuring the three-dimensional structure of the near fields of microwave radiators and scatterers is described. The amplitude and phase for different polarization components can be recorded in analog and digital form using a microprocessor-based system. The stored data are transferred to a large high-speed computer for bulk processing and for the production of isophot and equiphase contour maps or profiles. The performance of the system is demonstrated through results for a single conical horn, for interacting rectangular horns, for multiple cylindrical scatterers, and for the fields inside an absorber lined chamber.

Volumetric 1.9-GHz fields in a hospital corridor: electromagnetic compatibility implications

The current draft of the next medical-equipment EMC standard, IEC 60601-1-2, will recommend usage of free-space minimal separations (between RF sources of given power and medical devices of given immunity) to minimize EMI malfunction of medical equipment. We have previously reported that such separations were useful in most hospital corridors, but such reports were based on mid-corridor measurements. We now describe preliminary three-dimensional 1.9-GHz extensions of these reports. We found that mid-corridor path loss was less than that at corridor walls. Path loss near floors was much less then that at higher locations. Because medical devices are rarely placed on the floor, our previously reported minimum-separation recommendations are still likely to apply at most corridor locations.

The behaviour of space deployable, non-paraboloidal reflector antennas and the morphology of aperture illumination phase in relation to the far field

1) The one to one to one correspondance of the reflector shape to the aperture phase distribution then to the far-field pattern permits making an estimate of the effect of symmetric surface distortions on far-field patterns- Similar relationships can be expected for faceted reflectors as well. 2) Clearly as the number of gores (or facets) is increased, the effects of these distortions on the main beam structure is decreased and becomes evident only in the sidelobe structure. 3) It is evident that single scan (e.g. E-plane and/or H-plane) far-field patterns of such reflectors should not be used to characterize their behaviours since the scan can be chosen which could be misleadingly “good” or “bad”. The entire sectional plane contour plots are needed. Conversely, if in a particular application selective treatment of the beam illumination is needed, this might be achieved by an appropriate rotation of the reflector.