Inverse Fourier transform technique of measuring averaged absorption cross section in the reverberation chamber and Monte Carlo study of its uncertainty

The averaged absorption cross section (ACS) of a lossy object characterises its ability to capture power from diffused electromagnetic waves. The averaged ACS is very important in many EMC research areas such as indoor wireless channel modelling and human safety exposure study. The measurement of averaged ACS in a reverberation chamber can be achieved by measuring the rate of power loss in the time domain, however this technique requires dense frequency sampling for taking the inverse Fourier transform, which is very time consuming. A new scenario which accelerates the measurement speed is presented in this paper. It combines the technique of non-linear curve fitting to the power delay profile, segmented frequency sweeping and continuous mode stirring. The scenario was validated by measuring the averaged ACS of a hollow plastic sphere filled with deionized water in an EMC reverberation chamber. Measurement results showed a good accordance with the simulations and the measurement uncertainty was studied numerically with the Monte Carlo method

Efficient Determination of Reverberation Chamber Time Constant

Determination of the rate of energy loss in a reverberation chamber is fundamental to many different measurements such as absorption cross-section, antenna efficiency, radiated power, and shielding effectiveness. Determination of the energy decay time-constant in the time-domain by linear fitting the power delay profile, rather than using the frequency-domain quality factor, has the advantage of being independent of the radiation efficiency of antennas used in the measurement. However, determination of chamber time constant by linear regression suffers from several practical problems, including a requirement for long measurement times. Here we present a new nonlinear curve fitting technique that can extract the time-constant with typically 60% fewer samples of the chamber transfer function for the same measurement uncertainty, which enables faster measurement of chamber time constant by sampling fewer chamber transfer function, and allows for more robust automated data post-processing. Nonlinear curve fitting could have economic benefits for test-houses, and also enables accurate broadband measurements on humans in about ten minutes for microwave exposure and medical applications. The accuracy of the nonlinear method is demonstrated by measuring the absorption cross-section of several test objects of known properties. The measurement uncertainty of the method is verified using Monte Carlo methods

On the Meaning of Enclosure Shielding Effectiveness

This paper presents some thoughts on the meaning of shielding effectiveness (SE) for a real enclosure. It is demonstrated that the conventional measurement of SE is a value specific to the particular measurement and is not necessarily representative of the enclosure in any practical situation, including a different measurement. The SE of an enclosure depends both on the enclosure contents, including the measurement antenna and the effective transmission cross section of the energy coupling mechanisms such as apertures, penetrations, seams and diffusion through walls

Rapid and accurate broadband absorption cross-section measurement of human bodies in a reverberation chamber

A measurement methodology for polarization and angle of incidence averaged electromagnetic absorption cross-section using a reverberation chamber is presented. The method is optimized for simultaneous rapid and accurate determination of average absorption cross-section over the frequency range 1–15 GHz, making it suitable for use in human absorption and exposure studies. The typical measurement time of the subject is about 8 min with a corresponding statistical uncertainty of about 3% in the measured absorption cross-section. The method is validated by comparing measurements on a spherical phantom with Mie series calculations. The efficacy of the method is demonstrated with measurements of the posture dependence of the absorption cross-section of a human subject and an investigation of the effects of clothing on the measured absorption which are important considerations for the practical design of experiments for studies on human subjects

3D Diffusion Models for Predicting Reverberant Electromagnetic Fields in Loaded Enclosures

The power balance technique for the prediction of shielding effectiveness of reverberant enclosures is fast and simple to use. However, it assumes a uniform field in the enclosure, which has been shown to be incorrect in the presence of dissipative contents. The diffusion model is a generalization of the power balance method that can account for the field inhomogeneity due to the presence of losses with much lower computational effort than a full wave solver. Evaluation of a 2D diffusion model produced promising results compared to physical measurements. Here we present a 3D diffusion model applied to an enclosure with an aperture and dissipative contents. Comparisons between the 3D diffusion model, measurements and a full wave solver suggest that it is able to account for the variation of the electromagnetic field due to dissipative contents with far less computational effort than full wave solvers. The diffusion model allows rapid solution of the shielding effectiveness of enclosures with dissipative contents and arbitrary geometries. and reduces the time to model equipment enclosures from hours to minutes, whilst still determining the variation of field strength due to contents. In addition, the method may help predict field inhomogeneity in reverberation chambers

Predicting Shielding Effectiveness of Populated Enclosures Using Absorption Cross Section of PCBs

Shielding effectiveness (SE) is an important measure of how well an enclosure reduces the electromagnetic (EM) field incident upon it. Commonly, when the shielding effectiveness of an enclosure is stated it is for the case when the enclosure is empty. Including contents such as printed circuit boards (PCBs) in the enclosure will affect the shielding effectiveness as the PCB absorbs EM energy. One technique of determining how much energy a PCB absorbs is to measure its absorption cross section (ACS) using a reverberation chamber (RC). The measured ACS can be used to predict the shielding effectiveness of an enclosure when the PCB is inside it using power balance techniques. In this paper the ACS of a number of PCBs are measured both individually and in closely stacked groups. This information is then used to show how the ACS can be used to calculate shielding effectiveness and the results are compared to direct measurement of the SE of the enclosure containing a PCB. Knowledge of the ACS of typical or particular PCBs could be used by engineers to estimate the real shielding effectiveness of an enclosure with contents, when designing electronic systems

Experimental characterization of the response of an electrical and communication raceway to IEMI

This paper reports the results of two experimental campaigns aimed at studying the high-frequency response of a raceway containing low voltage power, telephone, and Ethernet cables, to external electromagnetic field illumination. The raceway was tested against HPEM transients inside a GTEM cell and low power fields inside a reverberation chamber (RC). The HPEM tests revealed that the low voltage power cables have the greatest coupling under a hyperband illumination, compared to telephone and Ethernet cables. The RC tests allowed the determination of statistical transfer functions from random incident field configurations into DM voltage in cable loads. The responses were found to be governed by the raceway under test at the lower frequencies (below 1 GHz). Between 0.2-1 GHz, the raceway gives about 10 dB higher coupling than a short patch cable. The difference is even greater at lower frequencies and for shielded cables. In the frequency band 1-3 GHz, little difference was observed between short patch cables and the full raceway, but both were still significantly higher than direct coupling to the measurement card. Beyond 3 GHz, the coupling is clearly dominated by the terminal equipment. The experiments performed in this work provide a better understanding of the expected induced voltages and currents in commercial cable systems when exposed to IEMI-like signals

Impact of opioid-free analgesia on pain severity and patient satisfaction after discharge from surgery: multispecialty, prospective cohort study in 25 countries

Background: Balancing opioid stewardship and the need for adequate analgesia following discharge after surgery is challenging. This study aimed to compare the outcomes for patients discharged with opioid versus opioid-free analgesia after common surgical procedures.Methods: This international, multicentre, prospective cohort study collected data from patients undergoing common acute and elective general surgical, urological, gynaecological, and orthopaedic procedures. The primary outcomes were patient-reported time in severe pain measured on a numerical analogue scale from 0 to 100% and patient-reported satisfaction with pain relief during the first week following discharge. Data were collected by in-hospital chart review and patient telephone interview 1 week after discharge.Results: The study recruited 4273 patients from 144 centres in 25 countries; 1311 patients (30.7%) were prescribed opioid analgesia at discharge. Patients reported being in severe pain for 10 (i.q.r. 1-30)% of the first week after discharge and rated satisfaction with analgesia as 90 (i.q.r. 80-100) of 100. After adjustment for confounders, opioid analgesia on discharge was independently associated with increased pain severity (risk ratio 1.52, 95% c.i. 1.31 to 1.76; P < 0.001) and re-presentation to healthcare providers owing to side-effects of medication (OR 2.38, 95% c.i. 1.36 to 4.17; P = 0.004), but not with satisfaction with analgesia (beta coefficient 0.92, 95% c.i. -1.52 to 3.36; P = 0.468) compared with opioid-free analgesia. Although opioid prescribing varied greatly between high-income and low- and middle-income countries, patient-reported outcomes did not.Conclusion: Opioid analgesia prescription on surgical discharge is associated with a higher risk of re-presentation owing to side-effects of medication and increased patient-reported pain, but not with changes in patient-reported satisfaction. Opioid-free discharge analgesia should be adopted routinely