Ian David Flintoft

A Method for the Measurement of Shielding Effectiveness of Planar Samples Requiring No Sample Edge Preparation or Contact

A method is presented for the measurement of shielding effectiveness of planar materials with nonconducting surfaces such as carbon fiber composites. The method overcomes edge termination problems with such materials by absorbing edge-diffracted energy. A dynamic range of up to 100 dB has been demonstrated over a frequency range of 1-8.5 GHz, depending on the size of the sample under test. Comparison with ASTM D4935 and nested reverberation measurements of shielding effectiveness shows good agreement.

Interference to medical equipment from mobile phones

Cellular mobile phones may interfere with hospital equipment. We irradiated five representative pieces of equipment using simulated phone signals and frequencies. Two (an oximeter and a syringe pump) were immune to electric fields of up to 40 V m-1. The most susceptible was a physiological monitor which showed effects at 10 V m-1. None of the equipment was affected by fields that could be produced at over one metre from a 2 W mobile phone.

A simple model of EMI-induced timing jitter in digital circuits, its statistical distribution and its effect on circuit performance

A simple model has been developed to characterize electromagnetic interference induced timing variations (jitter) in digital circuits. The model is based on measurable switching parameters of logic gates, and requires no knowledge of the internal workings of a device. It correctly predicts not only the dependence of jitter on the amplitude, modulation depth and frequency of the interfering signal, but also its statistical distribution. The model has been used to calculate the immunity level and bit error rate of a synchronous digital circuit subjected to radio frequency interference, and to compare the electromagnetic compatibility performance of fast and slow logic devices in such a circuit.

Broadband Measurement of Absorption Cross Section of the Human Body in a Reverberation Chamber

We present broadband reverberation chamber measurements of the absorption cross section (ACS) of the human body averaged over all directions of incidence and angles of polarization. This frequency-dependent parameter characterizes the interactions between the body and the enclosures of reverberant environments such as aircraft cabins, and is, therefore, important for the determination of the overall Q-factor and, hence, the field strength illuminating equipment inside such enclosures. It also correlates directly with the electromagnetic exposure of occupants of reverberant environments. The average ACS of nine subjects was measured at frequencies over the range 1-8.5 GHz. For a 75-kg male, the ACS varied between 0.18 and 0.45 square meters over this range. ACS also correlated with body surface area for the subjects tested. The results agree well with computational electromagnetic simulations, but are obtained much more rapidly. We have used the obtained values of ACS to estimate the effect of passengers on the Q-factor of a typical airliner cabin.

The re-emission spectrum of digital hardware subjected to EMI

The emission spectrum of digital hardware under the influence of external electromagnetic interference is shown to contain information about the interaction of the incident energy with the digital circuits in the system. The generation mechanism of the re-emission spectrum is reviewed, describing how nonlinear effects may be a precursor to the failure of the equipment under test. Measurements on a simple circuit are used to demonstrate how the characteristics of the re-emission spectrum may be correlated with changes to the digital waveform within the circuit. The technique is also applied to a piece of complex digital hardware where similar, though more subtle, effects can be measured. It is shown that the re-emission spectrum can be used to detect the interaction of the interference with the digital devices at a level well below that which is able to cause static failures in the circuits. The utility of the technique as a diagnostic tool for immunity testing of digital hardware, by identifying which subsystems are being affected by external interference, is also demonstrated.

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.

Measured Average Absorption Cross-Sections of Printed Circuit Boards from 2 to 20 GHz

Absorption by the contents of an equipment enclosure, particularly printed circuit boards (PCBs), affect the enclosures shielding performance. At high frequencies, this absorption can be quantified using the angle of arrival and polarization averaged absorption cross-section (ACS). However, there is no available data on the high-frequency absorption characteristics of modern PCBs. In this study, we apply a reverberation chamber to the determination of the average ACS of a large number of PCBs taken from contemporary information and communication technology (ICT) equipment to provide a unique and comprehensive dataset. The ACS was found to range from 4 × 10-4-10-2 m2 from 2-20 GHz and different classes of PCB could be identified according to their surface characteristics. The “shadowing effect” of densely packed PCBs was also quantified for a subset of the PCBs. It was found that the ACS of a PCB in the stack was reduced by 20%-40% compared to its value when isolated. By way of a review of the general power balance analysis of an electrically large populated equipment enclosure in an external environment, we show how the acquired data will be useful for future qualification methodologies for ICT enclosures and PCBs.

On the Measurable Range of Absorption Cross Section in a Reverberation Chamber

Average absorption cross section can be measured in a reverberation chamber with an uncertainty estimated from the number of independent samples of the chamber transfer function taken during the measurement. However, the current uncertainty model does not account the loading effect of the object being measured or the presence of nonstochastic energy in the chamber, as characterized by the Rician K-factor. Here, the uncertainty model for the absorption cross-section measurements has been extended to include both of these effects for the case of stepped mechanical tuning with a paddle and frequency tuning. The extended uncertainty formula has been applied to predict the range over which absorption cross-section measurements can be made with a defined relative uncertainty in a chamber of given geometry, using both simple models for the reverberation chamber and the measured quality-factor and K-factor. The model has been validated using measurements on a set of absorbing cubes of different sizes compared to Mie Series calculation on sphere of equivalent surface area. The extended error model is particularly utile for the design of reverberation chambers and experiments for absorption cross-section measurements across a wide range of application areas.

A Theory of Alternative Methods for Measurements of Absorption Cross Section and Antenna Radiation Efficiency Using Nested and Contiguous Reverberation Chambers

Average ACS can be measured in a reverberation chamber; however, the existing technique determines a value that includes the effects of the radiation efficiencies of the antennas used in the measurement. Correcting for these necessitates further complex measurements. Here, we present the theory of an alternative measurement methodology using two nested or contiguous reverberation chambers which is free from errors caused by the radiation efficiencies of the antennas. The new method is based on the theoretical average transmission cross sections (TCS) of circular holes in a metal plate between the two chambers. In fact, the method can be viewed as an accurate transfer calibration measurement between TCS and ACS that is independent of both the chamber and antenna characteristics. Further, since the existing method of measuring ACS includes the effects of the radiation efficiencies of the antennas, a comparison of the ACS of a reference object, measured using both the existing and new methods, also provides an alternative method of determining radiation antenna efficiency. Measurement uncertainties for both alternative measurement methods-ACS and antenna radiation efficiency-are also derived.

On measurement of reverberation chamber time constant and related curve fitting techniques

The reverberation chamber time constant quantifies how fast a reverberation chamber loses its stored energy at different frequencies, which makes it a very important parameter in many power related tests, such as the measurement of antenna efficiency, the measurement of absorption cross section, and the electromagnetic immunity test of electronic devices. The chamber time constant is usually obtained by doing regressions of power delay profile and calculating its gradient. But the shape of power delay profile can sometimes be distorted by the band limited window function applied in the frequency domain. A non-linear curve fitting technique which can cancel the effect of window function was developed, aiming to give a robust determination of the chamber time constant. With the help of this technique, window functions with much smaller bandwidth can be applied without introducing error in the evaluation of chamber time constant. In this paper, a 1 MHz wide window function in which only 10 samples of S21 are available was put under test and it was found a robust answer of chamber time constant can still be given by non-linear curve fitting techniques. Therefore the measurement time can be reduced and the frequency resolution of the chamber time constant can be increased at the same time.