A.C. Marvin

Analytical formulation for the shielding effectiveness of enclosures with apertures

An analytical formulation has been developed for the shielding effectiveness of a rectangular enclosure with an aperture. Both the magnetic and electric shielding may be calculated as a function of frequency, enclosure dimensions, aperture dimensions and position within the enclosure. Theoretical values of shielding effectiveness are in good agreement with measurements. The theory has been extended to account for circular apertures, multiple apertures, and the effect of the enclosure contents.

Optimization of stirrer designs in a reverberation chamber

This paper describes an investigation into the key factors which contribute to an effective mode stirrer. The work concentrates on the lower frequency range, since all stirrers have poorer performance at low frequencies. The stirrers shapes and sizes have been investigated, together with an optimization of the finer details in the stirrers shape. The modeling of the mode stirred chamber has been performed using the transmission-line matrix (TLM) method. Software has been developed which, for each position of the rotating stirrer, builds the shape of the stirrer using thin, perfectly conducting boundaries. Results indicate that the design of the stirrers basic shape has a small but significant impact on its performance. A genetic algorithm has been used to optimize certain parameters in the shape of the stirrer, and a fitness factor based on a free space model of the stirrer has been used. The free space model runs 1500 times faster than the model in the chamber. The optimization is shown to improve the stirrers performance in three different sizes of chamber. Computer modeling has been verified by measurements performed in the chamber at the University of York.

Use of Reverberation Chambers to Determine the Shielding Effectiveness of Physically Small, Electrically Large Enclosures and Cavities

With the proliferation of small electric devices in recent years, along with various other applications, there is a growing need to test and determine the shielding properties or shielding effectiveness (SE) of physically small (but electrically large) enclosures or cavities. In this paper, we discuss how a reverberation chamber technique can be used to measure the SE of such enclosures. The approach consists of placing the small enclosure inside a reverberation chamber and using frequency stirring to excite the reverberation chamber. A small surface probe (i.e., a monopole) is mounted on the inside wall of the small enclosure to measure the power level inside the small enclosure. We present measured data from various other reverberation chamber approaches obtained from various enclosure configurations. The data from these other reverberation chamber approaches are used to validate the proposed approach. We also compared measured data to theoretical calculations of the SE for two small enclosures with circular apertures. These various comparisons illustrate that the proposed technique is a valid approach for determining the SE of physically small (i.e., cubic enclosure dimensions of the order of 0.1 m and smaller), but electrically large enclosures (that support several modes at the lowest frequency of interest).

A fast multiple mode intermediate level circuit model for the prediction of shielding effectiveness of a rectangular box containing a rectangular aperture

This paper presents an intermediate level circuit model (ILCM) for the prediction of the shielding effectiveness (SE) of a rectangular box containing a rectangular aperture, irradiated by a plane wave. The ILCM takes into account multiple waveguide modes, and is thus suitable for use at high frequencies and/or relatively large boxes. Inter-mode coupling and reradiation from the aperture are taken into account. The aperture may be positioned anywhere in the front face of the box, and the SE at any point within the box may be found. The model is presented in such a way that existing ILCM techniques for modelling elements such as monopoles, dipoles, loops, or transmission lines may be seamlessly incorporated into the circuit model. Solution times using the ILCM technique are three orders of magnitude less than those required by traditional numerical methods such as FDTD, TLM, or MoM. Accuracy however is not significantly compromised. Comparing the circuit model with TLM over nine data sets from 4 MHz to 3 GHz resulted in an rms difference of 7.70 dB and mean absolute difference of 5.55 dB in the predicted SE values.

A proposed new definition and measurement of the shielding effect of equipment enclosures

This paper describes the rationale behind a new proposed measurement of the screening effect of an equipment enclosure that takes into account the contents of the enclosure. The method uses a set of representative contents for enclosures. The representative contents are equipped with surface field probes to measure the power entering the contents. The ratio of this power to the incident power density is used to derive a quantity with the dimensions of area, termed here the shielding aperture. The measurement technique is described and examples of measurements are given along with computed comparisons with the conventional shielding effectiveness of the enclosures used.

Uncertainty Analyses in the Finite-Difference Time-Domain Method

Providing estimates of the uncertainty in results obtained by Computational Electromagnetic (CEM) simulations is essential when determining the acceptability of the results. The Monte Carlo method (MCM) has been previously used to quantify the uncertainty in CEM simulations. Other computationally efficient methods have been investigated more recently, such as the polynomial chaos method (PCM) and the method of moments (MoM). This paper introduces a novel implementation of the PCM and the MoM into the finite-difference time -domain method. The PCM and the MoM are found to be computationally more efficient than the MCM, but can provide poorer estimates of the uncertainty in resonant electromagnetic compatibility data.

Model of the electromagnetic fields inside a cuboidal enclosure populated with conducting planes or printed circuit boards

We examine the extension of a simple and versatile model of the electromagnetic fields in an equipment enclosure with an aperture to include the effects of loading the enclosure with conducting planes or printed circuit board structures (PCB). Modeling results are compared with experimental measurements of the shielding effectiveness in a cuboidal enclosure loaded with both grounded and ungrounded conducting planes and/or PCBs with a range of grounded and ungrounded tracks. Measurement results are compared with full electromagnetic simulations and the simple model to demonstrate the accuracy and range of validity of the simple model.

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.

Electromagnetic coupling between wires inside a rectangular cavity using multiple-mode-analogous-transmission-line circuit theory

This paper examines the coupling between two arbitrarily positioned wire segments inside a rectangular enclosure. The enclosure is treated as a superposition of analogous transmission lines which have been short circuited at two positions on the propagation axis. Each analogous transmission line is associated with a particular waveguide mode in the cavity. Previous work has used this analogy to predict the coupling between two monopoles inside a small box using the dominant TE/sub 10/ mode. This paper considers the general case of high-frequency coupling between two wire monopoles in a large rectangular cavity, where several higher order modes are active. By taking into account higher order modes, and the mutual coupling between the modes, a simple equivalent circuit is presented which can give a prediction for the coupling between the monopoles. Experimental results for various monopole pair positions are shown, which indicate the success of the multimode theory. The technique requires far less computer resources than traditional methods for solving such a problem (e.g., MoM, TLM or FDTD), with solution times of less than a second on an average PC. In addition, considerable insight into the coupling process can be gained by including or excluding particular waveguide modes. This is not possible with numerical methods.

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.