Transverse Electromagnetic Cell Research Articles

Screened-Room Measurements on the NIST Spherical-Dipole Standard Radiator.

We report the results of a study of measurements of radiated emissions from the NIST sphcrical-dipole standard radiator in several screened rooms. The study serves as a demonstration of possible applications of the standard radiator as well as an investigation of radiated-emissions measurements in screened rooms. The screened-room measurements were performed in accordance with MIL-STD)-462 (1967). Large differences occurred in the field intensity measured at different laboratories and even on different days at the same laboratory. There was a systematic difference at low frequencies between the screened-room results and results obtained in a transverse electromagnetic (TEM) cell, open-area test site (OATS), and anechoic chamber. We also present the results of OATS tests confirming the temporal stability of the standard radiator and measuring the loading effect of a ground plane as a function of distance from the sphere.

FDTD evaluation of scattering by metallic objects in GTEM cells

The finite difference time domain (FDTD) technique, developed in a polar coordinate system, is used to analyse the scattering by metallic objects in a gigahertz transverse electromagnetic (GTEM) cell. The advantage of using the FDTD method is the noticeable mathematical simplification, with respect to the analytical approach, which allows one to treat general structures and electrical characteristics without further analytical effort, and use of computer resources. The analysis has been carried out both at discrete frequencies, to compare our results with those in the literature, and with pulse sources to extract, by means of a discrete Fourier transform, the frequency domain behaviour of the system. The measurements inside the GTEM cell show a good correlation with those made in anechoic chambers.

Electromagnetic field calculations used for exposure experiments on small animals in TEM-cells

Three-dimensional electromagnetic calculations for loaded transverse electromagnetic (TEM) transmission cells are presented. Based on those calculations a prediction of the perturbation of the standard uniform field in the TEM-cell, due to the scattering by inhomogeneous structures placed inside the cell cavity, is given. The influence of the dimensions of a lossy structure and its position in the TEM-cell on its absorption of the electromagnetic fields is presented. Knowing the perturbation of the uniform field is important for good interpretation of the biological experimental results.

Experimental studies of brain tumour development during exposure to continuous and pulsed 915 MHz radiofrequency radiation

It has been suggested that electromagnetic fields (EMFs) act as a promoter late in the carcinogenesis process. To date, however, noconvincing laboratory evidence has been obtained indicating that EMFs cause tumour promotion at non-thermal exposure levels. The effects of EMF exposure in a rat brain glioma model were investigated. The exposure consisted of 915 MHz microwaves, both as continuous waves (1 W), and modulated with 4, 8, 16 and 200 Hz in 0.5 ms pulses and 50 Hz in 6 ms pulses (2 W per pulse). Fischer 344 rats of both sexes, weighing 150–250 g, were used in the experiments. 5000 RG2 cells in 5 μ1 nutrient solution were injected by the stereotaxic technique into the head of the right caudate nucleus in 37 experimental rats and 37 matched controls. The exposed animals were kept unanaesthetized in well ventilated transverse electromagnetic (TEM) cells producing 915 MHz continuous or modulated microwaves. Exposure was started on day five after inoculation. The animals were exposed for 7 hd d −1 for 5 d per week during two to three weeks. The controls were kept in an identical TEM cell without EMF exposure. All brains were examined histopathologically and the tumour size was determined. Our study does not show a significant difference in tumour size between animals exposed to 915 MHz microwaves, and those not exposed. Our preliminary results do not support that even an extensive daily exposure to EMF promotes tumour growth when given from the fifth day after the start of tumour growth in the rat brain until the death of the animal which by then has a large brain tumour. Further studies with higher specific absorption rate levels are in progress.

Method of Moment Approach to Compute the Characteristic Impedance of TEM Cell

The characteristic impedance of asymmetric and symmetric transverse electromagnetic (TEM) cells are computed using Method of Moment (MOM). Quasi-static approximations are used for the operation of the cell at frequencies well below its cut off frequency. Accurate distribution of electric charge on the centre conductor and subsequently the characteristic impedance are obtained. The results are compared with those of others obtained by different methods. The method has the advantage that no assumptions are needed for the charge distribution on the septum.

Simulation of the effect of inhomogeneities in TEM transmission cells using the FDTD-method

The finite-difference-time-domain (FD-TD) method is applied to model transverse electromagnetic (TEM) transmission cells in three dimensions. The perturbation of the TEM mode and the standard field distribution in the TEM cell, due to inhomogeneous materials placed inside the cell cavity, is evaluated. Particularly, the dependence of the disturbance of the TEM mode on the dimensions of the material and its position in the TEM cell is studied. The absorption of the electromagnetic fields in the lossy materials, placed in the cell, is also calculated. >

Reduction of vector unknowns using geometric symmetry in a triangular-patch moment method for electromagnetic scattering and radiation analysis

A simplification method for the boundary element model developed by H. Tsuboi et al. (1990) is applied to the triangular-patch moment method in order to reduce the number of vector unknowns. The simplification of the computational model is performed by a variable transformation using the spatial eigenmodes, which is similar to transformation used in the boundary element case. However, the unknown vectors have to be arranged according to the geometry of the rotational symmetry because of the varying directions of the unknown vectors. The applicability of the simplification method was verified by a flat square plate model and a TEM (transverse electromagnetic) cell model. >

Time-domain calibration of field sensors for electromagnetic pulse (EMP) measurements

EMP (electromagnetic pulse) simulators built to test the hardening of electronic equipment against a high-intensity pulsed EMP produced by a nuclear detonation are discussed. In particular, a method for time-domain calibration of the field-strength sensors needed to carry out measurements is presented. The approach taken is to calculate the response of a simple electric and magnetic field sensor consisting of a closed circular loop (CCL) when placed in the electromagnetic field created by a small EMP simulator consisting of a transversal electromagnetic (TEM) cell. The results of measurements carried out to verify the field distribution within the TEM cell as well as the calculated sensor factor of the CCL sensor are presented. They show that the calculated sensor factor of the CCL sensor agrees reasonably well with practical measurements in an EMP field (to within 5%), making it suitable as a reference sensor to measure unknown pulsed electromagnetic fields or to compare with other sensors for which the sensor factor is unknown in the time domain. >

Higher‐order mode field distribution in asymmetric transverse electromagnetic cells

The higher‐order modes in asymmetric transverse electromagnetic (TEM) cells are considered; in particular, field expressions are developed and catalogued. These may be used to examine scattering and reflection from test objects and discontinuities. The results are applied to the termination reflection in a large tapered cell. Experiment and theory agree well and aid in understanding perturbations to the desired TEM mode field distribution.

Electromagnetic fields with arbitrary wave impedances generated inside a TEM cell

It is shown that arbitrary electromagnetic fields and wave impedances can be generated inside a transverse electromagnetic (TEM) cell for RF susceptibility testing. This is achieved by simply exciting one port and terminating the other port with appropriate loads. Thus, TEM cells are not limited to producing a planar field environment with the free-space impedance of 377 Omega , but can also be used to generate high-impedance or low-impedance fields for special testing needs. Experimental results for only three load impedances are described. In principle, many other loads including reactive terminations could be used to create a particularly desired field and wave impedance inside the TEM cell.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A broad-band resistively loaded V-antenna: experimental results

A broadband receiving antenna was realized using a resistively loaded thin-film V-monopole and a 500- Omega oscilloscope probe. The pulse-receiving performance of the antenna was evaluated in a time-domain antenna range, for various directions of the incident pulse. The antenna is capable of receiving a 520 ps electromagnetic pulse, incident within 45 degrees from the boresite direction, with good fidelity. The antenna was also tested in a transverse electromagnetic cell in the time and frequency domains. The frequency-domain receiving transfer function of the antenna was found to be within -57+or-3 dB in the 22 MHZ to 1.1 GHz frequency range.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Numerical solution of higher order mode cut-off frequencies in symmetric TEM cells using finite element method (EMI measurement applications)

The higher-order-mode cut-off frequencies in symmetric transverse electromagnetic cells are computed with the finite-element method. Transverse electric and transverse magnetic modes are considered. For symmetry, only one-quarter of the cross section is analyzed. Electric and magnetic walls are introduced in the cross section, which includes the effect of the gap between the center conductor and the sidewalls. This arrangement enhances the accuracy of the solution. The results obtained by this method are compared with those of other methods. Discrepancies observed in the results of other methods are explained. The present method is the more powerful one; with this method, mode identification can easily be made using an eigenvector solution.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Implementation of an automated system for measuring radiated emissions using a TEM cell

The transverse electromagnetic (TEM) cell is widely used to evaluate the electromagnetic characteristics of electrically small devices. The authors review the theoretical basis for a technique to quantify the radiated emissions from any such device in the cell. The technique is well suited to an automated test system provided that the mechanical motions required can be controlled by a computer. The difficulties associated with these mechanical motions are discussed and possible solutions are proposed. The measurement technique is also expanded to include multiple-frequency sources in addition to single-frequency sources.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A comparison between near-field shielding-effectiveness measurements based on coaxial dipoles and electrically small apertures

The near-field shielding effectiveness (SE) of a material can be measured by placing it between two closely spaced dipoles (electric or magnetic) and noting the resulting insertion loss. An alternative approach is to cover an electrically small aperture with the test material and measure the resulting loaded aperture polarizability (electric or magnetic), as is done in a dual TEM (transverse electromagnetic) cell. The author develops approximations leading to simple closed-form SE expressions by developing expressions that relate these two configurations. Small-aperture theory is discussed first, followed by a development of approximate dipole coupling expressions. The results are then compared with an experimental example.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A new sensor structure for the measurement of an electromagnetic pulse

Harry Diamond Laboratories (HDL), Woodbridge, VA, currently uses groundplane version electric- and magnetic-field sensors to measure the electric and magnetic fields produced during a simulated high-altitude electromagnetic pulse (HEMP). These sensors are also used for the measurement of such pulsed fields in the vicinity of metallic enclosures. A new sensor structure has been developed to measure the electric and magnetic fields accurately. The authors have recently fabricated and mounted a hollow metallic cylinder behind the sensor's 0.218-m (8 1/2 in) ground plane to house a fiber-optic transmitter, amplifiers, and other equipment necessary for data collection. The presence of this metallic cylinder increases (“enhances”) the amplitude of a signal when measured with this new sensor structure geometry (“sensor/cylinder”). The electric- and magnetic-field enhancements caused by the presence of this metallic cylinder were measured in both the frequency and time domains. The enhancement was also calculated numerically using a three-dimensional finite-difference code. To eliminate unwanted field coupling to the sensor's coaxial signal cable, the output of the sensor is transmitted through fiber-optic cables to remotely located recording equipment. In the frequency domain, the enhancements for both the electric and magnetic field were determined using a large transverse electromagnetic (TEM) cell. In the time domain, the enhancements were determined by comparing simulated HEMP fields measured with this sensor/cylinder to those measured with miniature electric- and magnetic-field sensors. The degree of enhancement of this sensor/cylinder will vary as a function of distance when the sensor/cylinder is placed next to a conducting object, such as a metallic shelter. These enhancements were also determined using miniature electric- and magnetic-field sensors.

A Boundary-Element Analysis of TEM Cells in Three Dimensions

Transverse electromagnetic (TEM) transmission-line cells are modeled and analyzed in three dimensions based on a mixed discretization using both constant and linear boundary elements. In particular, the distributed characteristic impedance along the line is evaluated for symmetrical square TEM cells of the type built at the National Bureau of Standards (NBS) in order to confirm the validity of the present numerical technique. Comparison with experimental results is shown. This paper shows that our numerical approach should contribute to TEM cell design.

Simple Approximate Expressions for Higher Order Mode Cutoff and Resonant Frequencies in TEM Cells

Simple approximate expressions for determining the cutoff frequencies of the first few higher order modes and the associated resonant frequencies in transverse electromagnetic (TEM) cells are presented. Both symmetric (seven TE and two TM modes) and asymmetric (three TE modes) cells are discussed.

Measurements of unintentional electromagnetic emissions

A summary of a new method for determining the radiation characteristics of leakage from electronic equipment or other unintentional radiators of interference is presented. The theoretical background and specific measurement procedures for the method using a transverse electromagnetic cell are outlined. The theory and measurements have been verified in referenced work by the results of a simulated theoretical example and an experiment using a spherical dipole radiator. Mathematical analysis of the uncertainties in the final, extracted results when the experimental data are degraded by the background noise and measurement imperfections is also available.

Comparison of Failure Mode Criteria in Electromagnetic Environments

Open field/ground plane test methods are generally used to evaluate the performance of equipment in electromagnetic environments. Laboratory enclosures with controlled environments in the form of transverse electromagnetic (TEM) cells, reverberation chambers, and anechoic chambers can overcome the disadvantages of remote outdoor test facilities.

Shielding-Effectiveness Measurements with a Dual TEM Cell

Small-aperture theory is used to investigate the dual transverse electromagnetic (TEM) cell. Analyzing coupling through an empty versus loaded aperture leads to a simple model of dual-TEM-cell material shielding-effectiveness (SE) measurements. Experimental data are compared to theory with good agreement in the case of an empty aperture. Some of the difficulties in analyzing a loaded aperture are discussed.