The use of single-port broad-band TEM cells for both near-field and far-field radiated emission testing is considered. The approach is to model the radiation from the device under test (DUT) as due to an equivalent set of multipoles. Assuming the DUT is electrically small only the initial multipole moments, the electric and magnetic dipole terms, need be retained. A sequence of TEM cell measurements is then used to determine the equivalent DUT dipole moments, The dipole model then allows one to simulate DUT emissions both in free space and over a ground screen. Thus, emission measurements over an open area test site (OATS) as called for by various standards may be simulated. Such measurement schemes have previously been successfully developed for standard two port TEM cells. However, certain broad-band TEM cells are single-port devices; thus, some modification of the previous approach is required. This paper reviews the basics of the multipole model as it relates to TEM cells, details various measurement schemes appropriate to single-port TEM cells, and presents examples of measured emission data, both near field and far field, in all cases considered, the correlation between emission data measured directly over a ground screen and simulated ground screen data based on TEM cell measurements is excellent. >

On correlating TEM cell and OATS emission measurements

A system for measuring the sensitivity of electric field sensors at UHF using a novel gigahertz transverse electromagnetic (GTEM) cell is described. The system was developed to enable calibration of UHF couplers that are used to detect insulation defects in gas insulated substations. The frequency response of a sensor is determined using fast Fourier transform (FFT) processing of its output voltage when it is subjected to a step electric field with a rise time of less than 300 ps. Provided the step response of the sensor is relatively short, the absorbing termination of the GTEM cell can be dispensed with, simplifying its construction. The need for impedance matching at the cell input can be eliminated by using a signal source that absorbs the reflected energy. In addition, the use of a wire septum offers the potential for improving field uniformity. A description of the measurement system is given, and some typical results are presented.

A pulsed GTEM system for UHF sensor calibration

The first eight to ten TEmnp resonances have been identified in one large (6.1 X 7.3 X 13.0 m) and one small (1.0 X 0.6 X 2.0 m) TEM cell. The resonant frequencies fit a new equivalent coaxial box model with the effective length of the box depending on the mode. Weakly propagating TEmn modes were also detected at frequencies above their respective first-resonance frequencies. A biological body or metal box at the center of the test zone interacts strongly with the TE10p and TE11p resonances, but not with the TE01p resonances. These interactions cause bandwidth limitations in the use of TEM cells for bioeffects dosimetry studies, EMC testing, and probe calibration work.

Bandwidth limitations of TEM cells due to resonances.

This paper presents a review summary of radiated emission and susceptibility measurement methodologies currently used for assessing the electromagnetic compatibility/interference (EMC/EMI) characteristics of electronic devices and systems. In particular, measurement methods using open sites, transverse electromagnetic (TEM) cells, reverberating chambers, and anechoic chambers are discussed, in light of their technical justifications and bases, their strengths and limitations, and interpretation of the measured results.

A review of electromagnetic compatibility/interference measurement methodologies

Civilian handheld global positioning system receivers were tested for their susceptibility against radiated electromagnetic disturbances of different characteristics and field levels. This susceptibility data were compared to the existing electromagnetic compatibility (EMC) immunity requirements for electronic devices. Some of the receivers were disrupted by continues waves at in- as well as out-of-band frequencies, at field levels far below the existing EMC requirements. The possible reason for this is discussed. In general, due to the ever-growing use of commercial-off-the-shelf equipment even for critical functions, certificates of conformity to current EMC requirements may be inadequate.

Susceptibility of Civilian GPS Receivers to Electromagnetic Radiation

This paper discusses the development of a modified (absorber-loaded) transverse electromagnetic TEM cell with expanded bandwidth for use in accurately characterizing electromagnetic interference (EMI) fields within a shielded environment. The cell is analyzed experimentally, both before and after the modification, to determine its radio-frequency (RF) characteristics, both as an RF transmission line and as an electromagnetic (EM) field simulator or detector. Comparative measurements are given to show the performance of the modified versus the unmodified cell in parameters such as voltage standing-wave ratio (VSWR), insertion loss, test-field uniformity, and reverse-coupling characteristics. The results of these measurements indicate an approximate two-fold increase in the upper useful frequency of the modified cell. An example of using the cell to evaluate the radiated emissions from a common electronic module (microprocessor timing circuit) is given. Finally, the technique of absorber loading is extended to larger cells, specifically a 3- × 3- × 6-m cell.

Expanding the Bandwidth of TEM Cells for EMC Measurements

This paper discusses the development of an electromagnetic simulator for accurate generation of broad-band suspectibility test fields within a shielded environment. The simulator consists of a large, 3 m X 3 m X 6 m, rectangular transverse electromagnetic (TEM) transmission cell that is loaded with RF absorber to suppress multimoding at frequencies above the cell's waveguide cutoff or resonant frequencies. The paper describes the measurement facility and technique, and the experimental verification of pertinent test paramenters such as system VSWR, insertion loss, and test field uniformity. The measurement system is anticipated to provide swept, automated susceptibility measurements of electronic equipment to CW, pulsed, and EMP fields within the frequency band, 10 kHz to 1 GHz.

John L. Workman

Papers

Expanding the Bandwidth of TEM Cells for EMC Measurements

Generation of EM Susceptibility Test Fields Using a Large Absorber-Loaded TEM Cell