Complications in correlatability between test techniques due to directional emission patterns

The main purpose of Fully Anechoic Chamber (FAC) as an alternative facility for EMC compliance testing is to accurately measure the radiated emission generated from equipment-under-test (EUT). The Open Area Test Site (OATS) is the standard test facility for radiated emission measurement, thus the FAC data have to be correlated to the OATS results. In this paper, it is statistically verified the newly proposed correlation algorithm between FAC and test facilities with ground plane such as OATS, semi-anechoic chamber (SAC) in order to use a FAC as an alternative facility for EMC compliance test. The algorithm is based on modeling of EUT as an equivalent set of multipoles. A sequence of E field measurements at FAC is then used to determine the equivalent dipole moments and relative phases for EUT. The dipole model then allows one to simulate the radiated emission from EUT over a ground screen. Thus, emission measurements over an OATS as called for by various standards may be simulated. This paper reviews the basics of the multipole model as it relates to FAC, the E field measurement scheme at FAC, and presents the results of EMI measurements at both FAC and SAC. The radiated emission test using the proposed algorithm requires fifteen E field values measured at nine positions around an arbitrary EUT located within FAC. Radiation from the reference radiator has been tested in order to statistically verify the proposed correlation algorithm between a FAC and the test facilities with ground plane.

Abstract. Due to the increasing importance of EMC problems through the last years there is a great interest in measurement devices such as GTEM-cells (Giga(Hz)- TEM cells). They promise to allow compact and low-cost emission- as well as susceptibility tests up to very high frequencies. Expensive measurement procedures in open-area test sites or within semi-anechoic chambers would become obsolete in many cases. To estimate the quality and reliability of GTEM-cell measurements it is necessary to have detailed knowledge about the processes within the cell and, in particular, about the interactions between the cell and the DUT (device under test). Due to the high frequency and the cell’s dimensions a purely numerical simulation while using standard techniques such as Finite Element Method, Method of Moments (MoM) or the Finite-Differ-Time-Domain (FDTD) method is inefficient and unnecessary since the GTEM-cell is a mostly empty homogeneous TEM-waveguide. Analytical models allow only the investigation of empty cells. As will be outlined in the following, a suitable way to reduce the numerical complexity of the general problem is the use of a hybrid method, such as the combination of a modal analysis with the MoM.

Results of many measurements, which show that antenna calibrations in a SAC are feasible, are shown. Measurements include reflections from the ground plane at an open area test site (OATS) comparing them as the site insertion losses (SIL) at antenna height changes without and with electromagnetic absorbers on the ground plane of the OATS at frequencies from 1 to 18 GHz. Similar measurements are carried out in semi-anechoic chambers (SAC) and the results are compared with those of OATS. A substitution method of antenna calibrations at SAC with absorbers on the ground plane is experimented, compared with those of SSM and discussed.

The author reports on the results of a correlation exercise comparing radiated emissions measurements obtained at three different kinds of facilities: a GHz transverse electromagnetic (G-TEM) cell, a semianechoic chamber (SAC), and an open area test site (OATS). A source with a data cable which must exit the cell is used. The test objects were digital phones. These telephones receive their power and communicate over an unshielded twisted pair cable. The cable must pass out of the cell and to a switch simulator, which maintains the telephone in its operation state. Several steps taken to improve the correlation between these various radiated emissions test facilities are reviewed. The correlation between the G-TEM and OATS is found to be generally better than 4.5 dB. For most frequencies the correlation is better than 2.5 dB. >

The open area test site (OATS) has been in common use for over 20 years since the introduction of electromagnetic interference (EMI) regulations around the world The OATS must have very low RF ambients however the introduction of digital TV broadcasting and the proliferation of mobile telephone transmitters are threatening the validity of the OATS. This paper discusses the increasing difficulties caused by the masking of EMI emissions by new ambient threats at the OATS. The characteristics of ambients are analyzed and recommendations given for EMI measurements in the presence of the more common OATS ambients. Ambient profiles of old and new OATS are compared and the changes discussed. It is shown that validity of OATS in high ambient urban areas is questionable.

The paper presents the main factors which contribute to uncertainties in measurements in screened rooms and in open-area test sites (OATS). EMC standards require test measurements to be made on equipment under specified conditions in order to demonstrate compliance with various national and international regulations. In order for these tests to be meaningful it is necessary that the EMC behaviour of equipment under test (EUT) in different environments should be predictable, consistent and repeatable. In particular, it is desirable that measurements done on the same EUT in different vendor's premises are broadly similar and that measurements done in screened rooms and in OATS can be correlated. Unfortunately, these aims cannot be met at present and hence a deeper understanding and control of the various test environments is necessary to improve the accuracy and consistency of measurements. In order to help with this process, the main issues involved are addressed. Conditions encountered in screened rooms, OATS and other test environments are examined.

At present there are several methods to do EMI measurements in GTEM cell, such as EDM, MDM, Linear, Wilsonpsilas method and so on. But the difference between the calculated results of these methods and the EMI results measured in OATS (open area test site) is very large in some situation. This article provides a new method to calculate electric field intensity in GTEM cell using the equivalent dipole of the radiator. And series of experiments were performed to prove the validity and advantage of this method. The experiment results show that the calculated results of new method is more agree with the measured results in OATS, which means that the new method can be considered as a more approached way to calculate the electric field intensity in GTEM cell for EMI measurements.

To provide a uniform RF field for immunity testing, a basic understanding of the antenna beamwidth radiation pattern is required. Ground floor and wall reflections and resonance contribute to the field nonuniformity. During the process of developing an automotive RF immunity test requirement, obstacles to meeting this requirement were encountered due to the absence of an absorber lined ground plane and the frequency limitations of the ferrite tiles placed on the walls and ceiling of the test chamber. The frequency range of this test is 200 MHz to 4 GHz. To visualize the actual effects of the floor and wall on beamwidth patterns, a series of measurements were taken in the semi-anechoic room. Mapping of the antenna beamwidth in a semi-anechoic chamber provides a method to determine the optimum distance to antenna and antenna/target orientation. A secondary benefit of beamwidth mapping is that the actual field strength can be measured and compared to the input power. This will help in determining if the intended amplifier is large enough to produce the required field strength. Additional testing using an open area test site (OATS) sheds light on the contributions of floor and absorber wall reflections. Through an experimental study, optimum antenna orientation is determined to obtain the targeted field uniformity.

Discusses the measurement of electric field strength of radiated emissions in screened rooms in the frequency range 30 MHz to 1 GHz. The use of screened rooms rather than open area test sites (OATS) is specifically called for in the American standard MIL-STD461 and the UK standard Def.-Stan. 5941. The main differences between the two types of facility are: a) the wall reflections in undamped screened rooms enormously increase the measurement uncertainty, b) the military specification limits are lower than the civil ones which leads to c) the requirement for testing at a distance of 1 m from the equipment under test (EUT) in contrast to 10 m on OATS. Besides the uncertainty caused by wall reflections, there are antenna related uncertainties. The uncertainty in the measurement of field strength is not simply the uncertainty of the antenna factor given in the certificate of calibration for the antenna. There are four additional sources of uncertainty, namely mutual coupling, phase centre, directivity and size. The case is presented for increasing the distance used in military testing from 1 m to 2 m or 3 m. As a consequence of this: a) measurement uncertainties are reduced and b) the measured field is more absolute in the sense that it is less influenced by near-field effects and there is a convergence with methods and results pertaining to civil testing.

A GTD model is proposed to discuss the normalized site attenuation (NSA) associated with an open area test site (OATS) with finite metallic ground plane. In this study, the NSA values for three OATS sites, based on GTD model and measurement, are carefully examined and agreement between calculated and measured results supports the usefulness of the proposed GTD model. The GTD model is then adopted to analyze the deviation in NSA values for several OATS structures. The GTD simulation has revealed that the key factor affecting NSA is the width of rectangle metallic plane which is essential in reducing the NSA deviation under the finite test site condition.

Open area test site (OATS) is the standard test facility on radiated emission testing. The necessity of alternative methods to OATS that meet the requirements of CISPR or EN standards and the time, cost and space limitations of traditional alternative facilities (as semianechoic chambers) are impelling new methods; the problem using those new facilities is the acceptance of the measurements. Currently, gigahertz transverse electromagnetic field cell (GTEM) is gaining acceptance in this area of EMC, and an FCC Public Notice, dated December 1993, has established a limited equivalence to open area testing. This paper presents an OATS/GTEM correlation study using three different devices included in two European standards; this correlation is supported by an original statistical analysis made for every frequency and device. The results are discussed.

Radiated emission test in a FAR (Fully Anechoic Room) site has been adopted as the latest version of CISPR32, IEC61000-6-3 and IEC61000-6-4 standards. Test in a FAR site without metal ground plane is expected to the measurement time reduction, since the height scan is not required to find the maximum emission because there is no reflective wave from the floor. Consequently, the severe limit of 5 dB than a test in OATS (Open Area Test Site) / SAC (Semi Anechoic Chamber) has been introduced to a test below 1 GHz in FAR. However, based on the comparative simulation results, the radiated emission limit in FAR site has been relaxed to plus 7 dB at 30 MHz from the 5 dB severe value at 230 MHz linearly with respect to the logarithm of the frequency, since the disturbance source in the equipment is affected by a metal ground plane of the test in OATS/SAC. The rationale of FAR test limit was indicated as a calculation result, but actual measurement data are not enough. Under this situation, we have performed comparison experiment measurement between SAC site and FAR site by using a same equipment to investigate the validity of the relaxed limit based on the calculation. From the experiment measurement result, the limit according to the polarization plane should be specified because it has a high possibility to mislead the product conformity assessment in a polarization of vertical to the ground plane.

The addition of an OATS (open area test site) radiated emissions correlation algorithm to a GTEM (GHz transverse electromagnetic) cell allows performance of radiated emissions testing, equivalent to that conducted at an OATS in the GTEM. The original correlation model and the computational algorithm based on this model accurately addressed the measurement of radiated emissions to commercial standards, over the frequency range of 30 MHz to 1 GHz. In the present work, the authors examine new additions and expansions to the original model and algorithm. New additions include measurements to telecommunications specifications, measurements from 9 kHz to 30 MHz for electric and magnetic field measurements, and measurements to MIL-STD requirements and measurements above 1 GHz. >

Measuring electromagnetic disturbance field strength often refers to open area test site measurements. Due to the largeness of open area test sites and their requirements to the environment they can rarely be found in middle-Europe. Additionally the need of the heightscan of the receiving antenna enlarges the necessary time of a measurement, so that alternative test sites were developed to avoid these disadvantages. Using test sites for emission testing that are in use for immunity tests seems to be a way to avoid the disadvantages of the open area test sites. In this context the correlation between measurements on different test sites becomes important. In this paper, a statistical approach including a generally valid model for all kinds of equipment under test is described. It is applied to the conversion of perfect fully anechoic chamber to perfect open area test site measurements.

Radiated emission measurements are normally performed in laboratories at Open Area Test Sites (OATS) or in anechoic chambers at 10m or 3m in accordance with the standard CISPR22 or other similar standards. However, it is not always possible to use laboratories because of some limitations. If the Equipment Under Test (EUT) has large dimensions or heavy, it is not possible to use laboratory facilities for tests. As a consequence, on-site radiated emission tests are required in industry. On the other hand, radiated emission tests have many challenges in industry such as background noise, uncontrolled reflections and narrow testing environment, for that reason use of some alternative methods are inevitable in industrial environments. In this research, we have experimentally investigated the surface wire method and its links to the standard laboratory methods in an attempt to bring the state of art researches one more step further.