On the Benefits of Using IEEE Std for Shielding Effectiveness Testing

Transcription

1 On the Benefits of Using IEEE Std for Shielding Effectiveness Testing Dale G. Svetanoff Lindgren RF Enclosures, Inc. 400 High Grove Blvd. Glendale Heights, IL USA Abstract - MIL-STD-285 and the origination of IEEE Std 299 go back several decades. More recent versions of Std 299 maintained technically sound test methods, but required that all frequency ranges be applied when testing enclosures. Some frequency ranges were not available for testing at all. This paper summarizes the benefits to shield owners and users that are incorporated in the current revision of Std 299. A new, menu-driven approach matches the test of sbieldiig effectiveness to the enclosure application. Tests are no longer limited to just high performance enclosures; testy subjects may be of any shielding material and construction, and they may be as small as 2 meters in any linear dimension. several different configurations. Further, enclosures which are as small as 2 m in any linear dimension may be tested, and it is planned that the next revision of Std 299 will provide test procedures for enclosures which are less than 2 m in linear dimension. This paper will detail the new menu-driven approach [l] which uses intended application of the subject enclosure as the driver to determine which thzquency ranges are to be used for testing. No longer must an enclosure be tested in all ranges unless the owner/user deems it necessary. The shield owner, in conjunction with the tester, may then select which h-equency (or frequencies) in the given range(s) should be used. The actual test methods are similar to those used historically. The widely used test standard of the RF shielding industry, MJL-STD-285, was released in the 1950 s, and remained unchanged until the DOD canceled official support for the document a few years ago. IEEE Std 299 originated in the late 1960 s, and served mainly as a detailed, thorough testing procedure for high performance shielding enclosures. Some significant changes were made in 1991, but it still remained oriented toward high performance enclosure testing. The current revision of IEEE Std 299, approved in late 1997 and published in April, 1998, makes several significant changes in the document which broaden the applicability beyond high performance enclosures, provide for testing of smaller enclosures, and move testing in accordance with this standard toward an economic par with that of MU,-STD-285. At the same time, thoroughness and accuracy of the testing were maintained, along with a greater emphasis on assuring test result repeatability. With the US DOD no longer supporting MIL-STD 285, it was apparent that an important fbnction for a revised IEEE Std 299 would be to encompass the base of shielded enclosures which have historically been tested using STD 285. With that end in mind, the current revision no longer limits test subjects to high performance enclosures. All references to specific performance requirements have been deleted, and enclosures may be constructed from any of a variety of materials and in Until the release of IEEE Std , the document had never provided test procedures in the range of 20 MHz to 300 MHz. This is the so-called resonant range in which most actual test subject enclosures are likely to have their first, or fundamental, resonant point, fr. A discussion will be included on the empirical work performed which permitted development of a test procedure for this range, which is important to shield users in the medical and communications industries. This additional range forced a re-structuring of the frequency bands as used in Std 299, but which is still applicable over the span of 50 Hz to 100 GHz. This paper will also discuss the change in equipment requirements, which are now based upon the expected dynamic range required to test the subject enclosure, not an arbitrary dynamic range based upon the highest performing shields available. Computations of shielding effectiveness values have been simplified, and supportive technical information in the annexes has been expanded. Specific procedures have been included to accommodate restrictions in testing access space and the presence of intervening materials between the test antennas and the shield. EVOLUTION OF THE CHANGES The Working Group, which produced IEEE Std , incorporated a number of revisions over earlier versions. However, IEEE 299 remained as a test document intended 1016

2 primarily for room-sized high performance electromagnetic shielding enclosures [2]. The abstract of also stated, The intent is... to provide a common reference for suppliers and users on the performance of shielding enclosures. In other words, the test standard was actually tezzing the users what level of performance they should expect to achieve for a given shielding technique or construction method [3]. The Working Croup that developed the current revision of 299 felt that a universal test specification should be just that; applicable to a wide range of electromagnetic shields, in size, materials, and construction methods. The Croup also felt that a test specification should provide definitive guidance on use and application of the test methods employed within it, but not attempt to define what the results of those tests should be. In June, 1994, then-secretary (US) of Defense William J. Perry issued a memorandum [4] which would cause a total revamping of the use (or non-use) of military specitlcations and standards. The author summarized the resultant actions of the Perry Memo [5], as mandating the following: A. The use of (product or system) performance and commercial specifications and standards, unless no practical alternative exists. B. In cases where no non-government standard exists or use of a non-government standard would not be cost effective, the use of military specifications and standards is authorized as a last resort (emphasis added). C. Outdated specifications and standards should not be used for new development and should be removed. D. Partnerships with industry associations should be formed to develop non-government standards for replacement of military standards where practicable. These mandates led to a situation in which IEEE Standard 299 would be poised to assume the replacement role for MIL- STD 285 as the universal or main electromagnetic shielding standard, both within the US Government and the shielding industry. Some of us who had been working within the shielding industry felt that there were some significant problems with IEEE Std , as it received very little active call within the commercial sector [6]. It was felt that 299 was too enclosure-specific, with too limited of an applicability range, and not very cost effective. These factors, and the rapidly approaching IEEE deadline for aflirmation, revision, or withdrawal of a standards document within a five-year time frame necessitated some decisions in 1996, both within the P299 Working Croup and the EMC-S SCOM. Those decisions included: make 299 applicable to a wide range of enclosures; improve its cost effectiveness for both shield owners (users) and testers; define applicable dimensions for enclosures which could be tested using the proscribed methodology. (Some Croup members represented organizations that wanted a standard applicable to electromagnetic enclosures that are less than room-sized.) The combination of all these parameters produced a major change in the direction of the P299 Working Croup and in the document itself. In the early stages of this revised effort, two major guidelines were set that paved the way for a significant rewrite of the standard: a) The current revision would be made applicable to mry electromagnetic shielding enclosure having a smallest linear dimension equal to or greater than 2 meters; b) Both cost-effectiveness and applicability might be better served if the test frequency ranges and methods were determined not by the enclosure construction, but by the intended enc17os~re application. THE MT3Nu SYSTEM The selection of 2.0 m as the smallest linear dimension was based upon being able to fit a typical bicone style antenna inside of an enclosure (with adequate clearance between the antenna elements and the conductive enclosure walls) to perform planewave testing down in the range of 30 to 50 MHZ. [7] (It is recognized that most enclosure tests in that frequency range do not result in sufficient distance between the source antenna and the enclosure to achieve classical planewave geometry of the radiated field.) Rigorous testing of physically small enclosures (those having linear dimensions less than 2.0 m) would involve evaluating a number of test methodologies to determine which are best suited to providing repeatable results under varying conditions of enclosure size and material. Those evaluations would have to wait for a later revision. The structure of 299 testing methods would now be determined by a menu system, explained in Annex D of the standard, which mandates preparation of a test plan by the shield owner or owner s representative (which could be the testing party) and which shall determine the pass/fail requirements of the enclosure. If the size of the enclosure being proposed for testing to this standard meets the > 2.0 m criteria, then the responsibility for determining the application of the standard s test methods also falls to the shield owner or his representatives [S]. Annex D, Clause D. 1 gives some guidance in determining how to make cost-effective testing decisions relative to the proposed enclosure and its intended applications. Preparation of this informative annex considered that electromagnetic shielding applies to a significantly larger enclosure market than the relatively high performance military one alone. Land mobile services, paging providers, cell phone and PCS systems, and medical MRI systems are all candidates for application of this standard. In many of these commercial applications, testing over a wide range of frequencies, well beyond the 1017

3 operational range of interest for the enclosure, is neither desired (for economic reasons) nor required. Thus, a copper shielded room intended for medical MEU applications need only be tested within the range of interest for the scanning machine. Operators of cell phone and paging systems usually specify testing at or near 1 GHz, and have little interest in low frequency H-field performance. Of course, a welded high performance enclosure may still be tested over a range as wide as 50 Hz to 100 GHz, as was possible using previous versions of this standard. Clause 5.2 provides tables with the standard and extended range of measurement frequencies. It is clearly stated that the actuae test frequencies shall be according to the approved test plan (approved between the shield owner, tester, and shielding provider/vendor). IEEE Std modified the test frequency ranges over the previous versions. They now are: the Low range (nominally 9 khz to 20 MHz); the Resonant range (nominally 20 MHz to 300 MFbz); and the High range (nominally 300 MHZ to 18 GHz). The Low range is not changed from previous versions, the Resonant range is new, and the High range encompasses the old Midrange and High range combined. machines may image near frequencies of 21,42, and 63 MHz, depending upon their magnetic field strength. Most of those machines require placement within a shielded enclosure offering significant attenuation to outside signals at and near the imaging frequency. Enclosures for MRI systems have been tested either at the imaging frequencies, or at the standard MRI default test frequency of 100 MHz, since the early 1980 s. All of these applications fall within the avoided ranges of prior 299 versions. Shielded enclosure users with a need to verity shielding performance have had little choice but to use a MlL- STD 285 type of test, of which none existed at those frequencies in MU,-STD 285. (The planewave test in MU,- STD 285 was given as being at 400 mc (MHz)). Some MRI manufacturers specify an enclosure test at 10 MHz; the high impedance E-field test of MU.,-STD 285 has been the usual means of doing such a test, although 10 MHZ was not a listed frequency in 285. It was decided that the Working Group would conduct actual testing within the resonant range area. The natural resonant frequency of an enclosure is given by the simplified equation: Each of the three main ranges is broken down into sub-groups (1) of frequencies. Although the standard suggests that one where frequency f?om each of the selected sub-groups should be used, the test plan takes precedence. Hence, it may be a is the longest dimension of the enclosure in meters acceptable to test at but one frequency, period. That is a b is the next longest dimension of the enclosure in common commercial practice for many medical and special use meters, and therefore enclosures. a2b RIERES~NANTRANGE IEEE Std 299 has never included a high impedance E-Field test procedure. MIL-STD-285 did, and it was for use at frequencies of mc (MHz), and 18.0 mc (MHz) [9]. The P299 Working Group felt that lack of a test procedure in the range of 20 MHz to 300 MHz left many potential applications for the standard unsupported. Previous versions of 299 stated that... in all cases the test frequency shall be at least triple the lowest natural resonant frequency of the enclosure [lo]. A nomograph and equation (both based upon enclosure interior dimensions) were provided to assist in avoiding tests within that range of any given room. In the United States, there are land mobile radio and some paging services operating in the low VHF band of 30 to 49 MHz and in the high VHF band of 150 to 174 MHz. Aeronautical voice and navigation systems operate in the 108 to 136 MHz range, and some military tactical radio systems operate between 225 and 300 MHz. Manufacturers and service providers of equipment for those applications need electromagnetically shielded enclosures in which to perform equipment alignment and repair operations. Medical MRI A detailed- discussion of cavity resonance considerations is given in Annex A.3.1 of Given the smallest possible dimensions (a 2.0 m cube) of an enclosure meeting the applicability of this standard, such an enclosure will have a fundamental resonant frequency of 106 MHZ. Larger enclosures will have lower resonant frequencies. Tests were conducted in an enclosure having a calculated natural resonance of 47.5 MHZ. [ 1 l] The subject enclosure was modified to have a definite RF leak in the door area. The transmitting source was located outside, and a receiving system set up inside [ 121. The receiving system included a bicone antenna, spectrum analyzer, and plotting system. The object of the testing was to determine what variable factors existed in a room with its cavity being excited in the region from near@ to approximately 2fi. For some tests, the equipment was kept in a stationary location and the antenna moved about the room. In others, both antenna and equipment were fixed in position, but the connecting cable from antenna to spectrum analyzer was moved, changed in length, or fitted with varying numbers of large clip-on ferrite beads over the outer jacket of the cable. 1018

4 Besides moving the antenna within the enclosure, it was observed that moving a given length cable, changing a cable length (such as from 3 m to 6 m), and varying the quantity and location of ferrite beads all had effects on the received signal amplitude. [13] Such effects, of course, alter the calculated value of shielding effectiveness ( SE ) for the test enclosure. Due to these direct and adverse effects, the following requirements were incorporated into the document text: a) The 5mdamental resonant.fiequency of the enclosure shall be calculated using (1) or the supplied nomograph, and be noted on the test data sheet. b) While the choice of test frequency or frequencies shall be determined by the approved test plan, all reasonable attempts should be made to avoid testing at or nearf,. c) The length and type of cable used between the receive antenna and measuring instrument shall be noted in the measurement results. d) The cable between the receive antenna and measuring instrument shall employ either continuous loaded ferrite jacketing or ferrite beads located at the ends and midpoint of the cable. e) Inclusion of a statement in the test data: Electromagnetic SE measurements made at a single frequency in this range may not be representative of measurements made at other frequencies within the range. There may be significant variations due to resonance or other reflective condition effects. [ 141 Annex A.3.3 of IEEE Std suggests the use of either frequency sweeps or discrete close-m frequency stepping from just below to just above a frequency of interest in this range if adverse SE results are found or a resonant effect is suspected. Such effects are considered significant (for this standard) if variations of apparent SE are greater than 6 db over the test frequency span. Thus, although there are significant problems associated with meaningful testing in the Resonant range, a viable test method is offered in conjunction with precautionary measures that are not mentioned in ML-STD 285. informative Annex E; in general, areas of low performance can be corrected while the shield test is in progress. Since prior versions of 299 were intended for high performance shielding enclosures, there was legitimate concern over the issue of adequate measurement dynamic range. IEEE Std addressed the issue by reqzdring the use of high power amplifiers (200 W) for the High range. Beyond the rather obvious fact that use of such power at microwave frequencies may produce field intensities which exceed certain regulatory agency safety limits for human exposure, there is the cost of such amplifiers, both in purchase price and in shipping (due to size and weight factors) from one job to another. Technology has improved the performance of spectrum analyzers and given us very low noise pre-amplifiers which can be placed ahead of the analyzers (or similar receivers) to yield a dynamic range increase. The other significant factor in considering system dynamic range is the expected performance of the shield itself Again the older versions of 299 were concerned mainly with high performance shields, those having from a minimum of about 100 db SE to those welded ones with an SE of 130 db, or more. The 1997 version considers alz commonly available electromagnetic shieldmg materials and construction methods, including screening, hardware cloth, metal foils, and shielding fabrics. Some of these materials may have an intrinsic, possibly frequency-dependent SE of only 30 to 40 db; if that is sufficient for the intended application, then the revised standard provides for testing it. For TEEE Std , it was decided that the only requirement for system dynamic would be that it exceed the SE to be measured by at least 6 db. Thus, once the tester knows what the desired or expected SE performance will be, per the approved test plan it is then necessary to supply and use only that test equipment which is required to achieve a measurement capability of SE value + 6 db. For those doing welded steel enclosures, the 200 W amplifier may still be needed. However, a single shield copper or cell type galvanized steel enclosure that will be a repair station for PCS or GPS applications can most likely be successfully tested with 5 watts, or less. IMPROVING COST EFFECTIVENESS Dynamic range has long been a problem of testers doing H- While the Working Group felt that the menu system of testing field measurements at lower frequencies, usually at or below 10 was in and of itself a signi&nt cost reduction over the kf-fi. Since the procedures of&is St&ad we usable down to previous versions of IEEE Std 299, which required uzz 50 Hz, consideration was given to practical means for frequency ranges to be run on a test enclosure, there were improving perfo~~ce at this end of spectrum. Both M& other ways in which the document could be improved to STD 285 and IEEE Std mandated use of a single reduce test time and equipment requirements. turn loop of@5 AWG wire, 0.3 m in diameter. Previous versions mandated that areas of poor shielding The 299 Working Group examined loop performance and performance be determined prior to the start of actual test data made two changes: a) There never was a clear mention in 299 collection. In effect, this meant an enclosure was tested twice. as to whether the loop antenna should be shielded or The pre-test scan is now optional, and is described in unshielded. Empirical tests run by the P299 Working Group 1019

5 indicated that there can be discrepancies of as much as 6 to 7 better understanding of the test methods and quicker db with unshielded loops because there is some response to the completion of test data runs in the field. E-field component when taking the reference measurement. [ 151 Use of electrostatically shielded loops for both It had been the experiences of many members of the P299 transmitting and receiving is mandated in b) The Working Group that meeting the separation distance single turn loop restriction is gone; multiple turn loops are requirements between the (usually) source antenna and the permitted, 0.3 m in diameter. The existing procedure for exterior shield surface was very ditllcult in most test verifying that the shield material is not being driven into circumstances. In general, if there is intervening material magnetic saturation (non-linearity) was kept and suggested for between the source antenna (e.g.: drywall board, studding, use. [16] etc.) and the shield surface, the material, and any RF losses through it, become part of the net SE reading. For Low range tests, the standard requires an attempt to determine the location of panel or weld seams by the use of construction drawings if the shield has been covered by aesthetic treatments prior to SE testing. The improved dynamic range from using multiple turn loops may reduce the need for amplifiers, thus lowering testing costs. It was recognized that there may not be shielded multi-turn loop antennas on the commercial market. Self-construction of these, as well as other antenna types used in the standard, is possible. Where necessary, critical dimensions or otherwise important parameters are given. Other antenna requirements were changed in the 1997 document. In the Resonant range, biconical antennas are specified for 20 MHz through 100 MHz. Half-wave dipoles are specified for 100 MHz through 300 MHz. In the High range, any linear type of antenna may be used as a source (transmit) antenna in the 300 MHz through 1000 MEiz range, but a % h dipole shall be used for receiving. Above 1 GHz, horn antennas are specified. Either standard gain horns or broadband double-ridged horns may be used as a source radiator, but standard gain horns only are specified for receiving. This is to reduce the variations in SE calculations that can occur between taking the reference measurement outside of the shield, where antenna gain factors apply, and the SE measurement inside of the reflective shield, where the antenna factors are reduced. Calculation of SE values was simplified by deleting the averaging of measurements, as had been required in earlier versions of IEEE Std-299. For both the reference measurement and the shielding measurement, the peak, or maximum, observed signal amplitude is recorded and used for obtaining the SE value. Actual SE value calculations may be based upon the use of linear values and then converting to logarithmic form, or by direct use of logarithmic values (dbbased), such as those generally obtained direct from the measuring instrument. AU existing annexes in IEEE Std were revised and expanded. Jn particular, discussions of cavity and slot radiator resonances in Annex A were enlarged. There is also supportive information regarding compensation for diierent antenna factor effects when inside of a reflective enclosure, and the inclusion of existing architectural treatments as part of the shield when they are in the direct test measurement area is discussed. Annex B includes a defh&ive description of dynamic range as applied in the standard. It is hoped that the expanded definitions and support information will result in In the Resonant and High ranges, separation distance for the reference measurements and SE measurements are specified as being a distance of 2 m, minimum, unless physical spacing limitations for either the reference level or SE readings preclude maintaining that spacing. In that event, maximum available separation shall be used, but shall not be less than 1 meter, and that separation noted on the test report and data sheets. [17] Notice that only a minimum separation distance is specified. The tester may use a greater separation distance if test site conditions and available equipment dynamic range permit it. Maximum spacing between test locations, for both horizontal and vertical directions on the shield, is specified for large enclosures. Empirical testing with the loop antennas revealed that there was some deficiency in the Low range test method for enclosure doors. In particular, results in door comers were not consistent when testing a door with an introduced discontinuity in a comer. It was therefore decided to add specific corner test locations for doors, and to check comers by placing the loop antennas in both horizontal and vertical planes. [ 181 In order to minimize the effects of reflective nulls that can occur within an enclosure, the test procedures for both the Resonant and Higb ranges call for the receive antenna to be swept in position throughout the shield interior; the receive antenna shall also be swept through various polarization angles, in an attempt to obtain the largest received signal response. This is to be done for each location of the transmit antenna. In keeping with the menu approach to testing in this standard, the shield owner can select even the form of the test results. The minimum requirement is a status letter, to be prepared by the testing agency and submitted to the shield owner. The status letter basically reports on the eequencies at which measurements were made and the SE value calculation. A detailed guide is offered for those instances in which the shield owner desires a fully detailed test report. A listing of 1020

6 minimum required information, such as 111 details on instrumentation used and locations of all test points, helps to assure consistency in the reports. CONCLUSION - GETTING THE MOST OUT OF members to the standard were instrumental in completing this work. RIBZXENCES The revised standard is capable of providing fast, cost-effective tests in those instances where less than full eequency range tests are required. The methods for full, high performance testing remain intact. The shield owner should work closely with the shield supplier/vendor to make certain that the electromagnetic shielded enclosure being proposed is capable of producing the required results. Following that, the shield owner (or user, acting as the owner s representative) should consult with the test agency and the standard. The tiequency range(s) to be used will determine which tests are actually required. The shield owner will need to specify how many frequencies in which of the three available ranges are to be used in the testing. The shield owner must also provide the pass/fail parameters for the shield system. The test plan is the key to effective use of this standard. Once the test frequency range or ranges isfare selected, the equipment can be selected and testing begun when the shield is completed. While a prehminary check for RF leaks is advisable, it is not required and leaks, if found, may be corrected while testing is in progress. Low range testing, when required, specifies measurements around all doors, at accessible penetrations, and at as many of the section or panel seams as are accessible. Test locations for the Resonant and High ranges are to be in accordance with the approved test plan. If certain portions of a subject enclosure can not be accessed for testing, they can be exempted by the test plan. The test procedures for both the Resonant and High ranges make provision for source antenna-to-shield surface distance down to 1 meter. It is the intent of the standard that if a minimum 1 m separation can not be maintained over some portion of the immediate exterior of the shield, then that portion is, in essence, not accessible for testing in those ranges. IEEE Std continues to provide a foundation for electromagnetic shielding effectiveness testing that is based upon decades of application and experience. The advent of technology-driven equipment improvements and a proliferation of shielding applications, well beyond the original high performance ones, were considered in development of a useful, effective standard. ACXNO~LEDGMENT The author wishes to thank Elite Electronic Engineering Co., and Lindgren RF Enclosures, Inc., for use of test facilities and equipment. Contributions of all P299 Working Group 111 As presented in IEEE Std , Annex D, Institute of Electrical and Electronic Engineers, Inc., 345 E. 47th St., New York, NY USA, PI IEEE Std , Abstract. r31 ibid., Table C3. c41 Perry, William J., Secretary of Defense, Specifications and Standards - A New Way of Doing Business, NARTE News, Vol. 12, No. 3, pp , July- September 1994, text derived from memorandum of 29 June, 1994.!A Svetanoff, Dale G., New Procedures for RF Shielded Enclosure Testing, IEEE Std 299 vs. M5STD 285: WI 171 PI PI WI Cl 11 PI P31 The Cost of Improvement, EMI/EMC Metrology Challenges for Industry, MST, Boulder, CO, January, ibid. Croisant, Wm. J., Jr., Private communication, 13 June, IEEE Std , Annex D, Clause D.2. MIL-STD-285, Figure 3, US Dept. of Defense, 25 June, 1956 IEEE Std , Clause Enclosure size was 7.52 m L by 3.48 m H by 2.92 m W. The normal test configuration for all enclosure measurements in IEEE Std is with the source outside of the enclosure and the receiving antenna and associated equipment inside of the enclosure. Tests were done using the swept-frequency method. In one case, doubling the cable length between the receive antenna and the spectrum analyzer caused an amplitude change of nearly 20 db in portions of the swept range, some of which were increases. Cl41 IEEE Std , Clause WI Testing was done using A vs. B comparison tests on a welded enclosure. Paired loops, electrostatically shielded, and non-shielded, were tested at 14 khz and at 150 khz. WI IEEE Std , Annex C, Clause C IEEE Std , Clause 5.7.4, WI ibid., Figure 2, Standard loop positions for low-frequency tests 1021