Basic Parameters of the Normalized Site Attenuation (NSA) Method

Basic Parameters of the Normalized Site Attenuation (NSA) Method for Open Area Test Sites (OATS) and Semi Anechoic Chambers (SAC) In CISPR 16-1-4 for EMC Testing for IEEE EMC Chapter Michigan Section by Louis A. Feudi Raymond EMC 1

Scope CISPR 16-1-4 Qualification of OATS Qualification of SAC or FAR 30 MHz-1GHZ Normalized Site Attenuation SVSWR will not be covered 2

CISPR and IEC Comité International Spécial des Perturbations Radioélectriques (English translation: International Special Committee on Radio Interference) Part of the International Electrotechnical Commission (IEC) CISPR drafts standards for controlling electromagnetic interference For Electrical and electronic devices 3

CISPR and IEC Two types of standards Guideline standards – provided for use by a product standard committee as a “basic framework” Product Specific Standards – Tailored to evaluate and verify that a specific product or product category is compliant with a specific set of requirements that can be impacted by environment, user interface, and safety requirements where applicable. 4

CISPR 16 Standard for “Specification for Radio Disturbance and Immunity measuring apparatus and Methods” Multiple Parts Part 2-3 describes methods of measurement of electromagnetic disturbances Part 3 provides radio disturbance information Part 4 covers uncertainties, statistics and limit modeling Part 1-4 titled “Radio disturbance and immunity measuring apparatus – Antennas and test sites for radiated disturbance measurements” 5

CISPR 16-1-4:2019 Most recent publication Guideline standard “for use by product committees of the IEC” “specifies the characteristics and performance of equipment for the measurement of radiated disturbances in the frequency range 9 kHz to 18 GHz.” Antennas and Test Sites 6

Compliance Test Site Environment that assures valid, reproduceable measurement results of the rf disturbance field strengths generated by UUT Open Area Test Site (OATS) Semi Anechoic Chamber (SAC) Fully Anechoic Room (FAR) Defined in CISPR 16-1-4 for both mechanical description and verification process to ensure that sites will provide reproduceable, valid results from RF Disturbance Measurements 7

Basic Requirements “Quiet Zone” or “EUT Volume” Contains EUT Produces results that will closely align with a theoretical “ideal open area test site” perfectly flat, perfectly conducting ground plane of infinite area, and with no reflecting objects except the ground plane method to evaluate OATS, SAC and FAR sites to as closely approximate one in an area that encompasses the EUT 8

EUT Volume Commonly referred to as Quiet Zone cylinder defined by the EUT boundary diameter and height, encompassing the EUT, cable racks, and 1.6m of cable length For measurement from 30 MHz-1GHz 9

OATS “an area characterized by cleared level terrain and with the presence of a conducting” (metallic recommended) “ground plane” Free of obstructions (trees, buildings, electric lines, fences, no underground pipes or cables (except as needed to power site) Optional weather protection to protect EUT and measurement antenna, rf transparent Encompasses EUT and measurement antenna Antenna mast , rf transparent, up to 4m Minimum dimensions for site dependent on measurement distance: d Presence of a turntable 10

Minimum Dimensions Directional emissions require measurement all around the unit Reflected off ground plane Up to a height of 4m To ensure maximization of emissions measured In the absence of a turntable, round OATS requiring antenna to be revolved around the EUT 11

Stationary EUT No Turntable Note that the minimum distance of the area boundary is 1.5 times the measurement distance d (often 3, 5, or 10m). 12

OATS sans Turntable 13

Oats with Turntable Allows rotation of the EUT Shaped as an Ellipse Length 2x distance d Distance d/2 between test sample antenna and boundary behind 14

Ellipse OATS dimensions 15

Ambient rf levels classification Class a:The ambient emissions are 6 dB or more below the measurement levels. Class b:Some ambient emissions are within 6 dB of the measurement levels. Class c: Some ambient emissions are above the measurement levels, but they are either widely spaced in time between transmissions to allow for a measurement to be made (aperiodic) or are continuous only on limited identifiable frequencies (i.e. FM bands). Class d: The ambient levels are above the measurement levels over a large portion of the measurement frequency range, continually transmitting. Quality classification d is unacceptable, yet with the increasing use of cellular towers, Wi-Fi hotspots, and other transmission sources, most OATS will need to be located in remote areas. 16

Alternatives to OATS SAC FAR Both are shielded enclosures constructed of steel, with a door, honeycombed vents Electrical power filters Pipe penetrations Bulkhead panels Shielding Effectiveness normally 90-110 dB from 15 kHz-40 GHz Eliminates or drastically reduces ambient emissions 17

SAC Shielded Enclosure Ferrite lined on walls and Ceiling Absorber partial lined on Walls and Ceiling Reflective Ground plane for 30 MHz-1GHz measurements Ferrite tile and absorber patch on floor for RF Measurements from 1-18 GHz RF Immunity generated uniform field 3, 5, and 10m measurement distances 18

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FAR Shielded Enclosure Absorber lined on walls, ceiling, and floor Simulates free space environment Only direct ray transmitted, intentionally or not, to the receive antenna Absorber reduces indirect and reflected ray waves on walls, floor and ceiling 23

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Normalized Site Attenuation “minimum site insertion loss between two polarization-matched antennas located at a test site when one antenna is moved vertically over a specified height range and the other is set at a fixed height”. Fixed height antenna located at center of OATS Variable height antenna at specified measurement distance from transmit antenna Broadband antenna method most commonly used for site validation Tuned Dipoles not described in 16-1-4, section 6.4 – Test Site Validations, but refers to other documents 25

Bi-Log antennas NOT preferred For lower measurement uncertainty, the value of E-field measured by a typical biconical antenna or LPDA antenna is preferred, in particular over hybrid antennas. Typical biconical and LPDA antennas are defined in Annex A and only calibrated antennas shall be used. NOTE 1 Improved uncertainties are achieved by using the biconical antenna over the frequency range 30 MHz to 250 MHz and the LPDA antenna over the range 250 MHz to 1 GHz. NOTE 2 The measurement uncertainty of radiated emissions from an EUT depends on many different influence factors such as the quality of the site, antenna factor uncertainty, antenna type, and the measurement receiver characteristics. The reason for defining low-uncertainty antennas is to limit other antenna influences on the measurement uncertainty, such as the effect of mutual coupling with a ground plane, the radiation pattern with respect to height scanning, and the variable phase centre position. Verification of effects of these influences is a comparison of the readings of the two antennas at the selected change-over frequency, which should give the same value of E-field within a margin of 1 dB. 26

Site validation methods for OATS, SAC, and FAR Applicability of site validation methods Test site type Tuned dipoles NSA Broadband antennas NSA Broadband antennas RSM (Reference Site Method) OATS Yes Yes Yes OATS with weather protection No Yes Yes SAC No Yes Yes FAR No Yes Yes 27

NSA Compares ideal or theoretical site attenuation between transmit and receive antennas with actual measured site attenuation Uses 2 co-polarized antennas Site attenuation obtained by the difference between source voltage level Vi applied to transmit antenna and maximum received voltage Vr measured at terminal of receive antenna (between 1 and 4 meters height) 50 Ohm System Recommended 10 dB attenuator at both transmit and receive antennas (during both direct and site voltage measurements) 28

Theoretical Normalized Site Attenuation 29

Site Attenuation SA Calculation Measured values compared to ideal OATS or measurand for site validations Where ΔAS is the SA deviation FaT is the transmit antenna factor FaR is the receive antenna factor AN is the theoretical NSA (as provided in Table 2 of the standard) VDIRECT is a direct measurement of the voltage value at the terminal of the transmit antenna VSITE is the measurement of the voltage on the terminal of the receive antenna ΔAS, shall not be more than 4 dB 30

Example of co polarization 31

Validation of a weather-protection-enclosed OATS or a SAC “Whenever construction material encloses a ground-plane test site, it is possible that the results of a validation measurement at a single location, as specified in paragraph 6.7, are not adequate to show acceptability of such an alternative site Single measurement insufficient to pick up possible reflections from the material surrounding an OATS Turntable, (or walls and ceiling of a SAC) Defines Test Volume – largest volume traced out by an EUT rotated 360 degrees Up to 20 SA measurements taken 32

Volumetric measurement 20 measurement positions would include the center of the turntable, and the forward, left, right and rear edges of the turntable (5 transmit antenna locations total), with horizontal and vertical polarizations (2 polarizations total) and for 2 heights of the transmit antenna (1m and 1.5m for horizontal polarization, and 1m and 2m for vertical polarization). 33

Vertical polarization measurement 34

Horizontal Polarization measurements 35

Volumetric Both Transmit and receive antennas moved to maintain test measurement distance For SAC, Horizontal 25cm spacing should be between antenna tip and cone tip or 1m between antenna midpoint and closest absorber tip FOR SAC, Vertical 25cm from lower tip of antenna to the floor, to prevent coupling with ground plane Even if center of antenna is higher than 1m For Weather enclosed sites, edge of turntable is usually selected as width of test volume, with 2m height 36

Example of test volume 37

Exceptions 38

Exceptions 39

Possible causes for exceeding site acceptability limits CISPR 16-1-4 recommends that the following items be rechecked if the site deviation, ΔAS, exceeds the 4 dB requirement: Measurement procedure Antenna factor accuracy Drift in the signal source Accuracy of the receiver or spectrum analyzer input attenuator Measurement device readings 40

Possible site faults causing failure Annex F of the standard describes errors that can occur in NSA measurement. If no errors are found, it is likely that the site is at fault, and should be investigated. Key items recommended by the standard include: ground plane construction inadequacies Undesired reflections from the perimeter of the site or from an allweather cover Poor or no continuity between the turntable and the surrounding ground plane when the turntable is flush-mounted and conductive Thickness of any dielectric ground plane covers Openings in the ground plane, like trap door seams 41

2019 FAR Site Validation change The new requirements for FAR site validation provides the addition of Table 2 that follows. This table limits the size of the test volume diameter and height, depending on the test measurement distance. This limitation did not exist in the previous standard publication, and we will delve into this further in future presentations. In the meantime, it is important to note that, if you wish, for example, to have a test volume greater than 1.5m in diameter, then you must construct a 5m chamber, as a 3m chamber will not meet this criterion. Again, this is only for FAR chambers and does not impact SAC and OATS requirements. Maximum diameter, dmax, and height hmax, of the test volume m Test distance dnominal 1.5 3.0 2.5 5.0 5.0 10.0 m 42

Questions? 43