Normalized Site Attenuation Calculations Using Standard Spreadsheet Analysis

Manny Barron Compaq Computer Corp

Tandem Division 103 00 N. Tantau Avenue

Cupertino, CA 95014 USA

Abstracf: Normalized site attenuation (NSA), has become the standard method for determining the adequacy of EMC test sites to perform EMI emission measurements. The measurement of the NSA parameter for a test site and the subsequent required data processing and analysis can sometimes be complicated, tedious, and time-consuming. This is particularly so for alternate test sites (e.g., semi- anechoic chambers) where ‘volumetric’ NSA analysis is required and also when swept frequency measurements are performed that contain large amounts of data. In cases such as this the management of the large amounts of data can be just as critical as the actual measurements. This paper proposes a method by which the large amount of NSA data and calculations can be managed in a reasonably simplistic manner through the use of standard spreadsheet analysis. This method has the distinct advantage (in contrast to special custom software) of providing an open framework model whereby all calculation details are readily available and visible for review and inspection, including all formulas used in the calculations. The foundation for this spreadsheet analysis discussion is based on the author’s recent work involving NSA qualification testing of a new 1 O-meter semi-anechoic chamber for a major manufacturer.

INTRODUCTION

There are numerous papers on the subject of site attenuation in the open literature, probably numbering in the hundreds. Site attenuation, or ‘normalized’ site attenuation (NSA), has become the standard method for determining the adequacy of EMC test sites to perform EMl emission measurements. The NSA parameter is a frequency response characteristic of a test site that is acquired via measurement on the test site in accordance with various standards, e.g., ANSI C63.4-1992 [l] plus others. This same standard also requires that the measured NSA values be compared to the NSA values of a ‘theoretical’ ideal site (which are calculated from a mathematical model) to determine the deviation from the ideal site. In the case of ANSI C63.4 the standard specifies a maximum allowable deviation of f4 dB over the frequency range of 30 - 1000 MHz for a given test site to be considered adequate for radiated emission test measurements. The final output data results of NSA

measurements are typically presented in NSA graphs which provide an excellent visual display and data reduction of the large amounts of data. Figures 5 and 6 are examples of typical NSA graphs. These plots show the NSA curves for five different receive antenna positions around the test site turntable - center, front, back, right, and left - for one of several test site configurations (Table 1).

Table 1. NSA Test Site Configurations

JO Meter Site 3 Meter Site

&Ant lkAnt &Ant l’kAnt

+ 12 deg: Horiz 1.0 m + 18 deg: Horiz 1.0 m Horiz 2.0 m Horiz 2.0 m Vert LOm Vert 1.0 m Vert 1.5 m Vert 1.5m

- 12 deg: Horiz LOm - 18 deg: Horiz 1.0 m Horiz 2.0 m Horiz 2.0 m Veri 1.0 m Vert LOm Vert 1.5 m Vert 1.5 m

This particuhu test site (IO-meter semi-anechoic chamber) was designed for use with a dual antenna system whereby two antenna towers with antennas are present simuhaneously and are located at fmed angles from site center-line in symmetrical fashion and can be used at 10 or 3 meter Wance with offset angles of +12 or PI8 degrees respectively. The shove tabb summarizes 16 NSA test configurations (16 plots) which are rub-divided into 4 groups of 4 plots, with each group covering a unique .eceive (Rx) antenna position (+12, -12, +18, or -18 degrees) and with each plot representing a unique transmit (TX) antenna height (1.0, 1.5, or 2.0 meter) and antenna polarization (horizontal or vertical) combination.

Data Management Problem

Performing NSA measurements for a given test site and the subsequent required data calculations, although explained fairly well and in sufficient detail in the literature, can be somewhat complicated, tedious, and time-consuming, particularly where large amounts of data are acquired for processing and analysis. The amount of measurement data tends to grow large in cases such as the following: 1) for alternate test sites (e.g., semi-anechoic chambers) where ‘volumetric’ NSA analysis is required (5 different antenna positions around the turntable plus 4 separate antenna heights), 2) when swept frequency measurements are performed (which typically contain large amounts of data

133

points, in the hundreds), 3) for alternate test sites having multiple receive antenna positions for the same test distance (e.g., +12 and -12 degrees offset Corn site centerline), and 4) for test sites having multiple test distances (e.g., 3 and 10 meters) on the same site. All of these have a direct impact on the amount of measurements to perform and the subsequent amount of data acquired for processing and analysis. An example of the magnitude of this data management problem is the author’s NSA project which involved all four of the above scenarios and which resulted in a final NSA test report that contained over 500 pages of graphs and data tables, all of which was generated from about 160 data files with each file containing several hundred data points from swept tiequency measurements. Table 1 provides an overview of the various NSA graphs (only one is shown, see Figure 1) which contained all of the previously mentioned scenario’s and which dramatically increased the amount of measurement data acquired for processing and analysis.

Spreacisheet Analysis Solution

This paper proposes a method by which to manage the large amount of NSA data and calculations through the use of standard spreadsheet analysis. The host software platform chosen for this method is Microsoft Excel 97 although other spreadsheet programs would probably work just as well (hence in this paper the standard filename extensions are not shown at the end of filenames which are discussed). This spreadsheet analysis technique has the following advantages: 1) it provides an open framework model (i.e., the spreadsheet) whereby all calculation details are open for review and inspection, 2) it provides a tile ‘linking’ method that automatically links data Corn one spreadsheet file to another and which can also provide automatic and immediate calculation updates whenever any data in linked files are updated, 3) it provides a ‘macro’ programming capability to perform practically any function available to the spreadsheet program, including access to external sources (such as the Microsoft Windows operating system environment and any external files), and 4) the spreadsheet model and associated templates can be used over and over again for subsequent annual site NSA measurements with minimal changes required (primarily for data input tiles).

Spreadsheet Model Overview

The spreadsheet analysis sub-divides the overall NSA calculation model into several separate spreadsheet files and then ‘links’ these separate files together to integrate the results and present them in the form of NSA graphs and data tables, which readily and clearly show the pass or fail condition of the test site, The basic spreadsheet file templates provide the computing power for the numerous repetitive and tedious calculations and data plotting while the file linking capability provides immediate and automatic calculation and plot updates whenever any of the individual linked data tiles are updated or modified in any

way. [Special Note: For Microsoft Excel this will occur immediately if all related linked files are simultaneously open or it will occur upon opening the linked file(s) if not open at the time]. Additionally, a spreadsheet model is also used for the more complex calculations of the NSA theoretical values for an ideal site. This is implemented through the use of ‘macro’ programming within the spreadsheet environment to provide the necessary extensive re-iterative calculations, including real-time plotting of the theoretical NSA values while the macro is running.

SPREADSHEET MODEL

The spreadsheet model which prompted this paper involved the authors recent NSA work and included the four scenario’s shown in Table 1, which resulted in a relatively large amount of measurement data that underwent processing and analysis. However the discussion in this paper focuses on the spreadsheet model for only 1 of 16 NSA graphs (see Figures 5 and 6) listed in Table 1. The basic spreadsheet model can be extended to the other test site con&urations easily by using the basic model as templates to construct the spreadsheet files for the other test site configurations.

Table 2. Spreadsheet File Summary

File Description

SA-Data site attenuation data

M-Data antenna factor data

File Links

output to NSA-Caic

output to NSA-Calc

I NSA-neo NSA theoretical model output to NSA-Calc output to Al-Data

Spreadsheet File Summary

The basic spreadsheet model is comprised of five different spreadsheet files as shown and described in Table 2 and discussed in further detail below. These files are generic in nature and are actually considered ‘templates’ which can be used to extend the spreadsheet models to other test site configurations. It is anticipated that the first two files, SA-Data and M-Data, would be updated annually while the latter three files would be used over and over again yearly without any changes other than new data input f?om the SA-Data and AF-Data files.

&l-Data: Site Attenuation Data

This spreadsheet file contains the raw site attenuation measurement data, VDIRECT and Vsrrs that was acquired during the site measurements. This data could be the

134

!dat ,ef vldat Ill4 fi x _ / vlEdat

perform the site attenuation measurements or it can simply

Figure 1.

be site attenuation data that was acquired via manual

Site Attenuation Spreadsheet

measurements and entered manually into this spreadsheet

output of the automated test software that was used to .

file. This data is not ‘normalized’ yet since it still contains the influence of the antenna factors that were used for the measurements. The site attenuation data contained in this particular file shows the combined VoMcr minus Vsrm data (although it could also have been shown separately). Figure 1 presents the details of this spreadsheet file, showing the combined Vn -Vs data for the 5 different transmit antenna positions around the turntable, all for a unique transmit antenna height and polarization. The data in this file is linked (as an output) to the NSA-Calc spreadsheet file and is used for farther calculations in that file, therefore any data changes made to this SA-Data file will automatically update the linked files. There are no data links (as an input) to this SA-Data tile since it is simply raw measurement data.

test]. Figure 2 presents the details of the N-Data spreadsheet file for a unique transmit antenna position, height, and polarization. The antenna factor data contained in this particular tile shows the combined AFrx + AFRx data (although it could also have been shown separately). In this particular model (Figure 2) the M-Data spreadsheet file is set-up for calibrating the antennas at five different transmit positions (along an arc) while maintaining the required 3m or 10m distance between transmit and receive antennas. The Al-Data spreadsheet model then calculates the average of the multiple antenna measurements in order to minimize the negative influence of any site imperfections at the OATS where the antenna calibration measurements are performed. Also, this particular spreadsheet model (Figure 2) is set-up for the two-antenna calibration method per ANSI C63.5 although the model could be easily modified for any other method. The data

in this Al?-Data file is linked (as an output) to the NSA-Calc spreadsheet file and is used for farther calculations in that file therefore any data changes made to the AF-Data file will automatically update the linked files.

. _.

There is also one data link (as an input) into this AF-Data file t?om the NSA-Theo file which contains the theoretical NSA values NSAth for an ideal site. The two or three antema calibration method per ANSI C63.5 utilizes a theoretical site parameter called the maximum received field Edmax which is determined theoretically from a mathematical model for an ideal site. For the two-antenna calibration method there is a direct relationship between Edmax and NSAth that can be expressed mathematically (which is the reason why NSAth shows up in the AF-Data spreadsheet model shown in Figure 2).

AF-Data: Antenna Factor Data

This spreadsheet file contains the antenna factor calibration data, AFrx and AFW from an antenna calibration performed elsewhere [at an outside area test site (OATS)], or this file may contain the raw site attenuation measurements Vn,,r and Vsrrs that are used to calculate the resultant antenna factor values AFTx and AFnx per ANSI C63.5-1988.[2] [Special Note: For the latter, this particular Vnmm and Vsrm data represents the site attenuation measurements at the OATS where the antennas were calibrated and is not the same as the data contained in the SA-Data file described previously which represents the site attenuation measurements of the site under Figure 2. Antenna Factor Spreadsheet

135

Figure 3. NSA Theoretical Calculation Spreadsheet

NSA- Theo: NSA Theoretical Model

This spreadsheet file contains the theoretical NSA values for an ideal site per ANSI C63.4. This tile could be simply a data file containing the NSA theoretical values copied directly from the ANSI C63.4 standard (or interpolated values from same) or it could also be setup to calculate the values from the basic mathematical model contained in ANSI C63.5. Either way, the purpose of this file is to provide a data input function of the NSA theoretical values to the AF-Data and NSA-Calc spreadsheet files for use in farther calculations. Figure 3 presents the details of this spreadsheet file, showing the NSA theoretical values as a function of frequency for a given site distance and transmit antenna height and polarization. This particular NSA-Theo tile is setup to calculate the NSA theoretical values from the basic math model as noted. The input variables to this model consist of the site distance between antennas, the transmit antenna height and polarization, the receive antenna minimum and maximum height range, and the ground plane conductivity, permittivity, and reflection coefficient. At each frequency of interest the “macro” program in this NSA-Theo spreadsheet file mathematically varies the receive antenna height as required by the model to maximize the received field level Ed eventually locating the specific height that produces Edma. This Edma value is then used to determine the NSA theoretical value which is then provided as an input to the M-Data and NSA Calc spreadsheet files for further calcul&ons. There are no data links (as an input) to this NSA Theo tile since it contains values acqu&ed either directly from the ANSI

standard or values calculated from a model, also contained in the ANSI standard.

NsA_Calc: NSA Calculations

This spreadsheet file contains the integrated final NSA calculations for the site under test based on the equations specified in ANSI C63.4(1992). The input data for these calculations comes from the SA-Data, AF-Data, and NSA-Theo files. This includes the (V, -Vs) site measurement data in SA-Data, the (AFm + AF& antenna factor calibration data in AF-Data, and the (NSAth) theoretical values in NSA-Theo. Figure 4 presents the details of this spreadsheet file for a unique receive antenna offset angle, test site distance, and transmit antenna position, height, and polarization. This calculation is repeated 5 x 4 = 20 times in this spreadsheet (although only one is shown) for 5 antenna

positions and 4 antenna heights. The equation for this calculation is contained in ANSI C63.4 and is fairly straight forward and easy to implement (although very repetitious and therefore tedious). The final results of this calculation are the measured NSA values (NSAmeas) for the site under test which are then compared to the theoretical NSA values (NSAth) from which a ANSA deviation is derived. This ANSA deviation is then required to fall within the maximum allowable (zk 4 dl3) as specified in ANSI C63.4. Not shown in this NSA Calc model (because it does not apply to the particular site configuration scenario) are the mutual impedance correction factors, however the spreadsheet model could be easily modified to include them where required. All of the data in this NSA-Calc file is linked from the other spreadsheet files noted previously,

Figure 4. NSA Calculation Spreadsheet

136

-20

Normalized Site Attenuation 10m site -12 deg Horiz Im

IO 100 1000

Frequency MHz -Center -------.Front -------,Sack - - -Right - - - -Left -NSA Limit: i4.0 dB

Figure 5. Final NSA Plot

NSA Deviation 10m site -12 deg Horiz 1 m

10 100

Frequency MHz -CCenter _______. ,Zro,,t -. .- _. Ba,& _ - _ Kg,,t - _ - _ Left

Figure 6. Final NSA Deviation Plot

1000

therefore any changes made in these other linked files will automatically update the data contained in this NSA-Calc file. This would occur during the annual antema factor calibration and annual NSA site re-check measurements.

NSA-Plot: Final Data Plots

This spreadsheet file contains the final results of all calculations in a pre-defined full color graphical format suitable for output to a printer in hardcopy or left as-is for softcopy electronic storage, transmission, or display. All of the data input required for this final data presentation is provided from the NSA-Calc file which contains an integration of all the required calculations and derivations from the other individual files, therefore any changes made

in these other linked files will automatically update the data contained in this final NSA-Plot file. This would occur during the annual antenna factor calibration and annual NSA site re-check measurements. The final NSA data in this NSA-Plot file is presented as a function of frequency for a unique receive antenna position (site offset angle), test site distance, and transmit antenna height, polarization and position around the turntable. The resultant graph is known as a ‘Volumetric’ NSA plot because of the volume covered with the transmit antenna over the turntable in the region where the equipment under test (EUT) would be located during actual emission testing. This volume includes four antenna heights (Horz lnl, Horz 2m, Vert lm, and Vert lm) and five antenna positions around the turntable (center, front, back, right, left). There are two basic output plots available in this NSA-Plot file: 1) a graph showing the actual NSAmeas data along with the maximum allowable deviation boundary (Figure 5), and 2) a graph showing a closer view of the NSA deviation data with respect to the maximum allowed deviation of Z!Y 4 dB (Figure 6). Hence, Figures 5 and 6 contain the same dam results but the data is plotted from slightly different perspectives; one shows the absolute NSA value while the other shows a deviation from the ideal value.

Integrated Spreadsheet Model

The discussion in this paper addressed five spreadsheet files (Table 2) that were used to derive one of sixteen NSA plots (Figures 5 & 6) and which was listed as one of the sixteen NSA test con&urations (Table 1). The integrated spreadsheet model to cover all sixteen test configurations will use the same AP-Data and NSA-Theo files without change but will utilize similar SA-Data, NSA-Calc, and NSA-Plot files that are structured appropriately for the various NSA test configurations. This would include other test sites (3m and 1Om) within the same site, other receive antenna offset angles (~~12 and f18 degrees), and the other receive antenna heights (1.0, 1.5, and 2.0 meter) and

137

antenna polarization (horizontal and vertical) combinations. For the integrated spreadsheet model the user could set up a filename convention to indicate the test configuration represented by the particular spreadsheet file. Example: a filename of “m12d-10m~H1” would indicate the receive antenna location was at the -12 degrees offset angle on the 10 meter site with the transmit antenna set up at 1 .O meter height in the horizontal polarization orientation.

OTHER SPREADSHEET MODELS

The spreadsheet models discussed in this paper were used by the author on his particular NSA project, however others may consider similar spreadsheet configurations derived from these models or their own models. An example would be to combine the input data into one spreadsheet file, i.e., combine SA-Data and AF-Data into one single file “In-Data”, thereby reducing the total amount of spreadsheet files fiom five to four. Another example would be to combine the output data into one spreadsheet file, i.e., combine NSA-Calc and NSA-Plot into one single file “NSA-Data”, again reducing the total amount of spreadsheet files. Also, the various data within a single spreadsheet file can still be linked together in a satisfactory manner (i.e., updating a particular ‘worksheet’ within a ‘workbook’ can automatically update another linked worksheet within the same workbook (Microsoft Excel terminology). Still another [extreme] example would be to combine all of the separate files into one giant spreadsheet file. However, combining a large amount of data into a smaller set of spreadsheet files will of course increase the size of the files and possibly increase the complexity of maneuvering through a larger file and managing the larger amount of data contained therein. This is a trade-off that the spreadsheet user must make when he sets up his spreadsheet models. Still other possibilities could be to implement exotic ‘macro’ programs within the various spreadsheet files to perform practically any functions that are available to the spreadsheet program, including access to external sources (such as the Microsoft Windows operating system environment and any external files). In summary, there are many possibilities to tailor the spreadsheet models to suit your particular needs.

CONCLUSIONS

Normalized site attenuation testing that involves volumetric measurements on multiple locations within the same test site can result in a significantly large amount of test data. The subsequent data reduction, data analysis, and the presentation of the final data results is very repetitious in cases such as this, and therefore is time consuming, tedious, and prone to human error. To overcome this potential problem the use of standard spreadsheet analysis was proposed as a software tool to manage the large amount of data. It was shown that the problem could be spread out

over several spreadsheet files with each file having a unique purpose and function and with all files electronically linked together in a manner that would facilitate rapid and error&e data transfers as needed. In this manner, the basic spreadsheet models could be used as templates over and over again during the annual measurements required for EMC emission test sites. Another distinct advantage of this spreadsheet analysis method is that it provides an open framework model whereby all calculation details are readily available and visible for review and inspection, including all formulas used in the calculations. Furthermore, the electronic form of this data management tool provides portability such that the spreadsheet models and/or the resultant data can be transported easily and rapidly via electronic means.

REFERENCES

[l] ANSI C63.4-1992, “American National Standard for Methods of Measurement of Radio Noise Emissions j?om Low-Voltage Electrical and Electronic Equipment in the Range of 9 KHz to 40 GHz ”

[2] ANSI C63.5-1988, “American NationaI Standard for electromagnetic compatibility - radiated emission measurements in electromagnetic interference (EM4 control - calibration of antennas ”

138