A Comparison of Radiated Emissions Testing to European Directive 95/54/EC Using an Open Field Test Site and a

Semi-anechoic Chamber.

Dr Peter Miller Motorola AIEG Tayh Road

Stotfold, Hitchin, Her& SG5 4AY, England

Abstract: Most automotive component level testing, and in particular European Directive 95154 requires that under some (in practice most) conditions radiated emissions measurements from Electrical/electronic Sub-Assemblies (ESA’s) arc made with the ESA under test mounted on an elevated ground plane Im high on a wooden table. It is shown in this paper that how this ground plane is connected to the test sites ground is very important, and that this is in fact a key issue in obtaining correlation between different open field sites and also to obtaining correlation to test results obtained in a semi- anechoic chamber.

INTRODUCTION

Many authors (for example [2],[3],[4],[5]) have presented results showing differences in radiated emissions results between test sites. In particular [5] raises many questions about correlation between sites used for vehicle component testing. Most vehicle component test specifications (e.g. CISPR-25, SAE J1113/41, European directives 95/54iEC and 97/24iEC and IS0 standards IS013766 and ISO14892)[5] require that the equipment to be tested is placed upon an elevated ground plane while the measurements are made. How this elevated ground plane is earthed adds an exha degree of variability, and seems to make to the correlation behveen results obtained on open area sites and in semi-anechoic chambers worse [5]. It should be noted that some tests defined in MIL-STD-462 also have a similar test set-up.

EXPERIMENTAL RESULTS

Measurements were made using a commercially available comparison noise emitter (CNE) (a wide band noise generator which is battery powered and fitted with a small top loaded monopole antenna) (see figure 1). Using a wooden table, the results obtained (figure 2) show a good correlation between the open field test site and the semi-anechoic chamber. However, when the ground plane was fitted to the top of the wooden table (earthed via a vertical length of braid in the open area site), a deep (>20dB) notch at just under 7OMHz was apparent in the free field measurements for horizontal

polarisation (figure 4 - this is also apparent to a lesser degree in vertical polarisation - figure 3), which was not present in the other measurements. A somewhat smaller notch is also visible at -140MHz. 95/54 measurements in a semi-anechoic chamber (here the table top was earthed by extending it backwards through the absorbers to the chamber wall) arc shown in figure 5.

Again, evidence of two notches is seen at approximately 55MHz and 11OMH.z. It should be noted this chamber is relatively small (6.7’3.7’3.2m) and is lined with 20in pyramidal absorbers. The general test set-up used is shown in figure 7, while figure 8 provides more detail. Figure 9 shows a photograph of the open area site during testing, the ground strap can just be seen on the back right hand comer of the table. Given the small (100mm long monopole) antenna used on the CNE it was not expected that this would directly be the cause of the observed notches, and the measured SWR plot on a ground plane (figure 10) confvms this. Some results have also been obtained using another open area

test site, these also appear to show a “notch” about 76MHz - these are not shown here because of space limitations.

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Further measurements on both open area sites and semi- major resonance at -1SOMHz [6]. It is however, possible to anechoic chambers are given in [S], however these tests used a see further evidence of the lower frequency notch, and 1.5m long horizontal antenna on the signal source which increased site-site variability in the region of this notch, in the therefore “overlaid” its own resonant emissions pattern on that results presented in [5]. of the test site. Simulations of this antenna structure show a

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Figure 3. CNE measured on a free field site at lm to 95/S, vertical polarisation.

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Figure 4. CNE measured on free field site to 95/54 with horizontal polarisation of CNE and measuring antenna.

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Figure 5. CNE radiated output to 95/54 in a semi-anechoic chamber, vertxal polarlsatlon.

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TABLE AS A TOP LOADED MONOPOLE?

A possible mechanism to explain the notch is that the elevated ground plane could be acting as an antenna, the notches being the fiat and 2nd resonance’s of this structure. If we consider the 95/54 open tield site set-up, then the vertical wire connecting the ground plane to “earth” could be acting as a top loaded monopole and resonance’s of this could be causing the deep nulls. Assuming a top loaded monopole strochue, reference [l] ~~2-40 shows that a vertical wire lm long and OSin diameter would be resonant at 68MHz with a top loading of -50pF. Tbe self-capacitance of the sheet used (1 l 1.15m) was calculated as 5OpF. This close agreement in frequency strongly suggests that this is the mechanism causing the notch.

SIMlJLATlONS

To investigate further, both the free field and the semi- anechoic chamber set-ups were simulated using a version of NEC. Modelling the table top as a grid of 5.5 wires, each wire being 10 segments long gave a resonant frequency of 72Mhz

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over a perfect ground plane - in good agreement with figures 3,4. The simulated impedance (looking from the bottom of the vertical wire connecting “real ground” to the tabletop ) is shown in figure 11 -which clearly shows the resonance.

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Figure 11. Simulated “table” impedance.

A similar simulation of the situation in the semi-anechoic room resulted in a simulated resonance at 54MHz, again in good agreement with the measurements in figure 5.

Clearly this basic structure is difficult to change, as any reasonable connection to “earth” in a free field site will result in a sort of “top loaded” antenna structure. This leaves the option of either using the same (“non-optimum”) earthing structure in the semi-anechoic chamber, or of including the effects of the structure resonance in the correlation required by 95154 (which treats the free field site as the reference, while not defining how the plate on the table is earthed). Given that the earthing arrangements of the tabletop ground plate do currently vary between different free field test sites, the correlation approach was chosen. To date correlation’s have been obtained for two NAMAS accredited open area sites, which are run by different companies. These correlation’s allow approximate correction of the results measured at a specific semi-anechoic chamber to those of the 2 open area sites.

CONCLUSIONS

Measurements and simulations have shown that the elevated ground plane can cause “notches” in the measured radiated emissions. It has been conjectured that this can be explained by treating the elevated ground plane as an antenna structure -

simulations based on this explanation have shown very good agreement with the measured results. Given the good explanation, and the lack of variability in the structures causing this, it is reasonable to apply a correction factor to see approximately what the measurements would be at another site. This is particularly useful when the semi-anechoic chamber is used for pre-compliance testing and open area sites are used for compliance testing. It should be noted that at present CISPR-25 requires correlation between a semi-anechoic chamber and an open area site be obtained before the semi-anechoic chamber is used for measurements to CISPR-25, but does not permit the use of a correction factor on the results. This paper has shown that to obtain such a correlation requires attention to many details not currently specified in CISPR-25. The author believes that similar simulations could also play a significant role in designing and tolerancing test set-ups so as to minimise variability between test sites.

ACKNOWLEDGEMENTS

The following people at Motorola inputted to this work, Bob Lempkowski and Jerry Meyerhoff re-enforced the idea that the table may be acting as an antenna which was the basis of this investigation, and Martin O’Hara performed the semi- anechoic chamber measurements and provided some of the drawings used in this paper.

REFERENCES

[l] “ARRL Antenna book”, 17th ed., 1994. [2] Heise, E., Heise, R., A method to compute open area test site uncertainty using ANSI C63.4 Normalised site attenuation measurement data, ~~505-507, IEEE 1996 International symposium on EMC. [3] Heise, E., Heise, R., A method to calculate uncertainty of radiated emissions, pp359-364, IEEE 1997 International symposium on EMC. [4] Kolb,L., Statistical Comparison of site to site measurement reproducibility, pp24 l-244, IEEE 1997 International symposium on EMC. [5] Swanson,D., Investigation of the Calibration Procedure from CISPR25 Annex B, for use with vehicle component testing, Proceedings 1998 IEEE International Symposium on Electromagnetic Compatibility, ~~368-374. [6] Miller,P., Bulk current immunity testing, its advantages and disadvantages for Automotive EMC testing, Proceedings AutoTech 96.

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