ARSAdvances in Radio ScienceARSAdv. Radio Sci.1684-9973Copernicus PublicationsGöttingen, Germany10.5194/ars-15-243-2017Results of an intercomparison for electric field strength measurements within the German calibration servicePapeReinerKarstenUweLindnerFrank-MichaelRittmannFrankvon FreedenJoachimhttps://orcid.org/0000-0003-4222-4494Kleine-OstmannThomasthomas.kleine-ostmann@ptb.deSchraderThorstenhttps://orcid.org/0000-0002-7073-2106Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, GermanyTESEQ GmbH, Landsberger Str. 255, 12623 Berlin, GermanyKalibrierzentrum der Bundeswehr, Sielower Landstr. 66, 03044 Cottbus, Germanysteep GmbH, Lise-Meitner-Strasse 6, 85521 Ottobrunn, GermanyNarda Safety Test Solutions GmbH, Sandwiesenstr. 7, 72793 Pfullingen, GermanyThomas Kleine-Ostmann (thomas.kleine-ostmann@ptb.de)25October20171524324821December20162August201722September2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://ars.copernicus.org/articles/15/243/2017/ars-15-243-2017.htmlThe full text article is available as a PDF file from https://ars.copernicus.org/articles/15/243/2017/ars-15-243-2017.pdf
In this paper we discuss the results of an intercomparison for electric field
strength measurements within the German Calibration Service (Deutscher
Kalibrierdienst – DKD). The comparison has been carried out on the field
strength value required to reach a display reading of 20 V m-1 of the
field probes for frequencies between 100 MHz and 18 GHz. Five laboratories
joined the intercomparison including the Physikalisch-Technische
Bundesanstalt (PTB), the German National Metrology Institute that keeps the
primary standard for electric field strength. As measurement artefacts both a
small 1-axis probe usually used as transfer sensor at PTB and a larger 3-axis
commercial field probe have been used. While the results agree well for the
small field probe and when the larger commercial 3-axis field probe is
oriented in the direction of the magnetic field, larger deviations occur,
when the larger 3-axis field probe is oriented into the direction of the
Poynting vector of the calibration field.
Introduction
Field probes are widely used to determine the electric field strength,
e.g. for guaranteeing personal safety in electromagnetic fields or for setting
the test levels during electromagnetic compatibility interference testing.
Both fields of application require accurate knowledge of the calibration
factor of the field probe to be able to measure field strengths traceable to
the SI units with known measurement uncertainty. In Germany, three
laboratories are accredited for electric field strength measurements by the
DAkkS – Deutsche Akkreditierungsstelle, the national accreditation body for
the Federal Republic of Germany. To assess the technical competence of the
accredited laboratories, an intercomparison was organised by the
Physikalisch-Technische Bundesanstalt (PTB), the German National Metrology
Institute, within the framework of the German Calibration Service (Deutscher
Kalibrierdienst – DKD), which is the association of the accredited
laboratories in Germany (PTB-Mitteilungen, 2015).
In addition to PTB, three accredited and one non-accredited laboratories
took part in the intercomparison. As measurement artefacts both a small
1-axis probe developed by PTB and a commercially available field probe
ETS-Lindgren HI-6053TM were used in different orientations. The field
strength values to generate a reading of 20 V m-1 were compared. A field
strength reading of 20 V m-1 represents a commonly measured field strength
value in personal safety assessment and electromagnetic compatibility
testing and is well above the display fluctuations observed at zero field
strength. The required field strength values to generate readings of 20 V m-1
differ for the different field probes and orientations. The 1-axis probe
oriented parallel to the magnetic field (PH orientation) was used for a
comparison between 300 and 1000 MHz in 100 MHz steps and for 1000 MHz to
6 GHz in 500 MHz steps. The field probe HI-6053TM was oriented both
parallel to the magnetic field (PH orientation) and parallel to the power
flux density vector (PS orientation). It was used for a comparison between
100 and 1000 MHz in 100 MHz steps and for 1000 MHz to 18 GHz in 500 MHz
steps. The intercomparison shows that all laboratories are able to generate
electromagnetic fields with the specified measurement uncertainty but that
calibrations in PS orientation can be problematic. The results are
supplemental to the outcome of the international key comparison
CCEM.RF-K24.F (Eiø et al., 2013) that indicates a problem with field
representation for some National Metrology Institutes, whereas this national
intercomparison suggests problems with dissemination in some cases. The
outcome of this comparison was one of the triggers for re-establishing the
standardization working group DKE GAK 767.4.3 “Feldsondenkalibrierung”
which works on a first draft on a new standard IEC 61000-4-26 “Field Probe
Calibration”. The goal of this working group is to overcome the
shortcomings identified in current standards and technical guidelines such
as IEEE Std 1309TM-2013 (IEEE 1309, 2013), IEC 61000-4-3:2006
(IEC 61000-4-3, 2006) and VDI/VDE/DGQ/DKD 2622 Blatt 10 (VDI/VDE/DGQ/DKD 2622, 2004)
by targeting all aspects of field probe calibration required for
accurate measurements, including field generators, calibration process and
the application of the field probe.
Small 1-axis probe “Transfer sensor” with electronic box for dissemination
of field strength developed by the Physikalisch-Technische Bundesanstalt.
ETS-Lindgren HI-6053TM 3-axis field probe.
Main orientations of the field probe axis parallel to the magnetic
field vector (PH), the Poynting vector (PS) and the electric field vector (PE)
of the electromagnetic wave. The yellow box with handle represents the field probe.
Results of intercomparison (field strength values provided by the
participating laboratories and CRV with uncertainty of CRV plotted as error bars)
for the transfer sensor oriented in PH direction.
In this paper we describe the travelling standards, the intercomparison
schedule and the approach taken for data evaluation in Sect. 2. In the
following Sects. 3 and 4 we then present the results for the two different
types of field probes. In the final section we draw conclusions regarding
the calibration procedures for field probes.
Results of intercomparison in terms of DoE and U(DoE) for the different
participants and measurement constellations for the transfer sensor in PH
orientation. (a) Teseq, (b) KalZ BW, (c) Narda,
(d) steep, and (e) PTB.
Results of intercomparison (field strength values provided by the
participating laboratories and CRV with uncertainty of CRV plotted as error bars)
for the ETS-Lindgren HI-6053TM 3-axis field probe in PH orientation.
Results of intercomparison in terms of DoE and U(DoE) for the different
participants and measurement constellations for the ETS-Lindgren HI-6053TM
3-axis field probe in PH orientation. (a) Teseq, (b) KalZ BW,
(c) Narda, (d) steep, and (e) PTB.
Results of intercomparison (field strength values provided by the
participating laboratories and CRV with uncertainty of CRV plotted as error bars)
for the ETS-Lindgren HI-6053TM 3-axis field probe in PS orientation.
Results of intercomparison in terms of DoE and U(DoE) for the different
participants and measurement constellations for the ETS-Lindgren HI-6053TM
3-axis field probe in PS orientation. (a) Teseq, (b) KalZ BW,
(c) Narda, (d) steep, and (e) PTB.
IntercomparisonTravelling standards
As measurement artefacts both a small 1-axis probe (see Fig. 1) and a
commercially available 3-axis field probe ETS-Lindgren HI-6053TM (see
Fig. 2) were used. The small 1-axis probe was developed by PTB and is
commonly used as a transfer sensor to establish traceability for larger
field generators. Here, it is chosen as it minimizes the field perturbation
during measurement and provides reasonable information about the field
generation without larger influence from the detector. The commercial field
probe ETS-Lindgren HI-6053TM represents a typical 3-axis field probe
with the sensor head connected to the readout unit by a rod. Such probes
need to be calibrated in accordance with their application in
electromagnetic compatibility testing or personal safety measurements. The
main orientations of the field probe with regard to the electromagnetic
field and its propagation direction are denoted with PE, PS and PH as shown in Fig. 3.
The field strength values to generate a reading of 20 V m-1 at the instruments
were compared. The transfer sensor in PH orientation was used for a
comparison between 300 and 1000 MHz in 100 MHz steps and for 1000 MHz to
6 GHz in 500 MHz steps. The field probe HI-6053TM was oriented both in
PH and PS directions. It was used for a comparison between 100 and 1000 MHz
in 100 MHz steps and for 1000 MHz to 18 GHz in 500 MHz steps.
Time schedule
The field probes were first measured at Physikalisch-Technische
Bundesanstalt (PTB) in Braunschweig, then at TESEQ GmbH (Teseq) in Berlin,
then at the Kalibrierzentrum der Bundeswehr in Cottbus (KalZ BW), after that
at Narda Safety Test Solutions GmbH, in Pfullingen (Narda), then at steep
GmbH in Ottobrunn (steep) and finally at PTB again.
The measurement campaign took place from October 2012 until March 2013.
Data evaluation
The intercomparison was evaluated based on the measurement results reported
by all laboratories that were able to calibrate the distinct field probes.
According to Cox (2002) a comparison reference value CRV and its
uncertainty U(CRV) were calculated based on the field strength values Ei and
the measurement uncertainties U(Ei) reported by the individual
laboratories in accordance with the “Guide to the expression of uncertainty in
measurement” (GUM, 1995):
CRV=∑iwi⋅Eiwithwi=∑i1U2Ei-1⋅1U2EiU(CRV)=∑iwi2U2Ei.
The weighting factor wi results in a higher contribution of the
laboratories with lower uncertainties to comparison reference value CRV.
From the field strength values Ei and the measurement uncertainties U(Ei)
reported by the individual laboratories and the comparison reference value CRV
a degree of equivalence DoE and its uncertainty U(DoE) has been calculated using
DoEi=CRV-EiUDoEi=U2Ei-U2(CRV)
as a measure for the deviation from the comparison reference value for each
laboratory. A deviation of a single calibration result from the consensus
value CRV that cannot be explained by the reported measurement uncertainties
can be easily observed when plotting the DoE and its uncertainty. In that case
the absolute value of the DoE is larger than its uncertainty. The calculation
is based on expanded uncertainties so that a confidence interval of 95 % applies.
The measurement results from PTB were treated in the same way as those of
the other participants, neglecting the fact that PTB provides traceability
to some of the other laboratories in some of the frequency ranges used in
this intercomparison. However, we assume that the errors due to correlation
are rather low. In case both measurements at the beginning and at the end
were available from PTB, the mean value was used as input to the data
evaluation. As no drifts of the measurement artefacts could be identified, a
drift correction was not applied.
Results for the small 1-axis probe
The calibration results for the transfer sensor are shown in Fig. 4. From
PTB calibration data was available for a limited frequency range from 300 to
1000 MHz at the end of the campaign, only, due to technical
limitations. The degrees of equivalence for the individual laboratories are
shown in Fig. 5. The results of all laboratories agree well with the
calculated CRV within U(DoE) calculated from the specified measurement uncertainty
of the laboratory.
Results for the 3-axis commercial field probe
In Fig. 6 the calibration results for the field probe ETS-Lindgren
HI-6053TM in PH orientation are shown. The corresponding degrees of
equivalence for the individual laboratories are shown in Fig. 7. Due to
technical reasons, PTB calibration data was not available from 4000 to
8000 MHz, whereas calibration data was not available from KalZ BW from 100 to
900 MHz. The results of all laboratories agree with the calculated
comparison reference value within their specified measurement uncertainties
at nearly all frequency points.
The calibration results for the field probe ETS-Lindgren HI-6053TM in
PS orientation are shown in Fig. 8. The corresponding degrees of
equivalence for the individual laboratories are shown in Fig. 9. As in the
case of PH orientation of the probe, PTB calibration data was not available
from 4000 to 8000 MHz, whereas calibration data was not available from
KalZ BW from 100 to 900 MHz. It can be seen, that the calibration
results of three laboratories do not agree with the calculated comparison
reference value considering their specified measurement uncertainties at
distinct frequencies around 8000, 14 000 and 16 500 MHz (if this is a
problem of the three laboratories, only, is not clear until the input
quantities are coherently identified. It is unlikely that the CRV represents
the true value, here). As can be seen from Fig. 8, these frequencies
correspond to oscillations in the calibration factor, which can be
attributed to standing waves in front of the field probe (Kleine-Ostmann
et al., 2007). Clearly, the measurement uncertainty is underestimated at the
affected frequencies when calibrating a field probe in PS orientation with
large housing that causes standing waves. Assuming the result changes
noticeable by an angular misalignment within a few degrees the laboratory
will not be able to declare the uncertainty with good confidence until the
angle dependence has been determined.
Conclusion
Generally results agree well for the small field probe and when the larger
commercial field probe is oriented in the direction of the magnetic field.
In these cases calibration curves are reasonably flat, as no problems with
standing waves in front of the field probe occur. The very good agreement
between the laboratories shows that the specified measurement uncertainties
are realistic and that no general problem with the representation of field
strength exists. However, when it comes to dissemination, the calibration of
larger field probes can be problematic at distinct frequencies when standing
waves occur due to orientation of the field probe in the direction of the
Poynting vector. It is very important that calibration laboratories identify
these problems in order to maintain their specified calibration
uncertainties or to take measures for a wise consideration of such
calibration results into the uncertainty budget. Some calibration
laboratories changed their calibration setups already, so that the situation
described in this intercomparison might have changed by now. The findings
will directly contribute to a new standard IEC 61000-4-26 “Field Probe
Calibration” for which a first draft is under development in the
standardization working group DKE GAK 767.4.3 “Feldsondenkalibrierung”.
The underlying data sets are property of the participating
laboratories and are not made available to the public.
The authors declare that they have no conflict of interest.
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