Field Uniformity and TEM Mode Verification in
GTEM 1000 Cell
Audrius Merfeldas, Darius Andriukaitis, Darius Gailius, Algimantas Valinevicius, Vytautas Markevicius,
Dangirutis Navikas, Mindaugas Zilys
Electronics Engineering dpt.
Kaunas University of Technology
Kaunas, Lithuania
audrius.merfeldas@ktu.lt
Abstract — This paper investigates GTEM cell field
uniformity and provides TEM mode verification for GTEM
1000 cell in the frequency range from 30 MHz to 1000 MHz.
Field uniformity is essential in electromagnetic compatibility
immunity tests. Uneven field distribution might lead to test
mistakes and decrease measurement accuracy. Verification of
TEM mode and field uniformity provides good representation
of cell manufacturing quality, materials and structure
imperfections or defects. GTEM cell capabilities for device
under test size limits could be expanded knowing exact field
distribution and uniformity at the reference plane.
Keywords—Electromagnetic compatibility, Immunity, GTEM
cell, Field uniformity, EMC
I.
INTRODUCTION
Field uniformity and secondary field components strength
are the key parameter defining GTEM cell performance for
EMC radiated immunity test [1], [2] as well as provide good
characterization of cell manufacturing quality. This research
investigates GTEM 1000 cell manufactured by Frankonia
field distribution in reference cross-section, defined in Fig. 1.
GTEM cells are widely used in electromagnetic
compatibility tests [3], [4], but main field of use is immunity
tests [5]. Significant advantage comparing to other chambers
is that here GTEM cell operates as wideband antenna so no
additional calibrated antenna needed. Secondly, GTEM cell
provides possibility to create high field strength with much
less power. While investigating immunity for radiated fields
device under test is placed inside GTEM cell and exposed
with electromagnetic field [4]. Monitoring device
performance immunity to external electromagnetic field is
investigated [6].
Field uniformity has great influence to EMC immunity test
results [2], [7] and can increase uncertainty due to some
sensitive parts of equipment under test (EUT) might be
exposed with much lower field strength comparing to the
desired values and even pass the test, when the other
laboratory could found problems. This might cause dispute
between laboratories. In order to prevent this situation and
reduce test uncertainty knowing field uniformity is crucial.
Another key parameter of GTEM cell is strength of
secondary field components [3]. TEM is waveguide mode in
which the secondary field components of the magnetic and
electric fields are much less in the propagation direction
comparing to primary field component [7]. In real life
material, cell geometry and manufacturing imperfections
cause field reflections that degrades uniformity of field and
cause unpredictable measurement errors [8].METHODOLOGY
To produce electromagnetic field inside GTEM cell signal
power P should be applied to it’s input3. Z0 is cell
characteristic impedance (50 Ω) and having spacing of
septum above cell floor, electric field strength can be
calculated
E=
1
P × Z0 .
h
(1)
Center point of the measurement cross-section was chosen
to be reference point. System was calibrating while changing
oscillator level to get total 10 V/m electric field strength at
the whole frequency range in the center point. Oscillator
levels at all frequencies are recorded in the computer memory
and all other measurements and same levels are used for
different reference plane grid points. Measured field strength
is recorded.
III.
EXPERIMENT SETUP
Research measurements were made using isotropic electric
field probe Narda EP-601. As less as possible sensor size and
no conducting fiber optics wires to output measurements
results reduces measurement errors and sensor influence to
measurement results. This sensor is battery powered and all
data transfer is established over non-conductive fiber optics
cable.
Fig. 1. GTEM cell internal structure, reference plane and measured field
direction.
Sensor during measurement was positioned at reference
plane in the grid of 10 cm both vertical and horizontal over
the whole cross-section area. 10 cm spacing was left from any
conductor due to conductor influence to measurement
uncertainty. Reference plane for GTEM 1000 cell is specified
978-1-7281-7542-3/20/$31.00 ©2020 IEEE
Authorized licensed use limited to: GLASGOW CALEDONIAN UNIVERSITY. Downloaded on August 13,2022 at 22:10:49 UTC from IEEE Xplore. Restrictions apply.
to be plane where septum distance from floor of the cell is 1
m. All measurements were taken in the reference plane as
shown in Fig. 1.
Research was based on measuring orthogonal components
of the electric field at the grid points of the reference crosssection of the GTEM cell and different frequencies. Wanted
and unwanted field components were registered.
Frequency range was chosen to cover 30 MHz up to 1
GHz. 30 MHz is lowest limit for radiated immunity tests
according to IEC 61000-4-20:2010, while highest frequency
range is limited by sensor size, grid size and number of points
to measurement. Step size chosen according same standard.
IV.
GTEM cell manufacturer and IEC 61000-4-20 standard
defines maximum testing volume H/3 x 0,6W x 0,6L that in
GTEM 1000 case is 700 mm x 700 mm x 330 mm. All nodes
from the measurement grid that fall within defined area were
taken in to consideration and results presented in the plots
(Fig. 3–Fig. 5).
Starting from Ey it can be clearly seen that field strength
varies between different space points of the reference plane as
well as frequency influence for electric field strength is
different from point to point (Fig. 3).
FREQUENCY AND FIELD UNIFORMITY
Electric field components values in the center point of
measurement plane provided in Fig. 1. Ey is y axis
component of the electric field and it is wanted component.
It’s value is very close to the main electric field strength for
which reference point was calibrated. It is important for this
plot to be as flat as possible. Excluding few frequency spikes
Ey trace flatness over whole investigated frequency range
does not exceed 2 V/m.
Secondly, unwanted field components Ez and Ex that are
respectively z and x projections of the electric field in GTEM
cell not required to be flat however small in values. Ex
component of electric field is low and almost never exceed 2
V/m, if few frequency steps over 800 MHz is not taken into
consideration. Unwanted field component Ez analysis shows
that this component is higher field strength compared to Ex
due to GTEM cell geometry. Ez component was chosen to be
measured parallel to the floor of the cell, while septum plane
is tilted from the floor point of view, causing more field to be
projected on z axis direction. Analyzing GTEM cell from
center point of measurement plane it can be clearly stated that
GTEM cell works in TEM mode, unwanted field components
are lower and main component field strength at different
frequencies has flat response.
Fig. 2. Electric field components strength versus frequency in the center
point of the reference plane.
Center point of the cell normally have no issues with TEM
mode or field strength regarding frequency. While evaluating
GTEM cell performance checking more points is crucial,
especially that points closer to outer conductors are suffering
from field reflections and imperfections of cell structure.
Following measurements in different points provides more
details on overall field uniformity (Fig. 2).
Fig. 3. Electric field Ey component strength versus frequency in the all grid
nodes that falls within standard defined area of the reference plane.
There are few frequency steps that has unexpected results,
but even standard doesn’t state very strict requirements for all
frequency steps. Field strength chart flatness is much better
at lower frequency range up to 300 MHz. With increasing
frequency trace has clearly peaks and dips that are quite
common for all measurement points. This effect could be
explained GTEM cell materials and construction
imperfections for higher frequencies.
Fig. 4. Electric field Ex component strength versus frequency in the
standard defined area points of the reference plane.
Ex unwanted field component analysis at different
measurement plane points shows that in different area points
behavior of the field is very similar, but unwanted field
strength increases going from the center to the outer conductor
of the cell. One measurement point has significantly higher
field strength peak at around 830 MHz (Fig. 4, Fig. 5).
Authorized licensed use limited to: GLASGOW CALEDONIAN UNIVERSITY. Downloaded on August 13,2022 at 22:10:49 UTC from IEEE Xplore. Restrictions apply.
Fig. 6. Electric field Ey component strength distribution in GTEM cell
reference cross-section at 30 MHz.
Fig. 5. Electric field Ez component strength versus frequency in the
standard defined area points of the reference plane.
As we can see from previous measurements this frequency
step was problematic before, but it shouldn’t have any
statistically significant influence to test results, having in mind
it is only one point.
V.
ELECTRIC FIELD IN VARIOUS POINTS OF REFERENCE
CROSS-SECTION
For visualizing field strength over whole reference crosssection all measured points were processed and placed on the
2D plot. To reduce number of measurement point half of the
reference plane was measured and the other half is mirror
reflection of the first one. Second side field was measured in
5 points to assure equality of both sides. Field values in
between measurement points were interpolated.
Starting from lowest frequencies field is quite even over
whole research area. There are two points on the top of the
image showing increased intensity of the Ey component of
the electric field (Figure 6). This is effect of the edges of
septum plane. In these areas field is curved. Investigating
various frequencies supplied to GTEM cell these uneven
areas might disappear due to current distribution in septum
plane. There are no uneven points on the bottom of the cell
due to GTEM cell construction has flat floor and geometrical
structure has no gaps to create uneven field.
Center part of the area is most important due to the fact that
all tests are carried in the center parts. This part looks even
and no noticeable peaks are spotted.
For higher frequency field uniformity appears to be more
uneven having much more peaks. 1 GHz electric field area is
shown in Fig. 7. On the bottom corners there is reduction of
the field strength from the floor corners. For lower
frequencies this effect was not noticeable due to much longer
wavelength is less sensitive to GTEM cell geometry. Center
area looks much more uniform due to lower geometry
changes and reflections.
Analysis of unwanted field component Ex distribution
shows similar tendency of the results however field strength is
lower, especially in the middle of test area (Fig. 8). At 30 MHz
frequency Ex field component has strongest values close to
the septum edges and gradually decreases. Still it is noticeable
Ex field on the side of the test area.
Fig. 7. Electric field Ey component strength distribution in GTEM cell
reference cross-section at 1 GHz.
Fig. 8. Electric field Ex component strength distribution in GTEM cell
reference cross-section at 30 MHz.
For higher frequencies Ex field strength becomes more
unequal, more field strength peaks are visible in the area (Fig.
9). The most important center area has more uniform field
strength distribution that is key important for the test results.
From unwanted field component Ez point of view field
strength looks much lower compared to Ey or even Ex
(Fig. 10). All field distribution imperfections can be noticed
on the top of the GTEM cell reference plane close to the
septum. Lower area looks very even.
Authorized licensed use limited to: GLASGOW CALEDONIAN UNIVERSITY. Downloaded on August 13,2022 at 22:10:49 UTC from IEEE Xplore. Restrictions apply.
Investigating higher frequencies at 1 GHz similar results
are obtained (Fig. 11). Field strength is much lower and
distribution is less even due to higher frequencies wavelength
is more similar to GTEM cell dimensions and all
imperfections are more noticeable.
CONCLUSIONS
Fig. 9. Electric field Ex component strength distribution in GTEM cell
reference cross-section at 1 GHz.
Field strength was measured in three different directions at
various point of the measurement plane of GTEM 1000 cell.
Three direction measurements allow not only get field
strength values in corresponding points, but measure
unwanted field strength that can influence immunity test
results and verify TEM mode of GTEM cell.
Field distribution strongly depends on frequency. For
lower frequencies up to 300 MHz field is very uniform. With
growing frequency field uniformity is more sensitive for
overall GTEM structure and possible imperfections of the
materials and geometry due to wavelength being close
dimension to GTEM cell structure.
Unwanted field components are more noticeable close to
the edges of measured area therefore influence to immunity
tests should be minor. Main Ey component of the field acts
similar as other components, but higher in value. Though
GTEM cell is not ideal environment to have excellent field
uniformity it is still reasonable good chamber for radiated
immunity test, having in mind that all imperfections of the
field should be corrected to make sure for all frequencies and
all area of device under test field strength is greater than the
limits required for the test.
REFERENCES
[1]
Fig. 10. Electric field Ez component strength distribution in GTEM cell
reference cross-section at 30 MHz.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Testing and measurement techniques - Emission and immunity testing
in transverse electromagnetic (TEM) waveguides (IEC61000-4-20),
2010.
A. Vie, B. Loader, D. Bownds, International Symposium on
Electromagnetic Compatibility EMC EUROPE, Wroclaw, Poland,
2016.
G. Shi, Y. Liao, X. Ying, Y. Zhang, Progress in Electromagnetic
Research Symposium, Shanghai, China, 2016, 603 p.
A. Nothofer, D. Bozec, A. Marvin, M. Alexander, L. McCormack,
Measurement Good Practice Guide, 2003.
P. M. Nicolae, I. D. Nicolae, D. G. Stanescu, IEEE International
Symposium on Electromagnetic Compatibility (EMC), 2012.
T. H. Jang, J. H. Lim, B. W. Lee, 7th Asia Pacific International
Symposium on Electromagnetic Compatibility, 2016.
M. A. Salhi, S. Cakir, M. Cinar, B. Tektas , M. Cetintas, International
Symposium on Electromagnetic Compatibility EMC EUROPE 2016,
Wroclaw, Poland, 2016.
D. Pouhe, B. Maier, International Conference on Electromagnetics in
Advanced Applications (ICEAA), 2012.
Fig. 11. Electric field Ez component strength distribution in GTEM cell
reference cross-section at 1 GHz.
Authorized licensed use limited to: GLASGOW CALEDONIAN UNIVERSITY. Downloaded on August 13,2022 at 22:10:49 UTC from IEEE Xplore. Restrictions apply.