Sources of Problems in the GTEM Field Structure and Recommended Sohitiohs
Diethard Hansenand Detlef Ristau
EURO EMC SERVICE Dr. HansenGmbH
D-14513 Teltow, PotsdamerStr. 18 A, Germany
William A Radaskyand Kenneth S. Smith
Metatech Corporation
358 Fairview Ave. Ste. E, Goleta, CA 93 117, USA
Abstract -- The E-field structure of a GTEM 1750 has been further
investigated in a number of relevant points within the test volume up
to one Gigahertz. An empty cell as well as a loaded cell have been
tested. The box under investigation was a metal cube of 30 cm. Additional support was gained from computer simulations, leading to
more detailed field contour plots. Based on this work a number of
improvementsand retrofits for existing GTEM cells is given.
The Gigahertz TEM Cell (GTEM) is a special TEM device, that
nowadays is used in about 300 test laboratories worldwide. This facility is utilized for both emission and immunity up to 1 GHz and
above. Based on previous comments and recommendationsin IEEE
EMC sessionsand including suggestionsfrom several members of
international standardizationgroups a very practical as well as challenging experiment was conducted.The GTEM cell is much smaller
than for example an anechoicchamber.Therefor it is expectedto find
in the GTEM cell a stronger interaction of the EUT with the shielding structure as in au anechoic chamber.The empty GTEM cell 1750
(8m long, 4m wide, 3m high) has been investigatedin the past, however further probing is necessaryin order to understandthe EUT interaction with the cell.
cube in the previously mentioned coordinates.Figure 1 shows the
field variation without the cube in place.
.._.......--_.....
-151
..-
-----
^.. -
..,,____-.
-_ .- ..-._
--
-
y.. .
behind --
-2o!
0
200
400
600
800
hiHz
f----c=-
Fig. 2: Variation of the E-field 30 cm before and behind the center
with cube in relation to the field in the center point of the test volume
:a
I
above -
EUT bfIER4mON
A metallic box (0.3 m cube) has been positioned in the center of the
typical test volume, which is down the center line along the floor of
the GTEM at 5.85 m from the apex at a height of 0.80 m above the’
floor. The septum height in this point is 1.6 m in total. The vertical
electrical field strength has beenmeasured30 cm perpendicularto all
6 surfacesof the cube using a small mini-dipole and an analog fiber
optic link systemas well as a spectrumanalyzer. Figures 2 to 4 show
these test results normalized to the non perturbed field without the
-8-I
0
200
400
600
800
I
MHz
f----
Fig. 3: Variation of the E-field 30 cm above and below the center
with cube in relation to the field in the center point of the test volume
u
v . ..~...._,_,,__
_._._
__.,__--5 :
0
200
400
600
800
MHz
f-
Fig. 1: Variation of the E-field 30 cm before and behind the center in
relation to the field in the center point of the test volume
O-7803-3207-5/96/$5.00
0 1996 IEEE
Fig. 4: Variation of the E-field 30 cm left and right of the center with
cube in relation to the field in the centerpoint of the test volume
48
Fig. 1 revealsthe typical spreadin field strengthvariation of about *
3 dB from 30 to 1000 MHz. Fig. 2 proves as expectedthe shadow
area behind the box. The most evident explanationis defractionby
the cube in the range below 300 MHz. In fig. 3 we find almost perfect symmetry.The curve labeledright is closerto the main access
door, causinga slight asymmetryto the surfacecurrents.
Em
14
E
V/m II
I
CELL BEHAVIOR
Due to the fine resonantstructuresin the loadedcell a more detailed
analysiswas made.In the position 5.85 m lengthand 0.8 m height we
took a referencemeasurementof the E field componentsabove the
center line (see fig. 5). The vertical field servesas the referencefor
the previous pictures and indicates2 peaksin the critical transition
areaof the absorbersection.The most affectedfrequencyrangeof the
working field componentis somewherein between60 and 100MHz.
In this range we also find the maximum of the mismatch in the
VSWR. In contrast to the expectedperfect TEM wave we find a
strong longitudinal componentat 123 MHz. Generallyone can state
that the overall decoupling of the longitudinal componentis fairly
weak. The transversecomponentis never a real problem and only
limited by our measurementdynamics of the FM 2000 Holaday
probe.
6
I
Ol
10
100
E-field components
E (V/m)
0....1,65
65....4,95
Volume Oriented Analysis at 123 MHz
95....8,25
Naturally one point is not sufficient, consequentlywe expandedinto
a triangular area over the center line towards the apex and a cross
section perpendicularto the center line through the referencepoint.
The incrementsin all caseswere 10 cm steps,defmed by the sensor
cube dimensions.In each measurementwe took the vertical and longitudinal component.Fig. 7 to 10 show the test results, Fig. 6 the
usedmagnitudescale.
25...11,55
55...14,85
85...18,15
Fig. 6: Used E-field magnitudescale
Fig. 7: E-field contour for the vertical componentin the section
towards the apex
Fig. 8: E-field contour for the longitudinal componentin the section
towards the apex
49
f/-MHz
1000
0.8
-0.8
-1.6
-0.8
0
0.8
1.6
-1.2 .a
-1.6
-0.8
0
0.8
1.6
Fig. 9: E-field contour for the vertical componentin the cross
section
0.8
-0.4
-0.8
Fig. 10: E-field contour for the longitudinal componentin the cross
section
Mapping the fields in figures 7 to 10 displaysa non TEM wave structure in some areas.Measuringthis field effects is extremely time and
data consuming.Therefor a computer simulation, using a time difference code,was performed and the resulting cross sectionscan be depicted from figure 11.
4
Calormap
E.7+
:’ 6to7
5 to
8 4to5
3 to
2 to
1 to
0 to
-1 to
-2 to
Key
Colormao
!!;;,7’.
6
4
3
2
1
0
-1
Kev
5 to 6
b>
I“.
-2
320-1
4to5
1 to
to2
to 431-1
too
Fig. 11 ComputedE-field contour in dB for the vertical a) and the
longitudinal b) componentin the cross section
Changing the Reference Point
The new point at 4.80 m length and 0.65 m height, where the septum
height above ground is 1.30 m, showsa better decouplingthan at 1.6
m height. Additionally the longitudinal peak is shifted higher in frequency,namely to 135 MHz (seefigure 12).
Moving in the other direction towardsthe absorberwall we have taken one more point (length 6.20 m, height 0.85 m). In figure 13 we detect below 80 MHz a very bad decoupling. The longitudinal
componentsupersedesthe vertical working component.
SOURCESOF
FIELDPROBLEMSANDRECOMMENDEDSOLUTIONS
Although the GTEM cell has proven to be a very useful tool for precompliance and compliance measurementsin EMC, there is still the
need for further improvements. Better than trying to improve the
emission correlation to open areatest sites by statisticsfor exampleis
it to find the sourcesof deviation inside the cell. For immunity tests
as one can seethere are some fairly sharpresonances.Comparingthis
fine structureof the fields, we find well designedabsorberchambers
still to be about 10 dB better in polarization decoupling. It becomes
evident from the abovementioned and the figures presentedthat one
of the main sources of problems in the GTEM cell is the “broad
band”terminator.
50
Fig. 12 E-field componentsin the point 4.80 m lengthand 0.65 m
height
3. Presentlythoseresistorsare installedon epoxyPCB with a typical
dielectric constant of 4.8. The conductivejunction islands on the
boardsbetweeneachsingle resistorare about 10 mm wide. This does
introduce too much capacitance.A further step could be, to cut out
the PCB material below the resistorbodies, leading to reduction of
capacitance.Reducingthe lead length of the resistorswith several
10thof nH will further improve the RF performanceof the boards.
4. Suppressingunwantedsurfacecurrentsin the comersof the cross
sectionsin the GTEM cell is going to improve the field quality [3].
5. As alreadyknown from EMP simulatorssince many years a tilted
resistive termination is very useful. The electric field lines would
than be in parallel to the resistors,aligning the natural flow of field
lines better.
Experimentsindicate that these measuresshould improve the field
quality in someareasof almost 10 dB.
REFERENCES
Fig. 13 E-field componentsin the point 6.20 m length and 0.85 m
height
r.11D. Hansen,D. Ristau et. al., “Analysis of the MeasuredField
1. Looking at the typical 5 dB reflectivity values for the 60 cm PU
cone absorbers at about 100 MHz one immediate improvement
would be adding ferrite tiles. This could gain another5 dB typically.
Foam and tiles, however,have to be impedancematched.
2. Reducingthe length of the foam absorberswill certainlyreducethe
capacitance.The resistor plates would be shorter and consequently
the inductanceof the plateswill decrease,preventingresonances.
51
Structure in a GTEM 1750”, in Proceedings of the IEEE EMC
Sym. 1994, Chicago,Aug. 1994,p. 144-149.
PI D. Hansen,D. Ristau et. al., “Expansionson the GTEM Field
StructureProblem”,in Proceedings of the IEEE EMC Sym.
1995, Atlanta, Aug. 1995, p. 538-542.
[31 L. Jendemalik,Zur Feldqualitat von TEM-Zellen (translated:
From the Field Quality in TEM Cells). Dortmund:
University Dortmund, Faculty of Electrical Engineering,dissertation, 1995