ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 90)

90 |
Radiated Emissions Test Performance of GTEM!
Ibid, pp 329-342.
Ibid, pp 152-154.
Ibid, pp 150-154.
C
HARACTERISTIC AND
W
AVE
I
MPEDANCES OF A
GTEM!
The characteristic impedance of the GTEM! is set by the internal dimensions,
namely the width of the septum and the location, relative to the cross sectional
dimensions of the cell. The GTEM! is an asymmetric transmission line, but the
derivation follows the same approach as that used for a coaxial transmission line.
For brevity and simplicity, the derivation of the impedances of a coaxial line is
shown, instead of that of a GTEM!. Numerous literature references describe TEM
and GTEM! cell characteristic impedance calculation; contact ETS-Lindgren for
details.
A traditional coaxial transmission line, with inner conduct of radius a and outer
conduct with inner radius b, has per unit length values of capacitance and
inductance given by the equations
m F
a
b
C /
ln
2
0








m H
a
b
L / ln
2
0 








Where
and
The value of the ratio of b/a can be selected to give a characteristic impedance of
50 ohms, while the wave impedance is 377 ohms between the conductors. This
is a condition for TEM operation. While the geometry and the calculations are
more complete, the same conditions hold true for the GTEM!. The characteristic
impedance of the GTEM! is set by the cross-sectional dimensions to 50 ohms,
while maintaining TEM operation with a wave impedance value of 377 ohms.
0
0
0
2
2
0
0
2
) / ln(
) 2 (
) / (ln(


  

a b a b
C
L
Z
  
) / ln( 60 / ln(
2
120
2
) / ln(
0 0
a b a b
a b
Z
  



