ETS-Lindgren 5411 GTEM! Test Cell - Manual (Page 91)

Radiated Emissions Test Performance of GTEM!
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T
ERMINATION
C
HARACTERISTICS
Having derived the transmission line characteristics of a coaxial transmission
line, it is postulated that the characteristics of a GTEM! match those of the
coaxial line. The characteristic impedance is 50 ohms, and the TEM mode exists.
The next step is description of the termination performance. It was stated
previously that the GTEM! is a doubly-terminated or hybrid-load device. The
two terminations are: resistive to match the current flowing in the septum, and
RF absorbers to terminate (absorb) electromagnetic fields propagating to the
termination.
The performances of these two terminations separately and in combination are
discussed in the following sections.
Load Boards
The GTEM! uses a set of large printed circuit boards, which support the large
number of resistors that compose the resistive termination. At low frequencies,
the match of the resistor boards is dominant and the return loss of the GTEM! is
that of the load boards by themselves. As frequency increases, the parasitic
elements of the resistor chains degrade performance, as does the termination of
the RF absorbers that appears as a capacitive element, causing the return loss
to increase.
RF Absorber
The second part of the GTEM! hybrid termination is the array of RF absorbers. At
low frequencies, the RF absorber shows very poor return loss. As frequency
increases, however, the RF absorber match improves with a consequent
improvement in the return loss due to the absorber.
Combined Performance
From a transmission line perspective, the two terminations act in parallel. The
load boards dominate the response at low frequencies, while the RF absorbers
dominate the response at higher frequencies. The transition between the
responses shows as an increase in the return loss at what is called the critical,
characteristic, or termination crossover frequency of the GTEM!. This critical
frequency is seen as an increase in the VSWR of the GTEM!. The critical
frequency depends on the size of the GTEM! and the volume of absorber.