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  Re-Distributed by http://www.tpub.com
Installation Comparability
Laboratory Measurements (C(f))
A major portion of the measurement effort was
All CWT FQIS components (including seven
to establish limits of comparability between the
fuel quantity probes, compensator probe, the FQIS
aircraft and laboratory FQIS installations. Once
connector, and terminal strip) were removed from
this was accomplished, laboratory measurement of
the retired aircraft in Roswell, New Mexico. The
RF power required for ionization events/heating of
components were reinstalled into a NASA LaRC
CWT FQIS components and wiring could be
reverberation chamber, providing a non-hazardous
correlated to the aircraft installation.
test environment.
RF input impedance measurements were
Careful consideration was required for
performed on the input terminals of the CWT FQIS
facilitating voltage and current measurements,
connection in a manner identical to those performed
monitoring for arcing/sparking, and detecting
on the actual aircraft. Analysis of this measurement
localized heating. These requirements needed to be
data was particularly challenging. Because the fuel
accommodated with an installation as electrically
quantity probes were designed to be capacitive, and
similar to the aircraft as possible. The system was
the wiring designed to be low-loss, the system
backed with an aluminum sheet. Fuel probes and
impedances were highly reactive. From 25 MHz to
wiring were installed with separation distances from
1 GHz, slight changes in wire routing in the
structure identical to that of the aircraft installation
laboratory installation could cause very large input
(to approximate the common mode impedances in
impedance variations at a given frequency. This
the aircraft). Nylon cable clamps recovered from
would also be the case for different aircraft
the actual aircraft CWT were used in the laboratory
installations. In fact, a laboratory test at the Naval
installation. The installation dimensions were
Surface Warfare Center (NSWC, Dahlgren, VA)
confined to fit within the field-of-view of infrared
demonstrated that fuel probe resonant frequencies
camera and ionization detection instruments (as best
changed dramatically with fuel level. Because of
as possible). The aluminum sheet was painted
these factors, it would be impossible to characterize
black to minimize infrared reflection. A shielded
the CWT FQIS at a specific frequency, and expect
box was installed to isolate the FQIS connection
similar results in the laboratory (or on another
from both the reverberation chamber (to prevent
aircraft, or even on the same aircraft with a different
interference with the approximated CWT RF
amount of fuel). Fortunately, the goal was not to
environment) and the RF generating equipment (for
characterize the FQIS at a specific frequency, but to
personnel safety). The final installation is shown in
establish electromagnetic comparability to the
Figure 4.
laboratory installation in a general sense (ie. as
good as any other aircraft installation). To
accomplish this goal, statistical analysis was
combined with a polar "Smith Chart" graphical
plotting technique, resulting in a novel analysis tool
for establishing comparability of input impedance.
This analysis was useful in demonstrating validity
comparing the laboratory CWT FQIS installation to
that of the aircraft. Details may be found in [5] and
[10].
Laboratory reverberation chamber cavity
coupling factor measurements were also performed
in a manner identical to those on the actual aircraft.
A scale factor was computed for comparing the
radiated field environment between the laboratory
chamber and the aircraft CWT from 25 MHz to 6
Figure 4. FQIS Components and Wiring
GHz. Details may be found in [5] and [9].
Installed in LaRC Reverberation Chamber B
6
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