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December 21, 2010


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Yep, even if you could have changed the dimensions of the probe you'd only just move its resonant frequency. Glad that they accepted the tank as a shield. I've had cases where I wasn't so fortunate, and had to add filters for signals that weren't a problem in the field. So, in a long way, I spell "relief" as "competent compliance decisions made by people who understand the whole system."

Sri Harsha

This is one of the good examples. I was wondering at what distance is the RF source from the UUT.

John D.

The distance from the RF source to the UUT was several feet, but for complete info, the test was being done at Retlif in Ronkonkoma, NY per MIL-STD-461C.

Brian D.

More than likely the fuel probe not changing was not because of fuel tank restrictions and more because the length of the probe and its capacitance value were already set contributing values to the overall probe array. To change the length would have resulted in other probes in the array having to change and then of course software that uses the capacitance data also having to change. And resultant requals all the way around.

Also, in addition to the metal skin of the air frame & fuel tanks their would be a fuel computer residing between the probe array and the "fuel gauge" and their would be EMI filtering capability most likely in a portion of the computer. aka - clean part of the box / dirty part of the box scenario. Thereby the passive probe array could never act as an antenna between aspects of filtering in the computer and shielding in the gauging harness and contributions from metal in the tank and aircraft skin. More than likely, the test configuration may not have been fully representative of the fielded configuration. At least it's a possibility. In either case the basis for the waiver is correct.

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