Radiated Susceptibility Problem and A Windom - John Dunn, Consultant, Ambertec, P.E., P.C.

You can fail a radiated susceptibility EMI test if the physical dimensions of the unit under test (UUT) are just right. Such was the case with an aircraft fuel gauge in which the fuel probe's dimensions had been designed  to match up with the aircraft's fuel tank.

At almost any irradiation frequency, everything worked okay, but not at 120 MHz.

 

It turned out that the probe's length was a half-wavelength at 120 MHz and worse still, there was a right-angle joint at the 37% of length point as shown above.

Dimensionally, this fuel probe assembly was a classic Windom antenna at 120 MHz and when it was irradiated at that frequency, the supporting electronics was thrown into all kinds of malfunctions.

None of the probe's dimensions could be changed because of fuel tank restrictions. However, since the fuel tank was metallic and therefore a good RF shield, a waiver was granted because there would never be any 120 MHz irradiation of the fuel probe in actual service.

How do you spell "relief"?

Comments

[Gerald]

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.