An aircraft is comprised of many pieces of metal with different lengths. The fuselage is the longest metallic piece. The fuselage is basically an antenna. The other pieces of metal inside the aircraft are also antennas.
Now, all solid objects have a sonic resonance frequency. Similarly, all metallic objects have an electromagnetic resonance frequency.
The fuselage has an electromagnetic resonant frequency. The ailerons, elevators, vertical stabilizers (probably metallic ribs), metallic wing ribs, metallic landing gear, etc. have their own resonant frequency.
With modern amplification and digital processing software (to exclude background noise), the detector is able to isolate the signal return from the aircraft due to half-wave and quarter-wave resonance of those metallic pieces.
There is no known way to stop a resonant frequency. It is a physical property. When the electromagnetic wave hits the piece of metal, it creates a standing wave. The electrons wiggle at the resonant frequency. This turns into a transmitter like an antenna. This is exactly how an antenna works. You pump a signal into an antenna. In the past, you couldn't distinguish the signal coming off resonance because it was near the noise level. Now, you can.
The only method is to stop the electromagnetic wave from reaching the piece of metal. The stealth coating is designed to interfere with the electromagnetic wave from reaching the piece of metal. However, the stealth coating only works at one (or maybe two) frequency band.
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