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大戦後期に連合軍で広く使用されたIFF Mk.3についてウィキペディアのこんな記述が
https://en.wikipedia.org/wiki/IFF_Mark_III
"Although very successful, Mark III had problems of its own.
Primary among them was that it would respond to any signal
across a wide variety of frequencies around 180 MHz.
An enemy that knew this could send out random signals on this band
and receive strong signals that could be used in a radio direction finder
to gain high-quality information about the position of any aircraft carrying a Mark III transponder. "