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There is a diaphram in the eye of eagle which get vibrated
when sound wave fall on it . It consisted of a tiny capacitive membrane
connected to a small quarter-wavelength antenna; it had no power supply or
active electronic components. The device became active only when a radio signal
of the correct frequency was sent to the device from an external transmitter.
Sound waves caused the membrane to vibrate, which varied the capacitance of the
circuit.When capacitance get changed , operating frequency get changed.When this
changed frequency supply reached Antenna circuit, different EM wave is produced
which get modulated with incoming EM wave and is get re-transmitted by the
Thing. A receiver demodulated the signal so that sound picked up by the
microphone could be heard, just as an ordinary radio receiver demodulates radio
signals and outputs sound.