Patent ID: 6894624

Claim:
A passive transponder identification system for detecting and monitoring the existing of automobiles and other object within a constant detection area comprising a transmitter 100 to radiate two different radio frequency (RF) signals within the area to a transmitting antenna 110 , either of the signals being modulated to a fixed low frequency (LF) and an antenna 210 modulated with two different center frequencies; wherein an impedance semiconductor circuit part 220 inserted into said modulated antenna combines said two RF signals; and the identification system further includes a transponder 200 having a tank circuit to contain a resonant frequency equal to sum of two different frequencies to re-radiate a data-tone carrier signal to such combined frequency and a narrow-bandwidth receiver 300 having a receiving antenna 310 to receive and demodulate said carrier signal by excluding the transmitting RF signal and its harmonics to detect LF modulating signal.