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In an electronic detection system comprising a transmitter for generating an interrogation field, said transmitter being coupled with at least one transmitting antenna coil; a responder with a receiving coil and a transmitting coil for transmitting a signal in response to said interrogation field; a

In an electronic detection system comprising a transmitter for generating an interrogation field, said transmitter being coupled with at least one transmitting antenna coil; a responder with a receiving coil and a transmitting coil for transmitting a signal in response to said interrogation field; and a receiver-and-detector coupled with at least one receiving antenna coil for receiving and further processing the signal transmitted by said responder; the improvement which consists in that said receiving coil and said transmitting coil of said responder are arranged in parallel to each other, and that said responder comprises a frequency divider connected between said receiving and transmitting coil and arranged to divide the signal frequency received by a factor N≥4.

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1. In an electronic detection system, comprising: transmitter means for generating an interrogation field, said transmitter means being coupled with at least one transmitting antenna coil; responder means including a receiving coil for receiving a signal having a signal frequency and a transmitt

1. In an electronic detection system, comprising: transmitter means for generating an interrogation field, said transmitter means being coupled with at least one transmitting antenna coil; responder means including a receiving coil for receiving a signal having a signal frequency and a transmitting coil for transmitting a signal in response to said interrogation field; and receiver-and-detector means coupled with at least one receiving antenna coil for receiving and further processing the signal transmitted by said responder means; the improvement wherein said receiving coil and said transmitting coil of said responder means are arranged in parallel to each other, and said responder means comprises a frequency divider connected between said receiving coil and said transmitting coil and arranged to divide the signal frequency received by a factor of N≥4. 2. A detection system according to claim 1, wherein the receiving coil and the transmitting coil of the responder means are jointly arranged on a single ferrite rod. 3. A detection system according to any one of claims 1 or 2, wherein the frequency divider is a divide-by-eight circuit. 4. A detection system according to claim 3, wherein the responder means comrpises a rectifying and smoothing circuit connected between the receiving coil and the frequency divider. 5. A detection system as claimed in claim 1 or 2, wherein the transmitter means comprises a high-frequency oscillator which, via at least one power amplifier, energizes the transmitting antenna coil and generates an output signal, and a modulation oscillator for frequency modulating the output from said high-frequency oscillator; and wherein the transmitting means includes a receiver portion having a synchronous detector, said modulation oscillator providing a reference signal to the synchronous detector of the receiver portion; the receiver-and-detector means comprising a filtering device selectively tuned to the frequency transmitted by the responder; said receiver portion including an FM demodulator connected to said synchronous detector and producing a demodulator output signal, said synchronous detector providing said demodulator output signal as a detector output if the demodulator output signal is sufficiently in phase with said reference signal provided to said synchronous detector by said modulation oscillator. 6. A detection system according to claim 5, wherein said synchronous detector comprises a synchronous switching device controlled by the reference signal and connected between a first terminal of an input to which the demodulator output signal is supplied and a parallel circuit comprising first and second branches, the first branch including a resistor and a diode, the second branch including a resistor and a Zener diode, said parallel circuit being further connected to a negative input of an operational amplifier connected as an integrator, the operational amplifier having an output which forms an output of the synchronous detector, and the operational amplifier having a positive input connected to a second terminal of said input. 7. A detection system according to claim 5, wherein the output of said synchronous detector is connected to a level detector which issues an energizing signal to an alarm signaling device as soon as the output signal of said synchronous detector reaches a set level. 8. A detection system according to claim 7, wherein said level detector is a monostable multivibrator. 9. A detection system according to any one of claims 1 or 2, wherein said transmitter means comprises a high-frequency oscillator which, via at least one power amplifier, energizes the transmitting antenna coil and generates a high-frequency output, and a frequency divider connected to said high-frequency oscillator for dividing the high-frequency output by said factor N by which the frequency is divided in said responder means to obtain a frequency divider output, the frequency divider output being supplied to a phase separator having four outputs carrying phase separator signals differing by 90° in phase relative to each other, said phase separator signals being supplied as reference signals to a synchronous detection device of the receiver-and-detector means. 10. A detection system as claimed in claim 9, wherein the synchornous detection device comprises four synchoronous detectors having outputs, each of the four synchronous detectors being supplied with one of the output signals of said phase separator and with the signal received by the receiving coil of said responder means, the outputs of said synchronous detectors being connected to an OR gate having an output which is connected to a level detector. 11. A detection system as claimed in claim 10, wherein said level detector is a monostable multivibrator with an adjustable detection level. 12. A detection system as claimed in any one of claims 1 or 2, wherein the transmitting antenna coil is also the receiving antenna coil with a separator being provided for coupling with the transmitter means and the receiver-and-detector means, respectively. 13. A detection system as claimed in claim 12, wherein said separator and at least one transmitting/receiving antenna coil form an antenna circuit, the transmitting/receiving antenna coil and a first capacitor forming a first LC circuit capable of resonating to the transmission frequency, and wherein a second LC circuit is cascade-connected to said first LC circuit, said second LC circuit being capable of resonating to the frequency transmitted by said responder means, said second LC circuit including a second coil, a second capacitor, and at least one transmitting/receiving antenna coil, and further including a third capacitor which, together with said second coil, forms a circuit resonating to the transmission frequency. 14. A detection system as claimed in claim 13, wherein an antenna circuit symmetrical relative to ground is formed by duplication of said second coil and said second and third capacitors, said antenna circuit being energized via a phase separator and two associated amplifiers by two signals differing by 180° in phase relative to each other. 15. A detection system according to claim 12, wherein two transmitting/receiving antenna coils are disposed substantially at right angles to each other, said coils being energized via associated antenna circuits, with signals being phase-shifted through 90° relative to each other for generating a rotary field. 16. A detection system as claimed in claim 6 wherein the output of said synchronous detector is connected to a level detector which issues an energizing signal to an alarm signalling device as soon as the output signal of said synchronous detector reaches a set level. 17. A detection system as claimed in claim 13, wherein an antenna circuit symmetrical relative to ground is formed by duplication of said second coil and said second and third capacitors, said antenna circuit being energized via a phase separator and two associated amplifiers by two signals differing by 180° in phase relative to each other. 18. A detection system as claimed in claim 13, wherein two transmitting/receiving antenna coils are disposed substantially at right angles to each other, said coils being energized via associated antenna circuits, with signals being phase-shifted through 90° relative to each other for generating a rotary field. 19. A detection system as claimed in claim 14 wherein two transmitting/receiving antenna coils are disposed substantially at right angles to each other, said coils being energized via associated antenna circuits, with signals being phase-shifted through 90° relative to each other for generating a rotary field. 20. A detection system according to any one of claims 1 or 2, wherein the responder means comprises a rectifying and smoothing circuit connected between the receiving coil and the frequency divider.