Detectable device and movable item detecting system

• US 5,465,099 A
• Filed: 08/26/1994
• Issued: 11/07/1995
• Est. Priority Date: 09/25/1991
• Status: Expired due to Fees

• Current Assignee: Nippon Information Industry Corporation
• Original Assignee: Nippon Information Industry Corporation
• Inventors: Mitsui, Tsutomu, Shimokawa, Yukitsugu, Akaba, Noriyuki
• Primary Examiner(s): Hajec, Donald T.
• Assistant Examiner(s): LE, HOANGANH T
• Application Number:
  US08/296,859
• Time in Patent Office: 438 Days
• Field of Search: 343/730, 343/701, 343/729, 343/720, 343/725, 343/726, 343/727, 343/728, 343/866
• US Class Current: 343/730
• CPC Class Codes: G01S 13/753 using frequency selective elements, e.g. resonator

Abstract

A detectable device attachable to a movable item for detecting movement. Radio waves having a predetermined frequency are received by the detectable device, and radio waves having a harmonic frequency of the received waves are reradiated from the detectable device. The detectable device comprises a dipole antenna for receiving radio waves with said predetermined frequency, a diode connected in parallel to feed points of the dipole antenna for generating an electric current having a harmonic frequency of said received radio waves. A loop antenna is provided whose feed points are connected in parallel to feed points of the diode for converting the electric current having the harmonic frequency to radio waves for transmitting the radio waves at the harmonic frequency.

Claims

1. A detectable device attachable to a movable item that receives radio waves at a predetermined frequency and, after receipt thereof, reradiates harmonic radio waves of said received radio waves, the device comprising;

a dipole antenna for receiving and converting said radio waves at said predetermined frequency to an electric current at said predetermined frequency, the length of said dipole antenna being selected to be less than one-half the wavelength of said predetermined frequency so that said dipole antenna acts as a capacitance at said predetermined frequency;

a diode provided at feed points of said dipole antenna for generating an electric current at a frequency that is a harmonic of said received radio waves; and

a loop antenna whose feed points are connected in parallel to said diode for converting the electric current at said harmonic frequency to radio waves and transmitting the radio waves at said harmonic frequency, said loop antenna having a length selected that is less than the wavelength of said predetermined frequency that will cause the reactance of said loop antenna to be positive so that said loop antenna acts as an inductance at said predetermined frequency;

a compound capacitance of said dipole antenna and said diode and the inductance of said loop antenna constituting a parallel resonant circuit resonating at said predetermined frequency;

whereby current flowing through said diode is substantially increased.

2. The detectable device as claimed in claim 1, wherein the dipole antenna is an asymmetric dipole antenna.

3. The detectable device as claimed in claim 1, wherein part of the dipole antenna is common to part of the loop antenna.

4. A movable item detecting system comprising;

a detectable device attachable to a movable item that receives radio waves at a predetermined frequency and reradiates radio waves at a harmonic frequency of said received radio waves, and movable item detecting equipment having a transmitter for transmitting radio waves at said predetermined frequency within a predetermined space and a receiver for receiving radio waves at said harmonic frequency from the detectable device existing in the space so that the movable item detecting equipment can detect the movable item, wherein the detectable device comprises;

a dipole antenna for receiving radio waves having said predetermined frequency and converting said radio waves to an electric current at said predetermined frequency, said dipole antenna having a length that is less than one-half the wavelength of said predetermined frequency so that said dipole antenna acts as a capacitance at said predetermined frequency;

a diode connected in parallel to feed points of the dipole antenna for generating an electric current that is a harmonic frequency of said received radio waves;

a loop antenna whose feed points are connected in parallel to said diode for converting the electric current at said harmonic frequency to radio waves for transmitting the radio waves at the harmonic frequency, said loop antenna having a length selected that is less than the wavelength of said predetermined frequency that will cause the reactance of said loop antenna to be positive so that said loop antenna acts as an inductance at said predetermined frequency;

a compound capacitance of said dipole antenna and said diode and said inductance of said loop antenna being a parallel resonant circuit resonating at said predetermined frequency;

whereby said electric flowing through said diode is substantially increased.

5. A detectable device attachable to a movable item to detect movement of said item, said detectable device receiving radio waves at a predetermined frequency and reradiating harmonics of said received radio waves, the improvement comprising;

a dipole antenna receiving and converting said radio waves at said predetermined frequency to an electric current at a predetermined frequency, said dipole antenna having a length that is less than one-half the wavelength of said predetermined frequency so that said dipole antenna acts as a capacitance at said predetermined frequency;

a diode connected in parallel to feed points of the dipole antenna for generating an electric current that is a harmonic frequency of said received radio waves; and

a loop antenna whose feed points are connected in parallel to said diode for converting the electric current at said harmonic frequency to radio waves for transmitting the radio waves at the harmonic frequency, said loop antenna having a length selected that is less than the wavelength of said predetermined frequency that will cause the reactance of said loop antenna to be positive so that said loop antenna acts as an inductance at said predetermine, frequency;

said compound capacitance of a dipole antenna and said diode and said inductance of said loop antenna being a parallel resonant circuit resonating at said predetermined frequency;

whereby said electric flowing through said diode is substantially increased.

6. A device according to claim 5 in which said dipole antenna is an asymmetric dipole antenna.

7. A device according to claim 5 in which part of said dipole antenna is common to part of said loop antenna.

8. A movable item detecting system comprising;

a detectable device attachable to a movable item to detect movement of said item, said detectable device receiving radio waves at a predetermined frequency and reradiating harmonics of said receiving radio waves;

detecting equipment for detecting movement of said detectable device having a transmitter for transmitting radio waves at said predetermined frequency in a predetermined space and a receiver for receiving radio waves at said harmonics of said predetermined frequency from said detectable device for detecting movement of a movable item in said predetermined space; and

detectable device comprising;

a dipole antenna receiving and converting said radio waves at said predetermined frequency to an electric current at said predetermined frequency, said dipole antenna having a length that is less than one-half the wavelength of said predetermined frequency so that said dipole antenna acts as a capacitance at said predetermined frequency;

a diode connected in parallel with feed points of said dipole antenna for generating an electric current at a frequency that is harmonic of said predetermined frequency of said received radio waves; and

a loop antenna having feed points connected in parallel with said diode for transmitting the harmonic frequency of said radio waves converted by said diode, said loop antenna having a length selected that is less than the wavelength of said predetermined frequency that will cause the reactance of said loop antenna to be positive so that said loop antenna acts as an inductance at said predetermined frequency;

a compound capacitance of said dipole antenna and said diode and said inductance of said loop antenna being a parallel resonant circuit resonating at said predetermined frequency;

whereby said electric flowing through said diode is substantially increased.