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A method and apparatus for tracking items automatically is described. A passive RFID (Radio Frequency IDentification) tag is used with a material tracking system capable of real-time pinpoint location and identification of thousands of items in production and storage areas. Passive RFID tags are att

A method and apparatus for tracking items automatically is described. A passive RFID (Radio Frequency IDentification) tag is used with a material tracking system capable of real-time pinpoint location and identification of thousands of items in production and storage areas. Passive RFID tags are attached to the item to be tracked, remote sensing antennas are placed at each remote location to be monitored, interrogators with several antenna inputs are connected to the sensing antennas to multiplex the antenna signals, and a host computer communicates with the interrogators to determine item locations to an exacting measure.

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1. An antenna, comprising:a single layer flat metallic coil; a capacitor coupled to the coil at a connection point; a diode having its anode coupled to the coil and to the capacitor at the connection point between the coil and the capacitor, the diode generating a rectified peak voltage indicative o

1. An antenna, comprising:a single layer flat metallic coil; a capacitor coupled to the coil at a connection point; a diode having its anode coupled to the coil and to the capacitor at the connection point between the coil and the capacitor, the diode generating a rectified peak voltage indicative of the load on the capacitor and coil; and a conductive plate positioned a predetermined distance apart from the coil. 2. The antenna of claim 1, wherein the conductive plate is ferrous.3. The antenna of claim 1, wherein the predetermined distance is approximately ⅜ inches.4. An antenna, comprising:a flat single layer coil of metal; and a conductive plate in the same plane as the flat single layer coil of metal and separated by a predetermined distance from the coil to preload the coil to reduce sensitivity of the coil to other ferrous material. 5. The antenna of claim 4, wherein the conductive plate is ferrous.6. The antenna of claim 4, wherein the conductive plate and the coil are separated by a distance of approximately ⅜ inches.7. The antenna of claim 4, wherein the flat single layer coil of metal is a flat single layer copper coil.8. An antenna, comprising:a substantially circular core, having an annular ring protruding substantially perpendicular to the plane of the core, and having a substantially centered core post extending substantially perpendicular to the plane of the core; and a coil wound around the post between the post and the annular ring. 9. The antenna of claim 8, wherein the antenna is configured to a size for operation with a tag having an operating range of about 0 to 20 inches.10. The antenna of claim 8, wherein the antenna is configured to a size for operation with a tag having an operating range of about 15 to 20 inches.11. The antenna of claim 8, wherein the antenna is configured to a size for operation with a tag having an operating range of about 2 inches.12. An antenna for connection to a mutiple input interrogator having a common detection circuit, the antenna comprising:a flat coil; a capacitor connected with the flat coil to form a tuned circuit; a diode having its anode connected between the coil and the capacitor, the diode generating a rectified peak voltage indicative of the load on the capacitor and coil; a conductive plate positioned a predetermined distance apart from the coil. 13. The antenna of claim 14, wherein the predetermined distance is approximately ⅜ inches.14. The antenna of claim 12, wherein the antenna is configured to a size for operation with a tag having an operating range of about 0 to 15 inches.15. The antenna of claim 12, wherein the antenna is configured to be driven with a signal operating at 125 kHz.16. An antenna for use in a RFID interrogation system, comprising:a substantially circular core, having an annular ring protruding substantially perpendicular to the plane of the core, and having a substantially centered core post extending substantially perpendicular to the plane of the core; a coil wound around the post between the post and the annular ring; and a capacitor connected in series with the coil. 17. The antenna of claim 16, wherein the antenna is configured to a size for operation with a tag having an operating range of about 0 to 20 inches.18. The antenna of claim 16, wherein the antenna is configured to a size for operation with a tag having an operating range of about 15 to 20 inches.19. The antenna of claim 16, wherein the antenna is configured to a size for operation with a tag having an operating range of about 2 inches.20. The antenna of claim 16, wherein the coil is a copper coil.21. An antenna for connection to a multiple input interrogator having a common detection circuit, the antenna comprising:a substantially circular core, having an annular ring protruding substantially perpendicular to the plane of the core, and having a substantially centered core post extending substantially perpendicular to the plane of the core; a coil wound around the post between the post and the annular ring; a capacitor connected to the coil to form a tuned circuit; and a diode having its anode connected between the coil and the capacitor, the diode generating a rectified peak voltage indicative of the load on the capacitor and coil. 22. The claim 21, wherein the antenna is configured to a size for operation with a tag having an operating range of about 0 to 20 inches.23. The antenna of claim 21, wherein the capacitor has a capacitance of about 3000 picofarads.24. The antenna of claim 21, wherein the antenna is configured to a size for operation with a tag having an operating range of about 2 inches.25. The antenna of claim 21, wherein the coil is a copper coil.26. The antenna of claim 21, wherein the antenna is configured to be driven with a signal operating at 125 kHz.27. An antenna array for use in a RFID interrogation system, comprising:a first antenna including a first coil of metal arranged in a flat radial single layer formation, the first coil connected in series with a first capacitor; and a second antenna including a substantially circular core, having an annular ring protruding substantially perpendicular to the plane of the core, and having a substantially centered core post extending substantially perpendicular to the plane of the core, a second coil wound around the post between the post and the annular ring, the second coil connected in series with a second capacitor. 28. The antenna array of claim 27, herein the first coil and the first capacitor form a first tuned circuit and the second coil and the second capacitor form a second tuned circuit.29. The antenna array of claim 27, wherein the second capacitor has a capacitance of about 3000 picofarads.30. The antenna array of claim 27, wherein the first coil has an inductance of about 800 microhenrys.31. The antenna array of claim 27, wherein the first antenna and the second antenna are configured to a size for each to operate with a tag having an operating range of about 2 inches.32. The antenna array of claim 27, wherein the first coil and the second coil are copper coils.33. The antenna array of claim 27, wherein the first antenna and the second antenna are configured to be driven with a signal operating at 125 kHz.34. An antenna, comprising:a single layer flat metallic coil; a capacitor coupled to the coil at a connection point; a diode having its anode coupled to the coil and to the capacitor at the connection point between the coil and the capacitor, the diode generating a rectified peak voltage indicative of the load on the capacitor and coil; and a means for reducing sensitivity of the coil to metals. 35. The antenna of claim 34, wherein the coil and the means for reducing sensitivity of the coil to metals are separated by about ⅜ of an inch.36. The antenna of claim 34, wherein the coil and the capacitor are configured as a tuned circuit with the means for reducing sensitivity of the coil to metals separated from the coil by a predetermined distance.37. The antenna of claim 34, wherein the means for reducing sensitivity of the coil to metals includes a means for magnetically preloading the flat coil.38. The antenna of claim 34, wherein the means for reducing sensitivity of the coil to metals includes a ferrous cover plate.39. The antenna of claim 1, wherein the coil is configured to a size to operate with a tag having an operating range of 20 inches or less.