System for minimizing coupling nulls
A system is disclosed for minimizing coupling nulls between an electromagnetic field derived from one or more sources and a plurality of randomly oriented RFID tags, wherein the electromagnetic field is rotated relative to the tags such that no tag is persistently located in a coupling null relative to the field. The source of the electromagnetic field may include a passive antenna or loop that changes its orientation relative to a direction of movement of the tags. The source of the electromagnetic field may further include an active antenna or loop that is electromagnetically coupled to the passive antenna or loop. A method for minimizing coupling nulls between the electromagnetic field and the randomly oriented tags is also disclosed.
1. A system for at least minimizing coupling nulls between an electromagnetic field and a plurality of randomly oriented RFID tags;
wherein said electromagnetic field includes a main axis that is oriented obliquely and is rotated relative to a direction of movement of said plurality of RFID tags;
said electromagnetic field being derived from an active fixed loop antenna having an inscribed circle having a first diameter and an axis, and a mechanically separate passive rotating loop antenna that lies in a non-parallel plane relative to said fixed loop antenna, said rotating loop antenna being circumscribed by an ellipse whose cylinder of rotation has a second diameter and is coaxially disposed about said axis of said fixed loop antenna; and
said second diameter being less than said first diameter such that said fixed loop antenna and said rotating loop antenna are electromagnetically coupled, said plurality of RFID tags being arranged to travel within dimensions of said fixed loop antenna and said rotating loop antenna such that no tag is persistently located in a coupling null relative to said electromagnetic field.
2. A system according to claim 1 wherein said main axis of said electromagnetic field is oriented at an acute angle relative to said direction of movement of said plurality of RFID tags.
3. A system according to claim 1 wherein said main axis of said electromagnetic field is oriented substantially at 45 degrees relative to said direction of movement of said plurality of RFID tags.
4. A system according to claim 1 wherein each of said plurality of RFID tags is arranged to rotate relative to said fixed loop antenna during movement of said RFID tag in said direction of movement.
5. A method for at least minimizing coupling nulls between an electromagnetic field and a plurality of randomly oriented RFID tags, wherein said electromagnetic field is derived from an active fixed loop antenna having an inscribed circle having a first diameter and an axis, and a mechanically separate passive loop antenna that lies in a non-parallel plane relative to said fixed loop antenna, said separate loop antenna being circumscribed by an ellipse whose cylinder of rotation has a second diameter and is coaxially disposed about said axis of said fixed loop antenna, said second diameter being less than said first diameter such that said fixed loop antenna and said separate loop antenna are electromagnetically coupled, said method including the steps of:
orienting a main axis of said electromagnetic field obliquely relative to a direction of movement of said plurality of RFID tags;
rotating said separate loop antenna to cause rotation of said electromagnetic field relative to said direction of movement of said plurality of RFID tags; and
passing said plurality of RFID tags within dimensions of said fixed loop antenna and rotating separate loop antenna such that no tag is persistently located in a coupling null relative to said electromagnetic field.
6. A method according to claim 5 wherein said main axis of said electromagnetic field is oriented at an acute angle relative to said direction of movement of said plurality of RFID tags.
7. A method according to claim 5 wherein said main axis of said electromagnetic field is oriented substantially at 45 degrees relative to said direction of movement of said plurality of RFID tags.
8. A method according to claim 5 including rotating each of said plurality of RFID tags relative to said fixed loop antenna during movement of said plurality of RFID tags in said direction of movement.