Apparatus and method for radio frequency transponder with improved read distance

• US 6,147,606 A
• Filed: 03/26/1999
• Issued: 11/14/2000
• Est. Priority Date: 03/26/1998
• Status: Expired due to Term

• Current Assignee: Intermec IP Corporation (Honeywell International Inc.)
• Original Assignee: Intermec IP Corporation (Honeywell International Inc.)
• Inventors: Duan, Dah-Weih
• Primary Examiner(s): Wu, Daniel J.
• Assistant Examiner(s): PHAM, TOAN NGOC
• Application Number:
  US09/277,271
• Time in Patent Office: 599 Days
• Field of Search: 340/572.1, 340/572.4, 340/572.7, 340/825.54, 340/505, 342/51, 343/867, 343/742, 343/701, 343/818, 343/802, 343/852, 343/860, 343/873, 343/793, 343/855, 343/895
• US Class Current: 340/572.7
• CPC Class Codes: G06K 19/0723 the record carrier comprising an arrangement for non-contact communication, e.g. wireless communication circuits on transponder cards, non-contact smart cards or RFIDs
• CPC Class Codes: G06K 19/07749 the record carrier being capable of non-contact communication, e.g. constructional details of the antenna of a non-contact smart card
• CPC Class Codes: G06K 19/07786 the antenna being of the HF type, such as a dipole
• CPC Class Codes: H01Q 1/2225 used in active tags, i.e. provided with its own power source or in passive tags, i.e. deriving power from RF signal
• CPC Class Codes: H01Q 1/38 formed by a conductive layer on an insulating support patch antennas H01Q9/0407; microstrip dipole antennas H01Q9/065; microstrip slot antennas H01Q13/106; transmission line microstrip antennas H01Q13/206; manufacturing reflecting surfaces using insulating material for supporting the reflecting surface H01Q15/142
• CPC Class Codes: H01Q 9/26 with folded element or elements, the folded parts being spaced apart a small fraction of operating wavelength resonant loop antennas H01Q7/00

Abstract

A radio frequency identification transponder (RFID tag) in accordance with the principles of the present includes a tag antenna having a gain and an input impedance. The tag also includes RFID circuitry, which may be in the form of an RFID tag integrated circuit, the RFID circuitry also being characterized, in part, by an input impedance. According to the principles of the present invention the tag'"'"'s read distance is maximized by tuning the tag antenna impedance and gain. In general, the imaginary part of the antenna impedance is chosen as the negative of the imaginary part of the RFID circuitry'"'"'s impedance. Additionally, the real part of the tag antenna'"'"'s impedance is selected by tuning those geometrical antenna parameters that affect both the antenna'"'"'s gain and impedance. For RFID tags wherein the geometrical parameters of the tag antenna do not affect the gain G_a of the tag antenna, the antenna input impedance may tuned to establish an antenna input impedance which is the complex conjugate of the RFID circuitry'"'"'s input impedance.

Claims

1. A radio frequency identification transponder (RFID tag) comprising:

a tag antenna having a gain G_a and an impedance Z_a ; and

radio frequency identification circuitry responsive to electromagnetic signals received at the tag antenna, the circuitry having an input impedance Z_l the antenna having geometric characteristics, the gain and the impedance of the antenna being a function of the geometric characteristics, the geometric characteristics of the antenna being tuned to maximize the value of [(Ra)(Ga)]^1/2 /|Za+Zl|.

2. The RFID tag of claim 1 wherein the imaginary part of the antenna impedance is the negative of the imaginary part of the load impedance.

3. The RFID tag of claim 2 wherein the load impedance is minimized.

4. The RFID tag of claim 1 wherein the geometric parameters of the antenna which affect the antenna impedance have a negligible effect for a particular application on the antenna gain and the antenna impedance Z_a is the complex conjugate of the load impedance Z_l.

5. The RFID tag of claim 4 wherein the geometric parameters of the antenna establish a real part of the antenna impedance which maximizes the expression [(Ra)(Ga)]^1/2 /|Za+Zl|.

6. The RFID tag of claim 4 wherein the negligible effect is less than or equal to five percent of the antenna gain.

8. The RFID system of claim 6 wherein the imaginary part of the antenna impedance is the negative of the imaginary part of the load impedance.

9. The RFID tag of claim 8 wherein the load impedance is minimized.

10. The RFID system of claim 9 wherein the geometric parameters of the antenna establish a real part of the antenna impedance which maximizes the expression [(Ra)(Ga)]^1/2 /|Za+Zl|.

7. An RFID system comprising:

a base station; and

an RFID tag, the tag including;

a tag antenna having a gain G_a and an impedance Z_a ; and

radio frequency identification circuitry responsive to electromagnetic signals received at the tag antenna, the circuitry having an input impedance Z_l, the antenna having geometric characteristics, the gain and the impedance of the antenna being a function of the geometric characteristics, the geometric characteristics of the antenna maximizing the value of [(Ra)(Ga)]^1/2 /|Za+Zl|.

11. The RFID tag of claim 7 herein the geometric parameters of the antenna which affect the antenna impedance have a negligible effect for a particular application on the antenna gain and the antenna impedance Z_a is the complex conjugate of the load impedance Z_l.

12. A method of maximizing an RFID tag read range, the tag including an antenna having a gain G_a and an impedance Z_a, the gain and the impedance of the antenna being a function of geometric characteristics of the antenna, and a load having an input impedance Z_l, comprising the steps of:

determining the impedance Z_l of the load; and

tuning the geometric characteristics of the antenna to maximize the value of [(Ra)(Ga)]^1/2 /|Za+Zl|.

13. The method of claim 12 wherein the antenna impedance is tuned such that the imaginary part of the antenna impedance is the negative of the imaginary part of the load impedance.

14. The method of claim 12 wherein the geometric parameters of the antenna which affect the antenna impedance have a negligible effect for a particular application on the antenna gain and the antenna impedance Z_a is tuned to be the complex conjugate of the load impedance Z_l.

15. The method of claim 12 wherein the load impedance is minimized.

16. The method of claim 15 wherein the geometric parameters of the antenna are tuned to establish a real part of the antenna impedance which maximizes the expression [(Ra)(Ga)]^1/2 /|Za+Zl|.