A Parametric Study on the Platform Tolerance of RFID Antennas and their Performance Enhancement with Artificial Magnetic Conductors A. S. Hoenshel and R. Mittra EMC Lab Pennsylvania State University University Park, PA 16802

RFID Antenna Applications Electronic Toll Collection Access Control Animal Tracking Inventory Control Tracking Runners in Races!

RFID Antenna Design Challenges Small Size Planar UHF Frequency Allocation – Europe MHz – North America MHz Impedance Matching – ASIC Chip: High Capacitive Value, Small Resistive Value Environmental Conditions

Scope of Presentation Mounting Materials Cardboard (  r =2.5) Glass with No Loss(  r =3.8) Glass with Loss(  r =2.5) and Loss Tangent Plastic (  r =4.7) Performance Enhancement using Artificial Magnetic Conductors (AMCs) Two Planar-Inverted F-Antenna Designs

Antenna Parameters Investigated Characteristic Impedance – Power 3-Dimensional Radiation Patterns Maximum Directivity where Z a = R a + j X a is the antenna impedance and Z s = R s + j X s is the source impedance and Z s = R s + j X s is the source impedance where U max is the radiation intensity in maximum direction and P rad is the total radiated power * Source: Constantine A. Balanis. Antenna Theory, Analysis and Design, New Jersey, (Third Edition) John Wiley and Sons. * *

ASIC Chip: Z c =10-j160 [  ] at 867 MHz Z c =10-j150 [  ] at 915 MHz Z c =10-j145 [  ] at 940 MHz 62mm 51mm = mm 5mm Dual-band Frequency Operation Gap Dimension and Stub Dimension Used to Tune Platform Tolerance Dominating Horizontal Current Distribution Open-Ended Stub PIFA Design * Source: M. Hirvonen, K. Jaakkola, P. Pursula, and J. Saily, “Dual-Band Platform Tolerant Antennas for Radio- Frequency Identification,” IEEE Trans. Antennas Propag., vol. 54, no. 9, pp , Sept

Mounting Materials Dimensions 900 mm x 900 mm (4 x 4 ) Thickness=13 mm Cardboard (  r =2.5) Glass(  r =3.8) Plastic(  r =4.7) Tag Dimension 62 mm x 51 mm Open-Ended Stub PIFA Design 62mm 51mm = mm 5mm

Impedance [867/915 MHz] Open-Ended Stub PIFA Design

Power Power (867 MHz) Power (915 MHz) Power (940 MHz) No Material Cardboard(  r =2.5) Amount Increased No Material Glass (  r =3.8) Amount Increased No Material Plastic (  r =4.7) Amount Increased *Design Goal: Power > 40% at Selected Frequencies

Radiation [867 MHz] Open-Ended Stub PIFA Design No MaterialCardboard GlassPlastic

Directivity Open-Ended Stub PIFA Design Directivity at 867 MHz [dB] Directivity at 915 MHz [dB] Directivity at 940 MHz [dB] No Material Cardboard (  r =2.5) Amount Increased No Material Glass (  r =3.8) Amount Increased No Material Plastic (  r =4.7) Amount Increased

Inductively-Coupled Feed Loop PIFA Design Dual-band Frequency Operation Achieved Gap dimension between loop and radiators is used to tune Platform Tolerance Reduced Current on Ground Plane ASIC Chip: Z c =10-j150 [  ] at 915 MHz Z c =10-j145 [  ] at 940 MHz

Inductively-Coupled Feed Loop PIFA Design Mounting Materials Dimensions 200 mm x 200 mm ( x ) Thickness=5 mm Cardboard (  r =2.5) Glass with No Loss(  r =3.8) Glass with Loss(  r =2.5) and Loss Tangent Tag Dimension 54 mm x 45 mm

Impedance Inductively-Coupled Feed Loop PIFA

Power Before and After Optimization Inductively-Coupled Feed Loop PIFA Power (915 MHz) [%]Power (940 MHz) [%] Free Space Cardboard Cardboard Optimized Free Space Glass Glass Optimized Free Space Glass with Loss Glass with Loss Optimized *Design Goal: Power > 40% at Selected Frequencies

Directivity & Radiation Inductively Coupled Feed Loop PIFA Directivity at 915 MHz[dB] Directivity at 940 MHz[dB] No Material Cardboard (  r =2.5) Amount Increased Directivity at 915 MHz[dB] Directivity at 940 MHz[dB] No Material Glass No Loss (  r =3.8) Amount Increased Directivity at 915 MHz[dB] Directivity at 940 MHz[dB] No Material Glass With Loss (  r =2.5) and loss Amount Increased MHz No Material 867 MHz Cardboard

Performance Enhancement with Artificial Magnetic Conductors PEC Ground – Reflects Half the Radiation Gain can be increased by 3 dB – Image Currents Can Cancel Currents in Antenna Limitation on distance between ground and radiating elements ( /4) – Reflection Coefficient of -1 PMC Ground – Image Currents In Phase with Original Currents PMC is reflective Low Profile Antennas – High Impedance Surface Current is filtered at selected frequencies so tangential magnetic field is small while electric field is still large Suppression of Surface Waves=>Minimizes Backlobe – Reflection Coefficient of +1

Fabrication of AMCs Configuration of AMC GA Input Parameter Output

FSS Layer FSS Unit Cell /2 x /2 FSS Layer Reflection Phase Crosses 0 at 939 MHz

Directivity AMC Directivity [dB] 867 MHz940 MHz PEC Ground AMC Ground Increased Directivity [dB] 915 MHz940 MHz PEC Ground AMC Ground Increased Open-Ended Stub PIFA Inductively-Coupled Feed Loop PIFA

Radiation [867 MHz] AMC Open Ended Stub PIFA Inductively Coupled PIFA PEC AMC

Optimization Open-Ended Stub PIFA Design

STUB 2.8Imaginary 867 MHz [  ] Real 867 MHz [  ] Power 867 MHz [%] Gap Gap Gap Gap Gap Note: 915 MHz and 940 MHz were not able to be sufficiently matched. *Design Goal: Power > 40% at Selected Frequencies

Dimension [mm] Imaginary 915 MHz [  ] Real 915 MHz [  ] Imaginary 940 MHz [  ] Real 940 MHz [  ] Power 915 MHz [%] Power 940 MHz [%] Optimization Inductively-Coupled Feed Loop PIFA Design *Design Goal: Power > 40% at Selected Frequencies

Work in Progress 16 mm x 16 mm Unit Cell 6 x 6 Unit Cells for /2 x /2 FSS 30 mm x 30 mm Unit Cell 3 x 3 Unit Cells for /2 x /2 FSS

Summary Open-Ended Stub PIFA Design showed to be platform tolerant in numerous cases – Each case was thoroughly examined Inductively-Coupled Feed Loop PIFA was very sensitive to platform – An optimization was done for each mounting material The AMC ground plane did improve the directivity and reduce the backlobe for both antenna cases An optimization needed to be done using the AMC for both antenna cases because the impedance was altered significantly in each case The Open-Ended Stub Design was optimized to sufficient operation but the Inductively-Coupled Feed Loop PIFA could not be tuned for sufficient operation