RFID DESIGN STUDIES

Dr. KVS RaoIntermec TechnologiesEverett, WA, USA

Prof. Raj MittraPennsylvania State UniversityUniversity Park, PA, USA.

Applications

• Electronic Toll Collection

• Access Control• Animal Tracking• Inventory Control• Tracking Runners in

Races!

Introduction

Radio Frequency Identification (RFID) System

- Background Information•Applications of RFID

• High Frequency (13.56MHz)• Supply Chain

• Wireless Payment

• Libraries Book ID

• Ultra High Frequency (902 – 928 MHz)• Supply Chain

• Sensors

• Libraries

• Microwave Frequency (2.45GHz)• Supply Chain

• Sensors

• Electronic Toll Payments

Design Challenges

• Small Size• Planar• UHF Frequency

Allocation – Europe 866-869 MHz– North America 902-

928 MHz

• Impedance Matching– ASIC Chip: High

Capacitive Value, Small Resistive Value

• Environmental Conditions

Antenna Parameters

• Characteristic Impedance– Power

• 3-Dimensional Radiation Patterns

• Maximum Directivity

224

( )a s

s a s a

R RP

R R X X

where Zwhere Zaa = R = Raa + j X + j Xa a is the antenna impedance,is the antenna impedance,

and Zand Zss = R = Rss + j X + j Xs s is the source impedance.is the source impedance.

radP

UD max

max

4

where Uwhere Umaxmax is the radiation intensity is the radiation intensity

in maximum direction, in maximum direction, and Pand Pradrad is the total radiated power. is the total radiated power.

Hybrid loop antenna

• Length of the antenna ~ one operating wavelength in free space

• Outer loop terminated by inner loop size reduction• Simple structure (one layer of dielectric substrate)• Antenna impedance must be highly inductive

Top View of the antenna

Hybrid loop antenna (cont‘d)

• Realize a high value for the inductance by: Changing the loop area (L ~ A) Changing the length of the perimeter

Top View of the modified antenna

Hybrid loop antenna : First design

• Perimeter of the loop antenna : 244 mm • Size used by the antenna : 39 x 40 (mm)• Resonance frequency : ~0.5 GHz and 1.26 GHz• Reactance +300 Ω at 1.01 GHz

Input Impedance

Hybrid loop antenna : First design (cont‘d)

• Not omnidirectional Pattern in the xy-plane

Non strictly symmetry of the geometry

Far Field Pattern (normalized)

xy-planexy-planexz- (blue) and yz-(red) planexz- (blue) and yz-(red) plane

Hybrid loop antenna : Parametric study

• Li = 16 (red graph) mm Li = 10 mm (blue graph)• Perimeter , Loop area : L • Perimeter 244 mm 232 mm

Changing the length of the inner loop Li

Hybrid loop antenna : Parametric study

• Wi = 28 (red graph) mm Wi = 30 mm (blue graph)• Perimeter , Loop area : L • Perimeter 248 mm 252 mm

Changing the width of the inner loop Wi

Hybrid loop antenna (cont‘d)

• Current distribution : small current in the top part of the antenna small influence on the inductance Meandering

Far Field Pattern and Current distribution at 910 MHz

Meandered Hybrid loop antenna

• Perimeter 252 mm 302 mm• Maximum percentage at 910 MHz

Top View of the meanderd antenna

Hybrid Loop Antenna

• Length of the antenna has a greater effect on the input impedance more than does the loop area

• Meandering technique reduces the size of the antenna• Small percentage power delivered to the antenna attributable to very small

resistive part of the input impedance• The developed design did not prove to be too useful

OBSERVATIONS

Dual cross-dipole

• Meandering dipole size reduction• Cross-polarization sensitivity dual dipole• Ground plane can act as reflector gain

Top and Side Views of the Antenna Structure

Dual cross-dipole : Design#1

• Length of the antenna : 218 mm ~0.66 λ (at 910 MHz)• Area used by the antenna : 51 x 51 (mm)• Reactance is too small in the desired frequency Length of the antenna • Resistive part is again very small

Input Impedance

Dual cross-dipole : Design#2

• “Load bar“ is added

• Length of the antenna : 258 mm ~ 0.78 λ (at 910 MHz)

• f300 at 900 MHz

Top View

Dual cross-dipole : Design#2

• ~80 % of the power is delivered to the antenna• Narrow bandwidth (10.5 MHz more than 50 % is delivered)• Min. AR 3 dB (860 MHz – 960 MHz)

Far Field Pattern/Power delivered to the antenna/ Axial Ratio

Dual cross-dipole : Parametric study

• Decreasing h, increases the resonance frequency• By varying the height, input impedance can be adjusted for a good matching

Influence of the height of the antenna

Dual cross-dipole : Parametric study

• Increasing the dielectric constant , drops the resonance frequency length of the antenna

• Area used by the antenna was decreased ~ 19 % by using a higher dielectric (4 instead of 2.2 )

• Max. Power delivered to the antenna was sligthly higher for the case with the higher dielectric constant (79 % vs 86 % )

• Bandwidth wasn‘t influenced

Influence of the dielectric constantInput Impedance and new design

Dual cross-dipole : New design

• Area used by the antenna reduced ~ 35 % compared to the inital design (second case) and ~21 % compared to the previous case

• Max percentage for the power plot : ~81 % (79 % second case / 86 % previous case)

• Bandwidth didn‘t change

New design/Power delivered to the antenna

Inductively coupled Feed

• Strength of the coupling depends on h2 and the size of the loop• Inductive coupling modeled by a transformer• Analyzing the input impedance by varying the size and shape of the and shape of the

looploop

Top View and structure

bodyloopin Z

MZZ

22

Inductively coupled Feed (cont‘d)

• Increasing the loop size, increase the inductance• With this method the reactance increases ~200 Ω• Two operating range frequency• Antenna size needs to be adjusted (increased)

Changing length of the loop

Inductively coupled Feed (cont‘d)

• Same experiment as before (changing the size of the loop)• For one design we realized a very high percentage of power

delivered (98 % at 899 MHz)• Bandwidth was narrow

Changing the shape of the feeding loop

Inductively coupled Feed (cont‘d)

• Narrow bandwidth• Operating frequency can be varied by changing the size of

the feeding loop • Antenna size must be increased to operate in the desired

frequency range if we use a square loop.

OBSERVATIONS

Antenna Measurement

Top View of the antenna

Input Impedance Comparison : Measurement et Simulation

Field Pattern normalized (910 MHz) - Comparison

Measurement Simulation

-Anechoic chamber not ideal for 910 MHz-Different feeding part (balun for measurement)-Infinite substrate size used for simulation

PLATFORM-TOLERANT RFID DESIGNS

Parameter Size (mm)

L 62

W 51.3

H 3

S 5

gap (867/915 MHz) 1

gap (867/940 MHz) 1.9

r 2.35

ASIC Chip:

Zc=10-j160 [] at 867 MHz

Zc=10-j150 [] at 915 MHz

Zc=10-j145 [] at 940 MHz

Dual-Band PIFA Design

•Dual-band Frequency Operation

•Open-Ended Stub

•Gap Dimension and Stub Dimension Used to Tune

•Platform Tolerance

•Dominating Horizontal Current Distribution

•Widening Short, Vertical Inductance Reduced, Antenna Lowered

Dual-Band PIFA Design

•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)

Dual-Band PIFA Design

Impedance [867/915 MHz] Dual-Band PIFA DesignImpedance [867/915 MHz] Dual-Band PIFA Design

Real Impedance

0

50

100

150

200

250

300

850 858 865 873 880 888 895 903 910 918 925 933

Frequency [MHz]

No Material Cardboard Glass Plastic

Imaginary Impedance

0

50

100

150

200

250

300

850 858 865 873 880 888 895 903 910 918 925 933

Frequency [MHz]

No Material Cardboard Glass Plastic

Power Dual-Band PIFA DesignPower Dual-Band PIFA Design

  Power (867 MHz)

Power (915 MHz)

Power (940 MHz)

No Material 83.49 64.92 74.07

Cardboard 54.53 86.28 80.5

Amount Increased

-28.96 21.36 6.43

       

No Material 83.49 64.92 74.07

Glass 54.81 80.72 72.9

Amount Increased

-28.68 15.8 -1.17

       

No Material 83.49 64.92 74.07

Plastic 58.3 85.72 72

Amount Increased

-25.19 20.8 -2.07

Radiation [867 MHz] Dual-Band PIFA DesignRadiation [867 MHz] Dual-Band PIFA Design

No Material Cardboard

Glass Plastic

Conclusions Dual-Band PIFA Design

  Directivity (867 MHz)

Directivity (915 MHz)

Directivity (940 MHz)

No Material 1.6841 1.832 1.8815

Cardboard 1.928 2.0704 2.3425

Amount Increased

0.2439 0.2384 0.461

       

No Material 1.6841 1.832 1.8815

Glass 2.4053 2.8135 3.9153

Amount Increased

0.7212 0.9815 2.0338

       

No Material 1.6841 1.832 1.8815

Plastic 2.9936 3.4036 4.1411

Amount Increased

1.3095 1.5716 2.2596

Environmental ChangeDual-Band PIFA Design

•Cardboard Box

• 900 mm x 900 mm

•4 x 4

•Thickness=13 mm

•Metal sheet

•450 mm x 450 mm

•2 x 2

•Height from Cardboard was Varied from 0 mm-20 mm

Radiation [867 MHz]

No Metal Metal 20 mm Under Cardboard

  Power (867 MHz)

Power (915 MHz)

Peak Directivity (867 MHz)

Peak Directivity (915 MHz)

No Metal 54.53 86.28 1.928 2.0704

Metal 0 mm 76.6 80.7 3.3105 3.0219

Amount Increased

22.07 -5.58 1.3825 0.9515

         

  Power (867 MHz)

Power (915 MHz)

Peak Directivity (867 MHz)

Peak Directivity (915 MHz)

No Metal 54.53 86.28 1.928 2.0704

Metal 10 mm 91.08 73.11 3.1872 3.0167

Amount Increased

36.55 -13.17 1.2592 0.9463

         

  Power (867 MHz)

Power (915 MHz)

Peak Directivity (867 MHz)

Peak Directivity (915 MHz)

No Metal 54.53 86.28 1.928 2.0704

Metal 20 mm 73.25 76.82 3.2213 3.0599

Amount Increased

18.72 -9.46 1.2933 0.9895

Environmental Change Dual-Band PIFA Design

Ground Plane Optimization Dual-Band PIFA Design

No Material Directivity (867 MHz)

Directivity (915 MHz)

Original GP 1.6841 1.832

1 Inch Larger 2.3265 2.3131

2 Inch Larger 2.9306 2.918

3 Inch Larger 3.6696 3.7427

10 Inch Larger 4.5087 4.4954

     

Cardboard Directivity (867 MHz)

Directivity (915 MHz)

Original GP 1.928 2.0704

1 Inch Larger 2.6915 2.7059

2 Inch Larger 3.2191 3.2618

3 Inch Larger 3.1907 3.3583

10 Inch Larger 4.6297 4.687

Glass Peak Directivity (867 MHz)

Peak Directivity (915 MHz)

Original GP 2.4053 2.8135

1 Inch Larger 2.5102 2.6094

2 Inch Larger 2.9178 3.0237

3 Inch Larger 2.7375 2.7357

10 Inch Larger 3.7178 4.1891

     

Plastic Peak Directivity (867 MHz)

Peak Directivity (915 MHz)

Original GP 2.9936 3.4036

1 Inch Larger 2.9787 3.0035

2 Inch Larger 3.0787 2.9965

3 Inch Larger 3.1032 3.0408

10 Inch Larger 3.0544 2.9356

Ground Plane Optimization Dual-Band PIFA Design

•Impedance Matching

•Inductively Coupled Feed Loop

•Gap dimension between loop and radiators is used to tune

•Designed to match Zc=10-j150 [] at 915 MHz

•Platform Tolerance

•Reduced Current on Ground Plane

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 0.002

Inductively-Coupled Feed Loop PIFA Design

Impedance

-10

10

30

50

70

90

110

130

150

170

190

0.825 0.845 0.865 0.885 0.905 0.925 0.945 0.965 0.985 1.005 1.025

Frequency [MHz]

Imaginary 9.75

Imaginary 9.5

Imaginary 9.25

Imaginary 9

Real 9.75

Real 9.5

Real 9.25

Real 9

  Power 915 MHz [%]

Power 940 MHz [%]

Average

Gap 9 mm 9.09 4.39 6.74

Gap 9.25 mm 86.09 41.45 63.77

Gap 9.5 mm 77.38 34.50 55.94

Gap 9.75 mm 15.28 13.00 14.14

Optimization of Impedance in Free Space

Directivity & Radiation

  Directivity (915 MHz) Directivity (940 MHz)

No Mounting Material 3.47 3.3

Cardboard (r=2.5) 3.43 2.94

Amount Increased -0.04 -0.36

     

  Directivity (915 MHz) Directivity (940 MHz)

No Mounting Material 3.47 3.3

Glass No Loss (r=3.8) 3.4 3.25

Amount Increased -0.07 -0.05

     

  Directivity (915 MHz) Directivity (940 MHz)

No Mounting Material 3.47 3.3

Glass With Loss (r=2.5) and loss 0.002

3.36 3.3

Amount Increased --0.11 0

867 MHz No Material

867 MHz Cardboard

Impedance

0

10

20

30

40

50

60

0.83 0.84 0.85 0.86 0.87 0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1

Frequency [GHz]

Gap 9.75 Gap 9.5 Gap 9.25 Gap 9 Gap 8.8 Gap 8.75 Gap 8.5

Optimization of Impedance for Cardboard

Impedance Inductively-Coupled Feed Loop PIFA

Impedance (Before Optimization)

020406080

100120140160180200

0.83 0.85 0.87 0.89 0.91 0.93 0.95 0.97 0.99 1.01 1.03

Frequency [GHz]

Imaginary No Material Imaginary Cardboard Imaginary No Loss Imaginary Glass with Loss

Real No Material Real Cardboard Real Glass with Loss Real No Loss

Impedance Inductively-Coupled Feed Loop PIFA

Impedance (Optimized)

0

20

40

60

80

100

120

140

160

180

200

0.83 0.85 0.87 0.89 0.91 0.93 0.95 0.97 0.99 1.01 1.03

Frequency [GHz]

Imaginary No Material Imaginary Cardboard Imaginary Glass No Loss Imaginary Glass With Loss

Real No Material Real Cardboard Real Glass No Loss Real Glass With Loss

  Power (915 MHz) [%] Power (940 MHz) [%]

Free Space 86.09 41.45

Cardboard 16.29 6.5

Cardboard Optimized 61.19 31.69

     

Free Space 86.09 41.45

Glass 24.06 9.48

Glass Optimized 56.59 69.36

     

Free Space 86.09 41.45

Glass with Loss 11.65 23.81

Glass with Loss Optimized 61.6 52.55

Power Before and After Optimization Inductively Coupled Feed Loop PIFA

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

Metamaterials

• Electrical Ground Plane Redirect one-half of the radiation gain can be increased by 3 dB Min. distance between antenna and ground : λ/4

• Image currents cancel currents in antenna poor radiation efficiency

• Metamaterials→Material that exhibit electromagnetic properties not found in nature

• EBG (Electromagnetic Band Gap) - Surface Subclass of metamaterials Can be designed to act as an AMC (Artificial Magnetic Conductor)

ground plane

ChallengesChallenges

Metamaterial

• Reflectivitiy “+1“ (reflection magnitude 1 and reflection phase 0°)• Can be achieved by utilizing periodic patch with via geometry or by planar

achitecture without the need of vias FSS (Frequency Selective Surface)• GA (Genetic algorithm) for an optimized FSS unit cell size,geometry and

dielectric constant and thickness of the substrate material

AMCAMC

Metamaterial - Simulations

FSS-designFSS-design

Unit CellUnit Cell

Antenna StructureAntenna Structure

FSS-Screen with AntennaFSS-Screen with Antenna

GA-Output ParameterGA-Output Parameter

Metamaterial – Simulations (cont‘d)

• Resonance frequency is decreasing by using a FSS-layer

Input Impedance- Comparison (with and without using an FSS-layer)Input Impedance- Comparison (with and without using an FSS-layer)

Metamaterial – Simulations (cont‘d)

• Higher Power transfer for the case with the FSS-layer (~94 % instead of 83 %)

• Directivity is alternating in the range between 900 and 950 MHz around 1.2

• Antenna could be made smaller future work

• Bandwidth sligthly smaller for the FSS-case

Bandwidth and maximum Directivity - Comparison Bandwidth and maximum Directivity - Comparison (with and without using an FSS-layer)(with and without using an FSS-layer)

Metamaterials (cont‘d)

• Using an FSS-layer drop the resonance frequency• Changing the size of the antenna to get a desired input impedance is

very difficult • Directivity behaviour changes sligthly• Higher power delivered to the antenna with the FSS-layer• Bandwidth slighly smaller for the FSS-case

Summary

Future work

Increasing Bandwidth Changing structure of the AMC instead of the antenna size Antenna attached to metal objects Performance will change Tunable antenna design provide tolerance for fabrication

Fabrication of AMCs

Configuration of AMC

GA Input

Parameter Output

FSS Layer

FSS Unit Cell

/2 x /2 FSS Layer

Reflection Crosses 0 at 939 MHz

Directivity AMC

  Directivity

  867 MHz 940 MHz

PEC Ground

2.5255 2.6126

AMC Ground

2.892 3.125

Increased 0.3665 0.5124

  Directivity

  915 MHz 940 MHz

PEC Ground

2.7074 2.421

AMC Ground

3.6855 2.9899

Increased 0.9781 0.5689

Dual-Band PIFA Inductively Coupled PIFA

Radiation [867 MHz]AMC

Dual-Band PIFA

Inductively Coupled PIFAPEC

PEC

AMC

AMC

Optimization Dual-Band PIFA Design

Impedance Stub 2.8

0

50

100

150

200

250

850 855 860 865 870 875 880 885 890 895 900 905 910 915 920 925 930 935 940 945 950

Frequency [MHz]

[]

Imaginary Gap 2.6 Imaginary Gap 2.5 Imaginary Gap 2.4 Imaginary Gap 2.2 Imaginary Gap 2.1

Real Gap 2.6 Real Gap 2.5 Real Gap 2.4 Real Gap 2.2 Real Gap 2.1

Optimization Dual-Band PIFA Design

STUB 2.8 Imaginary 867 MHz []

Real 867 MHz []

Power 867 MHz [%]

Gap 2.6 157.45 40.29 63.56

Gap 2.5 142.94 3.55 29.90

Gap 2.4 153.95 26.54 77.39

Gap 2.2 131.14 2.66 10.70

Gap 2.1 128.29 2.69 9.23

Note: 915 MHz and 940 MHz were not able to be sufficiently matched.

Optimization Inductively-Coupled Feed Loop PIFA Design

Impedance (Imaginary)

150

155

160

165

170

175

180

900 905 910 915 920 925 930 935 940 945 950

Frequency [MHz]

[]

Gap 9.5 Gap 9.4 Gap 9.3 Gap 9.25 Gap 9.2 Gap 9.1 Gap 8.9

Dimension [mm]

Imaginary 915 MHz

[]

Real 915 MHz

[]

Imaginary 940 MHz

[]

Real 940 MHz

[]

Power 915 MHz

[%]

Power 940 MHz

[%]

8.90 164.15 0.07 169.24 3.27 0.92 17.11

9.10 164.23 0.06 170.52 4.52 0.78 20.97

9.20 162.70 0.24 166.82 1.47 3.64 9.68

9.25 166.60 2.98 171.89 0.51 26.83 2.44

9.30 158.90 0.14 163.86 1.29 3.11 10.70

9.40 163.04 0.13 167.28 4.80 1.90 26.83

9.50 163.52 0.17 168.06 4.42 2.34 23.91

Optimization Inductively-Coupled Feed Loop PIFA Design

OBSERVATIONS

• Dual-Band PIFA Design showed to be platform tolerant in numerous cases

• Inductively Coupled Feed Loop PIFA was very sensitive to platform• An optimization was done for each mounting material with the

Inductively Coupled Feed Loop PIFA• The AMC ground plane did significantly improve the directivity and

reduce the backlobe in both antenna cases• An optimization needed to be done using the AMC for both antenna

cases because the impedance was altered• The Dual-Band PIFA Design was optimized to sufficient operation

but the Inductively Coupled Feed Loop PIFA was not

ALTERNATE PLATFORM-TOLERANT RFID DESIGNS

*courtesy of Prof. K.W.LeungCity University of Hong Kong

RFID Tag Design

- Background Information

•Inductive-coupled feeding design

Platform Tolerant

•Principle

-Using a patch antenna as the resonating element -The Tag antenna and the surface material are isolated by the ground plane

-The Tag has a stable performance regardless of the mounting surface

RFID Tag Antenna Configuration

First Design

Size of Ground Plane:Size of Ground Plane: 83.638 x 112.058mm (0.26λx 0.34λ)83.638 x 112.058mm (0.26λx 0.34λ)

Size of Printed Antenna:Size of Printed Antenna: 64.4 x 89.95mm(0.2λ x 0.27λ)64.4 x 89.95mm(0.2λ x 0.27λ)

Substrate thickness:Substrate thickness: 1.524mm1.524mm

Substrate dielectric constant:Substrate dielectric constant: 3.383.38

Substrate loss tangent:Substrate loss tangent: 0.00210.0021

RFID Tag Antenna Configuration

•Chip Impedance- 20.83 – j116.67ΩΩ

•Transmitted power of the Reader - 1W (30dBm)

•Gain of the Reader antenna- ~7.5dBi

RFID Tag Antenna Configuration

Simulated Antenna Gain

•Gain: ~ -8.6 to -0.77 dBi

RFID Tag Antenna Configuration

Current Distribution (First Design)

902 MHz

RFID Tag Antenna Configuration

Current Distribution (First Design)

915 MHz

RFID Tag Antenna Configuration

Current Distribution (First Design)

928 MHz

Result and Analysis

Measurement Method

-The Read Range was measured in the EMC Chamber

-Reader Antenna was moved inside the EMC Chamber

-Measure the maximum readable distance that the signal can be detected

RFID Tag Antenna Configuration

Measurement Method

- - RFID Tag was fixed by the foam stand and measured at different orientation angles (0 deg, 45 deg, 90 deg)

0 deg 45 deg 90 deg

Result and Analysis

Tag Antenna Configuration (by inductively-coupled feeding)

-The tag has similar performances for different angles

Result and Analysis

Second Design

-A Philips’s chip SL3S10 01 FTT is used

-Impedance of the chip: 16 – j380Ω (much more capacitive)

-Difficult to match using the first design

-Introduce a new feed network

RFID Tag Antenna Configuration

Second Tag Antenna Design

- - Directly connect the feed network to the radiating patch at several point

Size of Ground Plane: 83.638 x 112.058mm (0.26λx 0.34λ)Size of Ground Plane: 83.638 x 112.058mm (0.26λx 0.34λ)

Size of Printed Antenna: 54.45 x 93.3mm(0.17λ x 0.28λ)Size of Printed Antenna: 54.45 x 93.3mm(0.17λ x 0.28λ)

RFID Tag Antenna Configuration

Current Distribution (Second Design)

902 MHz

RFID Tag Antenna Configuration

Current Distribution (Second Design)

915 MHz

RFID Tag Antenna Configuration

Current Distribution (Second Design)

928 MHz

Result and Analysis

Platform Tolerance Test

-Following surfaces were used in the test:

-Acrylic (200 x 200 x 3 mm)

-Wood (200 x 200 x 3 mm)

-Aluminium (200 x 200 x 3 mm)

Result and Analysis

Platform Tolerance

-The tag has stable performance over different surfaces-The longest read range is obtained for the metal (Aluminium)

case because EM wave is reflected by the metal