Compact shorted c‐shaped patch antenna for ultrahigh frequency radio frequency identification tags mounted on a metallic (ngắn mạch cấu trúc ăng ten dạng c cho ứng dụng nhận dạ

The compact patch antenna consists of a C-shaped resonator and a ground plane connected to a small shorting wall and is connected to a feeding loop in the middle of the C-shaped resonato

R E S E A R C H A R T I C L E

Compact shorted C-shaped patch antenna for ultrahigh

frequency radio frequency identification tags mounted

on a metallic plate

1

Institute of Photonics Engineering,

National Kaohsiung University of Science

and Technology, Kaohsiung, Taiwan

2

Department of Mechatronics, Dong Nai

Technology University, Bien Hoa,

Viet Nam

3

Department of Avionics Engineering,

R.O.C Air Force Academy, Kaohsiung,

Taiwan

Correspondence

Hua-Ming Chen, Institute of Photonics

Engineering, National Kaohsiung

University of Science and Technology,

Kaohsiung 807, Taiwan.

Email: hmchen@nkust.edu.tw

Abstract This paper describes an ultrahigh frequency (UHF) tag antenna mounted on a metallic plate for radio frequency identification (RFID) The impedance of the proposed antenna can be tuned using various methods The compact patch antenna consists of a C-shaped resonator and a ground plane connected to a small shorting wall and is connected to a feeding loop in the middle of the C-shaped resonator Etching a couple of slits close to the shorting wall and a slot

in the center of the C-shaped resonator provided a flexible method for adjust-ment to match the conjugate impedance with the UCODE 8/8 m chip (13

−j191 Ω at 915 MHz) The optimal design with an overall size of

30 × 30 × 3 mm3(0.092 λ0×0.092 λ0×0.0092 λ0) yielded a high power trans-mission coefficient of 91% and reading range of 6.4 m for the effective isotropic radiated power (EIRP) of 4.0 W when the tag antenna was mounted on a

220 × 220 mm2 metal plate The proposed antenna was designed at standard frequency bands of the Federal Communications Commission (FCC, 902-928 MHz) for North America and Taiwan Antenna fabrication and testing were performed, which revealed that the measured data were in good agree-ment with the simulation results

K E Y W O R D S C-shaped patch, metal tag antenna, reading range, RFID tag antenna, shorting wall

1 | I N T R O D U C T I O N

Radio frequency identification (RFID) tags have been

used in a wide range of applications, such as in toll tag

systems, distribution logistics, the Internet of things,

sup-ply chain management, and security systems.1,2

Gener-ally, tag antennas comprise an antenna and an integrated

circuit (IC) chip and can operate in the ultrahigh

fre-quency band (UHF) Thus, the reading range can be

extended with a low-cost component.3,4 However, when

a tag antenna is placed on or close to metal objects,

antenna performance degrades because of its material characteristics, which negatively affects fundamental antenna properties including input impedances, power transmission coefficients (PTC), gains, and radiation pat-terns Several methods have been suggested to overcome this problem and improve tag performance Antennas with artificial magnetic conductors (AMCs) were pro-posed to enhance the gain and radiation patterns Such antennas consist of a radiating patch on the top and an AMC ground plane installed at the bottom.5-8However, most AMC antennas have a complex layout, large size,

Int J RF Microw Comput Aided Eng 2021;e22595 wileyonlinelibrary.com/journal/mmce © 2021 Wiley Periodicals LLC 1 of 13

and are not cost effective Therefore, these antennas are

impractical for applications that require a compact

antenna To reduce the profile of the tag antenna, a

pla-nar inverted-F antenna (PIFA) structure was introduced

A PIFA depends on a high impedance surface to

reduce the profile.9,10The input impedance of an IC chip

is generally designed with a high quality factor to

enhance its sensitivity, and that causes the designed

antennas have difficulty gaining the conjugate impedance

matching.11-13Specifically, studies14,15have attempted to

improve impedance matches based on a loaded bar and

conductive layer; however, the radiation efficiency is the

main problem, and the impedance tuning of tag antennas

is difficult Loaded via-patches with small antennas have

been proposed16,17to enhance radiation efficiency

How-ever, these antennas require parameter configuration at

the feed point and via-holes The position of the shorting

vias or via-patches affects the performance of PIFA tag

antennas Resonant frequency tuning is challenging, and

fabrication costs can increase considerably

Folded patch antennas have been developed on the

basis of the PIFA to enhance tag antenna performance

and reduce the size of the patch In,18a miniature folded

patch was proposed for designing a passive tag mounted

on metal objects The antenna consists of a square patch

that is equally separated into two rectangular patches

and connected to two ground planes using six thin

induc-tive stubs This structure is fabricated by using an

alumi-num patch, which decreases the performance of the

reading range because of the nature of the adhesive

bonding, and the aluminum film on the surface can be

easily oxidized.19Furthermore, the folded structures

reg-ularly use additional ground planes or radiating patch

layers, which increases the complexity and volume of tag

antennas.20,21 Unlike previous antennas, the antenna

proposed in22 is robust and does not require additional

grounds or radiating patch layers However, the use of

two shorting stubs in the middle of the patch to connect

the two separated ground planes increases the difficulty

in obtaining the resonant frequency

In this study, a tag antenna with flexible impedance

tuning methods on the metallic plate was developed The

proposed antenna is compact, low cost, low profile and

simple and does not require shorting vias or additional

layers (ground or radiating patch) Furthermore, the

pro-posed antenna can be adjusted easily by tuning the width

of the shorting wall, two slits, C-shaped arm, a slot in the

middle of the patch, and the position of the loop to

achieve conjugate impedance matching with a UCODE

8/8 m chip Antenna performance, including reflection

coefficients, gains, the power transmission coefficient,

and reading distances of the proposed tag antenna

mounted on the metal plate of 220 × 220 mm2, was

investigated All simulations were performed using Ansoft HFSS software

This paper is organized into five main sections Section II introduces the configuration of the proposed antenna Section III discusses the design procedure and current analysis Section IV describes the effects on parameters Section V presents the experiment results and compares the performance of the proposed antenna with those in previous studies

2 | A N T E N N A D E S I G N A N D

O P T I M I Z A T I O N

A compact shorted C-shaped tag antenna on the metallic objects (Figure 1) was proposed and designed at FCC standard frequency bands (902-928 MHz) for North America and Taiwan The tag antenna has a small gap (0.1 mm) and is mounted on a metallic object (220 × 220 mm2) The proposed antenna consists of a C-shaped resonator and a ground plane connected to a small shorting wall and fed by a loop in the middle of the C-shaped resonator The dimensions of the small short-ing wall are Sw × 3 mm2 The space between them is supported by a soft foam material (Polyethylene) measur-ing of 30 × 30 × 2.6 mm3, a dielectric constant of 1.03 and a loss tangent of 0.0001 Two open slits are etched around the shorting wall measuring Wn ×2.2 mm2 The

LC × 0.5 mm2, respectively, and are used to adjust the impedance matching between the proposed antenna and chip They can be easily made by the two FR4 substrates (the top substrate is used to the radiation layer and the other one is a metallic ground plane) with thickness of

F I G U R E 1 Schematic of the proposed antenna on the metallic plate

0.2 mm, dielectric constant εr of 4.3, and loss tangent of

0.025 The microchip used in this design is the UCODE

8/8 m, which has an input impedance of 13 − j191 Ω at

915 MHz, a read sensitivity of −22.9 dBm, and a write

sensitivity of −17.8 dBm over the frequency band of

840 to 960 MHz Notably, in this study, test pads TP1 and

TP2 were electrically disconnected and therefore could be

safely short-circuited to the RF pads (RF1, RF2) in the

UCODE 8/8 m These pads of the UCODE chip are

particularly described in Figure 2 and Table 1

In Figure 2A, the pads (TP1 and TP2) of chip are a

supporting pad for fix In Figure 2B, the feed pads of the

antenna are designed a single-slit assembly to make

easy to solder in the fabrication In this case, the

related increased input capacitance is canceled by

using optimize the antenna size Therefore, the tag

antenna was designed to match a conjugate input

impedance of 13 − j191 Ω of single-slit assembly at the

frequency of 915 MHz.23 The chip was soldered to the

middle of the short side of the rectangular feeding loop

connected to the center of the C-shaped radiator The

optimized design parameters of the proposed antenna

(listed in Table 2) were simulated using HFSS software

version 19.3.24

3 | D E S I G N P R O C E D U R E A N D

C U R R E N T A N A L Y S I S

To obtain an optimal tag antenna design, the design

pro-cedure used HFSS, and the characteristics of the input

impedance of the proposed antenna were investigated To

validate our design, an antenna prototype was also

fabri-cated (Figure 3) Because the frequency-dependent

com-plex impedance induced by a tag chip substantially

affects the performance of an RFID tag, the input

imped-ance of the chip was determined before RFID tag design

First, the method described in25 was used to determine the threshold power sensitivity, which was approximately

−22.9 dBm at 915 MHz Second, the threshold power sen-sitivity was adjusted to 915 MHz Figure 4A depicts that a balun probe was connected to a vector network analyzer (VNA) set up from 800 to 1000 MHz through a coaxial cable of 50 Ω to measure the input impedance of the chip

by observing the pointer position in the Smith chart cir-cle Furthermore, the balun probe was calibrated by using the open, short, and load before starting this mea-surement Third, the values of the chip resistance and reactance were measured and are presented in Figure 4B Finally, the tag antenna was executed by following design procedures The design required a conjugate impedance match between the tag antenna and the UCODE 8/8 m chip, which has a ZA value of 13 + j191 Ω at 915 MHz The design method entailed the four stages described in the following:

Stage 1: The C-shaped patch is connected to the loop

in the middle of the antenna, which does not have a shorting wall, slits, and a slot, in the early stage of design Figure 5A indicates that the resonant frequency of the antenna is higher than that required; the tag antenna achieves a reactance of j191 Ω at 1220 MHz because the high current density is only concentrated in the middle

of the C-shaped patch and loop, as depicted in Figure 5B The resonance frequency is shifted downward by increas-ing the current path

F I G U R E 2 Pads of the UCODE 8/8 m chip: A, standard assembly; B, single-slit assembly

T A B L E 1 Pads description of the bare die

Stage 2: The shorting wall is now studied to increase

the current density distribution or the current path of the

antenna, which decreases the resonant frequency; the

shorting wall is used to short the top patch and ground

plane Figure 6B displays a high density in the middle of

the C-shaped patch and loop and an extended current

path, which distributes strongly around the shorting wall

Therefore, the resonance frequency is reduced from

1220 MHz to 931.5 MHz (see Figure 6A)

Stage 3: To match the impedance of the chip, the

resonance frequency of the tag antenna is shifted

down at the desired frequency A slot is made in the

middle of the C-shaped patch The existence of the

slot causes the current distribution of the C-shaped

patch to focus on the contour of the edge of the

C-shaped left arm, as displayed in Figure 7B Therefore,

this method reduces the resonant frequency of the tag

to 920.5 MHz (Figure 7A)

Stage 4: Finally, the two slits are etched on the top

patch around the shorting wall, which makes the current

path longer than the case without the slits, as depicted in

Figure 8B Therefore, the resonant frequency shifts from

920.5 to 915 MHz; the result is excellent conjugate

impedance agreement between the proposed antenna

and the chip (Figure 8A) Furthermore, changing the

width of two slits slowly affects the frequency at a rate of

1 MHz per 0.3 mm

In summary, Figure 8B displays the surface cur-rent distribution on the proposed antenna mounted

on the 220 × 220 mm2 metallic plate and a resonant current flow on the radiating plate at 915 MHz Almost all the current is concentrated on the left arm

of the C-shaped patch, shorting wall, two slits, slot, and the contour of the loop, which suggests these are useful for tuning the resonant frequency of the tag antenna

design parameters

F I G U R E 3 Structure of the fabricated antenna

F I G U R E 4 Measured chip impedance vs frequency at the threshold power: A, Balun probe, calibration kit, and B, Cartesian coordinates

4 | P A R A M E T E R S T U D Y

In this section, the study of fundamental antenna

proper-ties, such as the input impedance, reflection coefficient,

power transmission coefficient, and gain, which was

undertaken to investigate the effect of key parameters on

the performance of the proposed antenna, is explained

The key parameters include the left arm size of the

C-shaped patch (W2), width of the shorting wall (Sw), size

of the two slits (Wn) and slot (Ws), and vertical length of

the loop (Lc) In all simulation cases, the tag was attached

at the center of the metallic plate measuring

220 × 220 mm2with a small gap of 0.1 mm The goal of

this study was to design a flexible antenna structure that

can easily match the input impedance of the UCODE

8/8 m chip by changing multiple parameters The effects

of the shorting wall were first studied (Figure 9) The

reflection coefficient is depicted in Figure 9B; the

reso-nance frequency of the tag is sensitive to the width of the

shorting wall The resonance frequency increased at a

rate of 8 MHz with an increase of 0.8 mm in Swbecause

the tag antenna became resistive and reactive when the

length of shorting wall decreased (Figure 9A) The input

impedance bandwidth for a 3 dB reflection coefficient

was 1.8%, ranging from 905 to 923 MHz Additionally, the performance of the tag antenna was 95% of the power transmission coefficient at the resonance frequency as illustrated in Figure 9c

The effects of the left arm of the C-shaped (W2) were then evaluated As illustrated in Figure 10A, the resistive and reactive characteristics largely increased every mm

as the left arm size decreased in W2, which shifted the resonance frequency downward quickly, as depicted in Figure 10B, whereas the power transmission coefficient was almost unchanged, as depicted in Figure 10c

The effects of the vertical length of the loop were then studied As illustrated in Figure 11A, the resonance fre-quency decreased at a rate of 8 MHz as the vertical length

of the loop (LC) increased from 12.5 mm to 14.5 mm, whereas the impedance bandwidth at 3 dB did not change Notably, the performance of the tag degraded in this scenario The power transmission coefficient decreased from 96.4% to 88% with an increment of 1 mm

in LC(Figure 11B) Next, the effects of the slot (WS) were evaluated (Figure 12) The results were similar to those of studies that analyzed cases of changing the loop and

F I G U R E 5 A, Resistance and reactance for the case not having

slits, a slot, and shorting wall; B, surface current distribution of the

shorting wall but not a slot and slits; B, surface current distribution

of the patch

increasing the effect of WS on tag resistance and

reac-tance, and that decreases the resonant frequency

(Figure 12A and B) Furthermore, the power

transmis-sion coefficient also slightly decreased as the width of the

slot increased by 0.8 mm

Finally, the effects of changing the Wnof the two slits

were analyzed (Figure 13) Figure 13A indicates that the

resistance and reactance increased slowly when the

width of slits was varied from 0.2 to 0.8 mm Therefore,

the resonant frequency of the tag only slightly decreased

at the rate of 1 MHz per 0.3 mm Figure 13C,D displays

the performance of the tag in terms of PTC and reveals

that the gain with 0.3 dB was unchanged in this case

The simulation results indicated that this slow shifting

was an excellent method to accurately adjust the

reso-nance frequency of the tag (Figure 13B)

5 | E X P E R I M E N T A N D

D I S C U S S I O N

The proposed UHF RFID tag antenna was fabricated by

using two FR4 substrates (thickness: 0.2 mm, dielectric

constant εr= 4.3, and loss tangent δ = 0.025), measuring

2.6 × 0.2 mm3for the C-shaped patch and ground plane and the side of the tag, respectively The space between the top and bottom layers was supported by a soft foam material (εr = 1.03), as displayed in Figure 3 The tag antenna was designed for a UCODE 8/8 m chip, which had an input impedance of 13 − j191 Ω and a power sen-sitivity of −22.9 dBm at 915 MHz The input impedance measurement of the tag was implemented using the VNA, which was connected to the balun probe through the coaxial cable with a resistance of 50 Ω Notably, the balun probe was calibrated through open, short, and loads before performing this measurement with reference

to Figure 4A Therefore, Figure 14 displays the compari-son of the measured and simulated input impedance of the tag antenna with the UCODE 8/8 m chip as the refer-ence The input impedance of the tag antenna in mea-surement and simulation were obtained at 13 + j215 Ω and 11 + j219 Ω, respectively, and slightly shifted com-pared with the chip at 915 MHz To verify a matching agreement between input impedance of the chip and the tag antenna, the performance characteristic parameters, including the reflection coefficient or return loss and

F I G U R E 7 A, Resistance and reactance for the case having a

shorting wall and slot but without slits; B, surface current

distribution of the patch

F I G U R E 8 A, Resistance and reactance for the case having a shorting wall, slits, and slot; B, surface current distribution of the patch

power transmission coefficient, were first implemented.

In,26a power wave reflection coefficient Γ is expressed as

follows:

Γ=ZT−Z

* A

ZT+ ZA, 0≤ Γj j≤1 , Return Loss dBð Þ = −20log Γj j

ð1Þ

where

ZT= RT+ jXTis the complex chip impedance

ZA= RA+ jXAis the complex antenna impedance The power delivered to the chip is expressed as follows:

Ptag − chip= 1 − Γ
j j2

Ptag − ant, ð2Þ

where P is the power received by the tag antenna

F I G U R E 9 Effect of changing Swof the shorting wall: A,

Resistance and reactance; B, reflection coefficient; C, power

transmission coefficient

(A)

(B)

(C)

F I G U R E 1 0 Effect of changing W 2 of the C-shaped left arm: A, Resistance and reactance; B, reflection coefficient; C, power transmission coefficient

The power transmission coefficient can be expressed

as follows:

τ=Ptag − chip

Ptag − ant

= 1 − Γ
j j2

RA+ RT

ð Þ2+ Xð A+ XTÞ2, 0≤τ≤1

ð3Þ

The reflection coefficient and PTC were then

calcu-lated by replacing the measured results of the

resis-tance and reacresis-tance into Equations (1) and (3), as

depicted in Figure 15, which illustrates the reflection

coefficient between the simulation and measurement

A good match occurred between the input impedance

of the tag antenna and the chip at the resonance

fre-quency However, the 3-dB bandwidth of the measured

result is larger than the simulated result This

discrep-ancy was caused by the fabrication tolerance in the

etching process and cable influence in the

measure-ment procedure, which are problems also identified in

other studies.25,27Moreover, the measured PTC values were obtained from 60% to 91% at the operation fre-quency band of FCC (902-928 MHz) for North America and Taiwan, meaning that the impedance matching between the tag antenna and the chip was acceptable (Figure 15)

Another crucial parameter that can be used to evalu-ate the performance of the tag antenna is the reading

(A)

(B)

F I G U R E 1 1 Effects of changing L c of the loop: A, reflection

coefficient; B, power transmission coefficient

(A)

(B)

(C)

F I G U R E 1 2 Effects of changing W S of the slot: A, Resistance and reactance; B, reflection coefficient; C, power transmission coefficient

distance or the read range The maximum reading

dis-tance for a radio power link was obtained when Ptag-chip

was equal to the threshold power of the microchip, P

tag-threshold, which is the minimum threshold power to

acti-vate the microchip on the RFID tag26:

R= λ 4π

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

P reader-tx G reader-ant G tag-antτ

Ptag-threshold

r

where

F I G U R E 1 3 Effects of changing Wnof the two slits: A, Resistance and reactance; C, power transmission coefficient; D, antenna gain

F I G U R E 1 4 Measured and simulated input impedance of the

proposed antenna vs the chip

F I G U R E 1 5 Measured and simulated reflection coefficient of the proposed antenna

Preader-tx and Greader-antare the power and gain of the reader antenna, respectively,

Gtag-antis the gain of the tag antenna, and Ptag-threshold

is the minimum threshold power necessary to provide sufficient power to the chip

To verify the tag performance, Equation (4) was used

to calculate the reading distance The fabricated tag antenna attached to the metallic plane was measured inside the anechoic chamber with a measurement system according to the method described in.25,28Figures 16 and

17 display the measurement of the tag antenna inside the anechoic chamber and outside the free space, respec-tively The RFID measurement system included a com-puter, a reader controller module (Favite FS-GM-201), a reader antenna, which has a circular polarization, and the proposed antenna Figure 18 depicts the angular sen-sitivity patterns of the tag in the yz and xz planes To obtain the angular sensitivity patterns of the tag, the measurement system was set up to rotate the tag antenna around the fixed axis in the yz and xz planes from 0 to

360 with 15 increments at 915 MHz, as depicted in Figure 18A and B, respectively The proposed antenna exhibited superior input power sensitive values in the yz plane compared with the xz plane at 915 MHz Figure 19A displays that the maximum readable distance was achieved at θ = 0 with a value of 6.4 m when the tag antenna was placed on the yz plane However, the maximum reading range was obtained with a value of 5.1 m if the tag antenna was placed in the xz plane (Figure 19B) An explanation for this phenomenon is that

a slight misalignment occurs between the reader and the tag antenna, which is unavoidable during the measure-ment process

Table 3 displays a comparison between the proposed antenna and previously studied antennas mounted on a metallic plate In,18,22for the tag antennas, shorting stubs were used to connect the top patch to ground layers

to decrease the resonant frequency By contrast, the

F I G U R E 1 6 Measurement setup of the RFID tag antenna for

reading range inside the anechoic chamber

F I G U R E 1 7 Measurement setup of the RFID tag antenna for

reading the range outside the free space

F I G U R E 1 8 Angular sensitivity patterns of the proposed tag antenna at

915 MHz in A, yz and B, xz planes