Low-profile, Electrically Small Meander Antenna Using a Capacitive Feed Structure

Kazuki Ide and Takeshi Fukusako

Department of Computer Science & Electrical Engineering,

Kumamoto University,

2-39-1, Kurokami, Kumamoto, 860-8555, Japan

ide@st.cs.kumamoto-u.ac.jp

Abstract – The present paper describes a capacitive feed (C-feed) for small, low-profile linear antennas. A

meander line, which is the radiating element, in such antennas with a metallic back conductor

electromagnetically couples a metallic feed plate placed between the meander line and the back metal

(ground). This antenna has a very small leakage current on the outer surface of the coaxial cable and can

control the imaginary part of the input impedance. The impedance characteristics and the antenna gain of

-10dBi are discussed based on simulated and measured results, which demonstrate successful achievement in

the impedance and antenna gain for a RFID tags etc.

Index Terms— Electrically small antenna, Low-profile antenna, Meander line, Capacitive feed, Impedance

matching, Leakage current

I. INTRODUCTION

Small, low-profile antennas with back reflectors have been investigated extensively in recent years

[1]-[3], and the reduction of the electrical effects generated by the backing material when the antenna

is installed on IC chips, the human body, or any metallic or lossy material has been a subject of

interest [4]-[6]. In low-profile linear antennas with back conductors [4]-[6], the real part of the input

impedance becomes zero at most frequencies other than parallel resonant frequencies when the

distance between the linear antenna and the back conductor is smaller than one-quarter wavelength.

It is usually difficult for such antennas to operate at the parallel resonance frequency [1], [6]. Using a

folded line, such as a meander line, and incorporating capacitances are effective ways to construct

electrically small antennas [6]-[10] that can be operated at the serial resonance frequency. A small,

low-profile antenna having an electrically small, low-profile structure that uses a capacitive coupling

with a back conductor and that can be operated at the serial frequency was presented in [6]. However,

a coupling terminal with a small area easily generates leakage current on the feeding coaxial cable

unless a ferrite choke on the cable is used, and this coupling terminal cannot control the imaginary

part of the input impedance when the antenna is connected to larger grounds, such as SMA

connectors. The leakage current existing on the outer surface of a coaxial cable causes drastic

changes in antenna characteristics [11], [12] when the feed is unbalanced.

The present paper proposes a novel feed structure using a capacitive feed (C-feed) technique. A

metallic feed plate placed between the back metal (ground plane) and the meander line provides

capacitance to the input impedance. The meander line, which is the radiating element, is

electromagnetically coupled to the feed plate. The antenna can easily control the imaginary part of

the input impedance even if the antenna is connected to larger grounds, such as SMA connectors. In

addition, the leakage current on the outer surface of the coaxial cable is very small because the

antenna has a large ground plane.

II. DESIGN OF A HALF-WAVELENGTH, SMALL, LOW-PROFILE ANTENNA WITH A

METALLIC REFRACTOR

A low-profile linear antenna with a reflector has been reported in [6]. The serial resonance can

exist near the parallel resonance having a peak of the real part of the input impedance by shifting the

feeding point of a dipole antenna from the center of a half-wavelength linear element. As a result, the

input impedance can be matched with the characteristic impedance of a feeding transmission line at

the serial resonance frequency even though the antenna is near the back conductor.

Figure 1 shows a meander line antenna (MLA) fed at the edge of a linear element with unbalanced

feeding. The metallic back conductor of the antenna acts as a ground. Figure 3 shows the input

impedance characteristics of the antenna. The structure is simulated using HFSS 10.1. The feed point

has high impedance at the parallel resonance frequency and high inductance around the parallel

resonance because the structure is equivalent to that of an open-ended micro-strip line. Therefore, the

equivalent circuit of the structure is an inductor. In order to cancel out the inductance, capacitance

should be introduced in the structure. For this antenna element, the CPW-like feed structure shown in

Fig. 2 was proposed in previous study [6]. The antenna uses an RT/Duroid 5880 substrate with a

thickness of 1.6 mm, a permittivity (εr) of 2.2, and a dielectric loss (tanδ) of 0.001. The substrate

dimensions are fixed at 22.5 mm × 14 mm (0.075λ0 × 0.047λ0) and satisfy the condition of an

electrically small antenna (ka = 0.3< 0.5). The meander line has a width (Wm) of 1 mm and gaps (Wd)

of 0.5 mm between adjacent lines. The ground planes (GP) of the CPW have a length (gl) of 3 mm

and a width (gw) of 1 mm. The feed point is coupled with the back conductor to form a capacitive

gap with the ground planes (GP) and the back conductor. Thus, this offset-fed structure yields

capacitive impedance at low frequency and approximates the parallel resonant frequency as the serial

resonant frequency. Figure 3 shows the input impedance characteristics of the antenna. The ground

of the CPW-like structure provides capacitance to the input impedance. The antenna has a simple

printed structure, and the impedance matching can be easily achieved. However, when the antenna is

connected to larger grounds, such as SMA connectors, it is difficult to control the impedance

characteristics because the ground of the antenna is much smaller than the ground of the connector.

In addition, the small ground area easily generates large leakage current on the outer surface of the

coaxial cable unless a ferrite choke on the coaxial cable is used. Therefore, a novel feed structure is

presented in the next section.

III. PROPOSED DESIGN AND STRUCTURE

Figures 4(a) and 4(b) show front and cross sectional views of the offset capacitive feeding

(C-feeding) meander antenna. The antenna has a thickness of 2 mm (ka = 0.37). The ground planes

(GP) of the CPW are removed from the antenna, and a metallic feed plate is installed between the

meander line and the back conductor. The back conductor, which acts as the ground plane, is 22.5

mm × 14 mm. The meander line has the same dimensions as in the CPW feed antenna. The feed plate

has a length (fl) of 14 mm and a width (fw) of 2 mm. The imaginary part of the input impedance of

the antenna is controlled primarily by varying fl and fw, substrate thicknesses Th1 and Th2, and the

length of the extended meander line (ml), as shown in Fig. 4. The equivalent circuit of the antenna is

shown in Fig. 4(d). Two capacitances are incorporated in the structure to achieve the required

impedance of 50 Ω. The first capacitance is located between the feed plate and the back conductor,

and the second capacitance is located between the feed plate and the meander line.

IV. SIMULATED AND MEASURED RESULTS

A. Impedance Matching Technique

There are several parameters that can be used to control the imaginary part of the input impedance

characteristics because the antenna has two capacitances, as shown in Fig. 4(d). Many applications

require the imaginary part of the input impedance to be controlled so as to maintain good impedance

matching. The input impedance characteristics of the C-feed antenna for different values of fl, fw, ml,

Th1, and Th2 are shown in Figs. 5(a) through 5(e), respectively. At a particular frequency, the

impedance becomes increasingly capacitive as fl, fw, and ml decrease and Th1 and Th2 increase. The

imaginary part of the input impedance can be independently controlled using these parameters,

which can be designed freely using a Smith chart.

Figures 6(a) and 6(b) show the input impedance characteristics of the CPW-feed and C-feed

antennas simulated with a SMA connector. The C-feed antenna can easily control the imaginary part

of the input impedance even if it is connected to a larger ground and shows good impedance

matching for the case in which fl is 14 mm.

B. Leakage Current

The simulated current distribution on the outer surface of the coaxial cable is shown in Figs. 7(a)

and 7(b) for the CPW-feed and C-feed antennas, respectively. The feed is given at the far end of the

coaxial cable. The CPW feed antenna shows a large leakage current as compared to the C-feed

antenna. The gain characteristics of both antennas are shown in Figs. 8(a) and 8(b). Figure 8(a),

which shows the gain characteristics of the CPW-feed antenna, shows that the antenna has high gain

when connected to a coaxial cable because the signal radiated from the coaxial cable contributes to

the antenna gain. However, the maximum gain of the C-feed antenna is constant even in the presence

of the coaxial cable. This indicates that the current on the coaxial cable has been suppressed

sufficiently, and the cable has little effect on the antenna characteristics.

C. Measured Results

Figure 9 shows the fabricated antenna. The simulated and measured results for the S11

characteristics of the antenna are shown in Fig. 10. The effect of touching the SMA connector with

the human hand is also investigated, and the results are shown in Fig. 10. The antenna shows stable

S11 characteristics, even if the SMA connector is touched with the hand. Figure 11 shows the

radiation pattern of the antenna. Although the antenna has the same radiation pattern as a dipole

antenna, the front-back ratio can be improved by using a larger back conductor. The simulated

maximum realized gain is -9.8 dBi, and the measured gain is -13.9 dBi, which is a practical value for

short-range wireless tags. This difference is probably due to fabrication error and a slight leakage

current. The antenna gain can be improved by improving the antenna element structure.

V. CONCLUSION

A capacitive feed structure for small, low-profile antenna with ka = 0.37 is simulated, fabricated,

and measured. The proposed C-feed antenna provides good impedance matching characteristics even

if the antenna is connected to larger grounds and shows a small leakage current on the outer surface

of the coaxial cable. The proposed antenna has potential applications in RFID and mobile terminals.

REFERENCES

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Antennas Propag. Mag., vol. 46, No. 6, pp. 9–22, Dec. 2004.

[2] K. V. S. Rao, P.V. Nikitin, and S.F.Lam, “Antenna design for UHF RFID tags: a review and a practical

application”, IEEE Trans. Antennas Propag., vol. 53. No. 12, pp.3870-3876, Dec. 2005

[3] Christopher T. Rodenbeck, “Planar minitature RFID antennas suitable for integration with batteries”,

IEEE Trans. Antennas Propag., vol. 54. No. 12, pp.3700-3706, Dec. 2006

[4] T.Tsukiji and Y. Kumon, “Modified transmission line type antennas for mobile communication”, IEICE

Trans. Commun. E75-B, 8, pp.775-780, Aug. 1992

[5] A. Thumvichit and T. Takano, “Ultra low profile dipole antenna with a simplified feeding structure and a

parasitic element,” IEICE Trans. Commun. E89-B, 2, pp.576-579, Feb. 2006

[6] T. Terada, K. Ide, K. Iwata, and T. Fukusako “Design of a small, low-profile print antenna using a peano

line,” Microw. Opt. Technol. Lett., vol. 51, No. 8, pp.1833-1838, Aug. 2009

[7] W. Choi, S. Kwon, and B. Lee, “Ceramic chip antenna using meander conductor lines” , Electron Lett.,

vol. 37, No. 15, pp.933-934, July 2001

[8] J. Zhu, A. Hoorfar, and N. Engheta, “Peano antennas”, IEEE Antennas Wireless. Propag. Lett., vol. 3, pp.

71–74, 2004.

[9] J. Zhu, A. Hoorfar, and N. Engheta, “Bandwidth, cross-polarization, and feed-point characteristics of

matched Hilbert antenna”, IEEE Antennas Wireless. Propag. Lett., vol. 2, pp. 2–5, 2003.

[10] C.R. Rowell and R.D. Murch, “A capacitively loaded PIFA for compact mobile telephone handsets”

IEEE Trans. Antennas Propag., vol. 45. No. 5, pp.837-842, May 1997

[11] S.Sekine. and H. Shoki, “Characteristics of T-type monopole antenna with parallel resonance mode”

IEICE Trans., J86-B, 2, pp.200-208, Aug. 2003.(in Japanese)

[12] C. Icheln, J. Krogerus, and P. Vainikainen, “Use of balun chokes in small- antenna radiation

mesurements”, IEEE Trans. Instrum. Meas., vol. 53. No. 2, pp.498-506, April. 2004

Figure Captions

Figure 1: Offset-fed meander line antenna

(a) Top view

(b) Cross-sectional view

(c) Equivalent circuit

Figure 2: Offset-fed meander line antenna with a CPW-like structure

(a) Top view

(b) Cross-sectional view

(c) Equivalent circuit

Figure 3: Effect of the CPW structure on input impedance

Figure 4: Proposed small, low-profile antenna using C-feed

(a) Top view

(b) Cross-sectional view

(c) Extended meander line

(d) Equivalent circuit

Figure 5: Effect of parameters on input impedance using a Smith chart. The frequency range is from

0.75 GHz to 1.5 GHz

(a) Variation in fl

(b) Variation in fw

(c) Variation in ml

(d) Variation inTh1

(e) Variation in Th2

Figure 6: Variation in input impedance of antennas with SMA connectors

(a) Variation in length of ground plane (gl) of the CPW-feed antenna

(b) Variation in length of feed plate (fl) of the C-feed antenna

Figure 7: Current distribution on the outer surface of the coaxial cable

(a) CPW-feed antenna

(b) C-feed antenna

Figure 8: Simulated absolute gain characteristics

(a) CPW-feed antenna

(b) C-feed antenna

Figure 9: Photograph of the proposed antenna

Figure 10: S11 characteristics with and without touching the SMA connector

Figure 11: Radiation patterns of the proposed antenna

(a) yz-plane

(b) xz-plane

1.60 mm

Meander line

Dielectric substrate

Ground

(b)

λ/4 < l < λ/2

Back metal - Ground

Zc

(c)

Figure 1

(a)

y

xz

yx

z

(a)

Back metal - Parasitic metal

Zc

λ/4 < l < λ/2

(c)

1.60 mm

Meander line GP

Dielectric substrate

Parasitic metal layer

(b)

Figure 2

y

xz

yx

z

-6000-5000-4000-3000-2000-1000

0100020003000400050006000700080009000

10000

0.75 1 1.25 1.5Frequency [GHz]

Inpu

t im

peda

nce

[Ω]

Re (Printed MLA)

Im (Printed MLA)

Re (Printed CPW-MLA)

Im (Printed CPW-MLA)

Figure 3

Figure 4

(a)

y

xz

yx

z

z

yx

Dielectric substrate

Dielectric substrate

Parasitic element

Feed plate

Ground

2.0 mmZc Th1

Th2

(b)

Back metal - Ground

Zc

λ/4 < l < λ/2

(d) (c)

Dielectric substrate

Dielectric substrate

Parasitic element

Feed plate

GroundZc

2.0 mmTh1

Th2

(a)

(c)

(d)

(e)

Figure 5

(b)

-3000-2500-2000-1500-1000-500

0500

1000150020002500

0.8 0.9 1 1.1 1.2

Inpu

t im

peda

nce

[Ω]

Frequency [GHz]

Re (gl=1 mm)Im (gl=1 mm)Re (gl=3 mm)Im (gl=3 mm)Re (gl=5.5 mm)Im (gl=5.5 mm)

(a)

-160-140-120-100-80-60-40-20

020406080

100

1 1.1 1.2 1.3 1.4

Inpu

t im

peda

nce

[Ω]

Frequency [GHz]

Re (fl=2 mm)Im (fl=2 mm)Re (fl=7 mm)Im (fl=7 mm)Re (fl=14 mm)Im (fl=14 mm)

(b)

Figure 6

(b)

(a)

Figure 7

l = 100 mm

l = 100 mm

SMA connector

SMA connector

Feed

Feed

-25

-20

-15

-10

-5

0

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2Frequency [GHz]

Abs

olut

e ga

in [

dBi]

without coaxial cable

with coaxial cable

(a)

-60

-50

-40

-30

-20

-10

0

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2Frequency [GHz]

Abs

olut

e ga

in [

dBi]

without coaxial cable

with coaxial cable

(b)

Figure 8

Figure 9

[mm]

SMA connector

y

xz

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1 1.1 1.2 1.3 1.4

S11

[dB

]

Frequency [GHz]

Sim

Meas-Without touching SMA connector

Meas-Touching SMA connector

Figure 10

Figure 11

0-10

-20

-30

-40

90

60

30

0

330

300

270

240

210

180

150

120

(dB)

sim-co_polsim-cross_polmea-co_polmea-cross_pol

(b)

0-10

-20

-30

-40

90

60

30

0

330

300

270

240

210

180

150

120

(dB)

sim-co_polsim-cross_polmea-co_polmea-cross_pol

(a)