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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
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
[1] S. R. Best, “A discussion on the properties of electrically small self-resonant wire antennas”, IEEE
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)