Proceedings of Asia-Pacific Microwave Conference 2007

Design and Performance of an Electrically Small
Antenna with Matching Circuit
H. Kanaya, R. Nabeshima, R. K. Pokharel, and K. Yoshida
Graduate School of Information Science and Electrical Engineering
Kyushu University
744, Motooka, Fukuoka, 819-0395, JAPANkanaya@ed.kyushu-u.ac.jp
Abstract—We designed and fabricated an electrically small
antenna (ESA) with coplanar waveguide (CPW) matching circuit.
Matching circuit is realized by using an interdigital gap and atransmission line. We designed ESA with the aid of thecommercial three-dimensional electro magnetic field simulator.
We also made experiments on the ESA with CPW matchingcircuits using patterned circuit board.

performances of the ESA with the CPW impedance matchingcircuit are calculated by using the transmission line model aswell as the commercial electro-magnetic (EM) field simulator(Ansoft; HFSS ver.10). In order to demonstrate the theory, we
also carried out experiments on the ESA at 2.4 GHz, which has
the size of 22.3 mm x 8.204 mm, with the pole number of n = 1
matching circuits using FR4 printed circuit board.

Keywords-Slot antenna, electrically small antenna, coplanaer
waveguide, matching circuit.

INTRODUCTION

Studies are made of an electrically small antenna (ESA),
i.e., the antenna whose dimension is much smaller than onewavelength, towards further reduction of the antenna size inorder to realize ubiquitous wireless systems [1, 2]. It is widely
known, however, that in order to realize the miniaturized
antenna.
Moreover, we must simultaneously realize abroadband impedance matching circuit, which compensates thenarrow bandwidth peculiar to the small antenna with low
radiation resistance, and we must attain large impedancematching ratios to connect with semiconductor amplifiers withhigh internal impedances. Also, the small antenna is sensitive
to the conductor resistance because of its low radiation
resistance, and the decrease of the radiation efficiency often
makes serious problem.
Slot antennas with coplanar waveguide (CPW) feed line are
preferable for monolithic microwave integrated circuit(MMIC’s) and RFIC (radio frequency integrated circuit) sinceno via holes are required for integration with devices [3-5].
In our previous works, we designed a one-wavelength slot
dipole antenna and a half-wavelength slot loop antenna with abandpass filter using CPW lines, which acts as an impedance
matching circuit as well [6-9]. Also, we have designed a slot
dipole antenna whose dimension is much smaller than one
wavelength with aid of EM-simulator, and carried out theexperiments using high temperature superconductors YBCO
thin film on the MgO substrate with relative permittivity of 9.6
in the 5.0 GHz band [10].
In this paper, we design the impedance matching circuit,which connects an ESA fabricated on the normal metal to asemiconductor amplifier. The proposed matching circuits have
performances similar to those of the n-pole bandpass filter(BPF) [11]. By using the quarter wavelength transmission line
and admittance (J) -inverter, we can reduce the total antenna
size more than one-wavelength. In the beginning, theoretical

The conventional slot antenna is a one-wavelength center
feed slot antenna as shown in Fig. 1. The designed center
frequency is 2.4GHz-band. The substrate has dielectric
constant εr=4.25 and tanδ = 0.015. The thickness of thesubstrate and copper top metal is 0.8 mm and 18 μm,
respectively. Three-dimensional EM simulator simulates the
RF properties. Fig. 2 shows the return loss of the standard slot
antenna. The input impedance is almost 50 Ω around 2.4 GHz,and –10 dB bandwidth is 165 MHz.

74.0mmW
feed⎧ L = 38 .0mm
⎪W = 18 .0mm⎪
Cross section
⎨
feed
=
5
.
00
mm
⎪
0.018mm⎪⎩edge = 1.00 mm

Y
Z

X

edge
copper0.80mmFR4: εr=4.25
tanδ=0.015(@1.8GHz)

Figure 1. Layout of the standard slot dipole antenna.

Fig. 3 shows the simulated layout of an electrically small
antenna (ESA). The physical properties of the substrate are thesame as those of standard antenna. The antenna size is 0.18λ0
and 0.067λ0, respectively, where λ0 is the wavelength in thevacuum at 2.45 GHz, which is especially smaller than that of
the standard dipole slot antenna. Fig. 4 shows the Za

This work was partly supported by a Grant-in-Aid for Scientific Research(C) from the Japan Society for the Promotion of Science (JSPS).

1-4244-0749-4/07/$20.00 @2007 IEEE.

L

24.0 mmI.

DESIGN OF THE ELECTRICALLY SMALL ANTENNA
(ESA)

II.

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(Za=Ra+jXa) of the slot antenna, where Ra represents radiation
resistance and metal loss, and Xa is the reactance of the antenna.
Because the quarter-wavelength parallel resonance appears at
2.74 GHz, the size of this antenna is electrically smaller thanquarter wavelength when the antenna works at 2.4GHz. Za is42.5+j385 Ω at 2.4 GHz, which is far from 50Ω, so that, it isshown that most RF signals reflect for the impedance missmatch.

The present matching circuit is based on the bandpass filter(BPF) composed of the transmission line and J-inverters. Fig.
5(a) shows the n=1 BPF, where, Y0 is the admittance of theload and B1 is the susceptance in the parallel resonator with asusceptance slope parameter b1. The design parameters of then=1 BPF are
J 01 = w

Y0b1
bY, J12 = w 1 0g 0 g1g1 g 2(1)0where, w is the relative bandwidth and gi is the filter
parameter. Fig. 5(b) shows the equivalent circuit model of theFig. 5(a) at the center frequency. In the figure, G’S and G’L,
and Qei are the equivalent conductance and external quality
factor, respectively.
Fig. 6(a) shows the equivalent circuit model for the smallest
matching circuit, corresponding to BPF with n=1. In thisfigure, Za (=1/Ya) denotes the input impedance of ESA and ZL(=1/YL) represents the input impedance of LNA or the output
impedance of PA. Z01 (=1/Y01) and θi are characteristic
impedance and electrical length of the transition line. Fig. 6(b)shows the equivalent circuit model of the Fig. 6(a) at the center
frequency. In Fig. 6(a), Y’ is the admittance for looking theantenna from A-A’, and Y’in is the admittance for looking
amplifier from A-A’ and given by,

Rreturn Loss [dB]

-5
-10
-15
-20
-25
-30
-35

2

2.2

2.4
2.6
Frequency [GHz]2.8

3

Figure 2. Return loss of the standard slot dipole antenna.

22.30mm(0.18λ0)8.204mm(0.067λ0)

Y' = G '+ jB ' = Y01

Z

Finally, the comparison of the Fig. 5(b) and 6(b), theproposed design value θ01, Z01, θ02, Cm are led in the numericalvalue as follows:

4000
Antenna Impedance (Za) [Ω]

. (3)

X

Figure 3. Layout of the electrically small slot dipole antenna.

Ra

3000

Im B ' ω =ω = Im Bin ' ω =ω = 0 (θ 01 , θ 02 ≤ π 2)

Xa

0

2000
1000
0

-1000
-2000

(2)

1
+ jY01 tan(−θ 02 )Z L + 1 / jωC m
Yin ' = Gin '+ jBin ' =1
Y01 + jtan(−θ 02 )Z L + 1 / j ωC m

Za=Ra+jXa

Y

Ya + jY01 tan θ 01
Y01 + jYa tan θ 01

2

2.2

2.4
2.6
Frequency [GHz]

2.8

(4)

b'
⎛ g g ⎞
= Qe1 ⎜ = 0 1 ⎟
G'
w ⎠
⎝

(5)

b'
⎛ gg ⎞= Qe 2 ⎜ = 1 2 ⎟
Gin '
w ⎠
⎝

(6)

b' =

3

0

ω 0 ∂B' ( Z 01 ,θ 01 )
. ∂ω
.
2
ω =ω

(7)

0

Figure 4. Input impedance of the ESA without matching circuit.

III. DESIGN OF THE IMPEDANCE MATCHING CIRCUIT
BETWEEN ESA AND SEMICONDUCTOR AMPLIFIER
In order to connect between ESA and RF front-end,
impedance matching must be attained between the antenna andamplifiers.

Fig. 7 shows the simulated layout of the ESA with CPWtransmission line which has Z01 and θ01 (See A-A’ in Fig. 6 (a)).
Fig. 8 shows the frequency dependence of the Y’. We can see
the parallel resonance around 2.4 GHz.
Fig. 9 shows the layout of ESA with CPW matching circuit.
In order to realize θ02 and Cm, we adopted the interdigital gap
and CPW transmission line, where ZL is assumed to be Z0=502072

Ω by the experiment to be convenient. Fig. 10 shows the input
impedance (Zin), which is the impedance for looking theantenna from B-B’ in Fig. 6 and Fig. 9. Fig. 11 shows thereturn loss and Smith chart of the ESA with CPW matchingcircuit. Zin is almost 50Ω around 2.4 GHz, so that, return loss
is –32.58 dB at 2.4 GHz. Fig. 12 shows the simulated radiation
pattern of the ESA with CPW matching circuit.

Layout

jB1

J0,1

L2

B

1mm
Inverter

(a)
Y0

(b)
G s′ =b1
Qe1

G L′ =

jB1

b2
Qe 2

Zin

W

B’

⎧ L1 = 10.0mm⎪ L = 4.60 mm⎪ 2⎪⎪W = 1.00 mm
J-Space⎨⎪ J − Space = 0.300 mm
Tooth Length ⎪⎪ToothLength = 0.200 mm⎩⎪ToothWidth = 0.148 mm
Tooth Width

Y0

J1,2

22.30mm (0.18λ0)8.204mm(0.067λ0)

L1

Figure 9. Layout of the ESA with CPW matching circuit.
300

(a)

Y’

A

Yin’

Antenna

ZL

θ01
θ0

(b)

Amp.

B

Cm

Z01

Za

Zin

Input Impedance (Zin) [Ω]Figure 5. Circuit model of the n=1 bandpass filter (a) and equivalent circuit
model at center frequency (b).

A’Im [Zin]

100
0

-100
-200

B’

−θ02

Re[Zin]

200

-300

2

2.2

2.4
2.6
Freqency [GHz]2.8

3

Figure 10. Input impedance of the ESA with matching circuit.
G’

jB’

Yin’

0

Return Loss [dB]

Figure 6. Circuit model of the n=1 impedance matching circuit (a) andequivalent circuit model at center frequency (b).

22.30mm

(b)

(a)

-5
-10
-15
-20
-25

8.204mm

-30
-35

A

Y’

2

2.2

2.4
2.6
Frequency [GHz]2.8

3

Figure 11. Return loss (a) and Smith chart (b) of the ESA with CPW matchingcircuit.

A’

Figure 7. Layout of the electrically small antenna with CPW feed line.

Input admittance (Y') [mS]

4

Z

3

θ

2

φ =0°

1
0

Y

-1

φ

-2
G'
B'

-3
-4

2

2.2

2.4
2.6
Frequency (GHz)

2.8

X

3

φ =90°
Figure 12. Simulated radiation pattern of the ESA.

Figure 8. Input admittance (Y’) of the ESA with CPW transmission line.

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θ

IV.

EXPERIMENTAL RESULTS AND DISCUSSIONSAn ESA is fabricated on FR4 substrate by using the print
board making equipment (MITS; FP-21T model 40), which has
100 μm-diameter milling cutter.
Fig. 13 shows thephotographs of the ESA with CPW matching circuit. In the
figure, an interdigital gap is also shown. RF signal is input
through MMCX connecter, which has characteristic impedance= 50 Ω. We measured the S-parameters by using a GP-IB
controlled vector network analyzer (HP; HP8722C).
Fig. 14 shows the experimental results of the return loss of
the ESA with CPW matching circuit.
The observed
experimental results are caused by an error in edge part of theinterdigital gap (see Fig. 13), an error of the dielectric constantof the FR4 substrate and residual loss of the connection
between connector and antenna. So, simulated results taking
account of the permittivity error are also shown. In EM
simulation, we made the εr from 4.25 to 4.05, so that, theexperiment results and the simulation values are corresponding
well.
Fig. 15 shows the comparison of the standard slot antenna
with the ESA. The antenna size can be reduced to about 90 %and bandwidth design becomes possible.

the amplifier. As a result, we designed a small-size (22.30 mm
x 8.204 mm) slot antenna. Moreover, ESA with CPW
matching circuit was fabricated and measured the RF
properties, thereby we demonstrated frequency characteristics
as expected.

Figure 15. Comparison of the standard slot antenna with the ESA.

REFERENCES[1]

N. Behdad, and K. Sarabandi, “Bandwidth enhancement and further size
reduction of a class of miniaturized slot antennas”, IEEE Trans.
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421-429, March 2003.
[3] B.K. Kormanyos, W. Harokopus, L. Katehi, and G. Rebeiz, “CPW-fed
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[4] S. S. Garcia, and J. J. Laurin, “ Study of a CPW Inductively coupled slot
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InverterFigure 13. Photograph of the ESA.

0

Return Loss [dB]

-5

Exp.

-10
-15
-20
-25
-30

Sim.

-35
2

2.2

2.4
2.6
Frequency [GHz]2.8

3

Figure 14. Experimental results and simulated results by taking account ofpermittivity error.

V.

CONCLUSIONSIn this paper, slot dipole antenna with a bandpass filter
have been designed and tested. We succeeded in realizing thecircuit which matches the small radiation resistance of ESA to2074