878

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007

Electrically Small Superconducting Antennas
With Bandpass Filters
S. Oda, S. Sakaguchi, H. Kanaya, R. K. Pokharel, and K. Yoshida

Abstract—In order to reduce the size of a wireless system, we
have proposed the design formulas for an electrically small antenna(ESA), i.e. an antenna whose dimension is much smaller than awavelength, with a miniaturized matching circuit which connects
external circuit. Theoretical performances of the slotto a 50
dipole antenna with the impedance matching circuit designed by
the present theory are studied by the electrical circuits using transmission lines (Transmission Line Model) as well as the electromagnetic (EM) field simulator. We designed a slot dipole antenna withthe aid of the simulations using the electrical circuits as well as theEM field simulator. The size of the designed antenna including the1.9mm on MgO substrate with relmatching circuit is 4.1mm
ative permittivity of 9.6 at the center frequency of 5GHz, and thedesigned fractional bandwidth is 13%. In order to demonstrate the theory, we also carried outexperiments on the slot dipole antenna with the matching circuitsof pole number
using high temperature superconductors
YBCO on MgO substrates in the 5 GHz band.

@3 dB(Return Loss) ==2

Index Terms—Bandpass filter, Coplanar waveguide, miniaturized impedance matching circuit, slot loop antenna.

I. INTRODUCTION
URING recent years, miniaturized planar antennas are essential to radio communication devices such as wirelessLAN, MIMO and RF-ID in information society, because thesize of antenna often has a great influence on the whole size ofa wireless system. There are many reports on electrically smallantennas (ESA), i.e., antennas whose dimension is much smaller
than the wavelength of interest [1]–[5].
The characteristic of antenna is closely concerned with its dimension. A 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. High temperature superconductor (HTS) antennas, whose electrode resistance is negligibly small, are useful for miniaturization [6]–[11].
In order to realize an ESA, it is also necessary to realize a broadband matching circuit that compensates the narrow bandwidth

DFig. 1. Equivalent circuit of the antenna with n-pole bandpass filter.

peculiar to small antennas with an low impedance. We must
attain large matching ratios to connect to semiconductor devices
with high impedances.
Coplanar waveguide (CPW) fed antennas have advantages
such as low attenuation, less distortion, and uni-planar configuration. Furthermore, this feature provides much conveniencein system integration for that the input of the antennas can directly connect to the CPW outputs of semiconductor devices.
Many slot antennas suitable for a CPW-fed have been reported[3]–[5].
In our previous works, we designed the slot dipole antenna
whose length is one wavelength with a bandpass filter using
CPW lines [8], [9] which acts as an impedance matching circuit as well. In order to reduce the total size of antenna, the slot
loop antenna was also designed whose length is one wavelength
in circumference [10], and it was integrated to a low noise amplifier with the matching circuit for interconnecting them [11].
In this paper, we designed a slot dipole antenna whose dimension is much smaller than one wavelength with aid of EM-simulator and carried out the experiments on the slot dipole antenna with 2-poles bandpass filter, whose total size is 4.1 mm
1.9 mm, using high temperature superconductors YBCO thinfilm on the substrate with relative permittivity of 9.6, in the 5.0
GHz band.

II. THE THEORY OF ESA WITH BANDPASS FILTERS
Manuscript received August 29, 2006. This work was supported in part by aGrant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS) and by a grant of Fukuoka project in the Cooperative Linkof Unique Science and Technology for Economy Revitalization (CLUSTER) ofMinistry of Education, Culture, Sports, Science and Technology (MEXT).
The authors are with the Department of Electronics, Graduate School of Information Science and Electrical Engineering, Kyushu University, Motooka747,
Nishi-ku, Fukuoka 819-0395, Japan (e-mail: yoshida@ed.kyushu-u.ac.jp).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2007.898273

Fig. 1 shows the circuit model of the proposed ESA with-pole bandpass filter, which was obtained by modifying thedesign theory given in [10], [11]. In the figure,andrepresent the internal impedance of the antenna and that of the external port. In the present design, we use
slot dipole antennas whose length is much smaller than a wavelength, whose reactanceis assumed to behave inductively.

1051-8223/$25.00 © 2007 IEEEODA et al.: ELECTRICALLY SMALL SUPERCONDUCTING ANTENNAS WITH BANDPASS FILTERS

879The proposed theoretical values for the electrical length and) andcharacteristic impedance of transmission line ( ,-parameters of inverters ( ,) [12] are given by [13]Fig. 2. Structure of the slot dipole antenna.

with

where
is the normalized filter element,is the normalized bandwidth [8],andare the unloaded
Q factor of the antenna and the external Q factor, andis the susceptance slope parameter. The equations,and ’were obtained with the condition
for, i.e., the antenna impedance is sufficiency low.
It is noted that, because the present design formulas are derived from the conventional design theory of Chebyshev BPF[12], and it is possible not only to match the impedance but also
to adjust the bandwidth.
III. THE DESIGN OF SLOT DIPOLE ANTENNAS
Antennas with a bandpass filter were designed using HTS
films on MgO substrate in 5 GHz band. In the present design, weuse a slot dipole antenna whose length is much smaller than thewavelength, as shown in Fig. 2. In order to reduce the total sizeof antenna, the end of each slot, which is assumed to slightly
andcontribute to radiation, is bended.
are obtained for the length of slot,
and the width of slot
, by EM
field simulator.
Figs. 3(a) and 3(b) show the layouts and the designed valuesof the slot dipole antennas with a bandpass filter, in the case ofand, respectively. In these design, -inverters
consist of interdigital gaps in the center signal lines of CPW.
In order to keep the total size small even if the number of poleincreases, CPW lines are of meandering geometry.
Figs. 4 and 5 show the frequency dependences of the reflecwhich are calculated by EM-field simulator and transtion
mission line model in the case ofand, respectively.

Fig. 3. Layouts and design values of the slot dipole antennas with bandpass
filters in the case of n = 1 (a) and n = 2 (b).

These simulation results are in good agreement with transmission line model results. The desired bandwidths are obtained,and good impedance matches between the antenna impedance
and the feeder impedance
are attained in the passband.
Figs. 6 and 7 show the normalized radiation pattern and the, respectively. The dimagnetic current in the case ofrectivity looks like that of a typical slot dipole antenna, since
it seems that the radiation from the bending parts of the slot issufficiently small as shown in Fig. 7. The same result was also.
obtained in the case ofIncreasing the pole number , we can obtain wider bandwidth, theat the sacrifice of the size. With the port number ofbandwidth could be 8.6% at.

880IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007

Fig. 7. Magnetic current ofthe slot dipole antenna (EM field simulation).

Fig. 4. Frequency dependence of return loss of the slot dipole antenna with a1-pole bandpass filter [(a) Return Loss, (b) Smith Chart].

Fig. 8. Photograph of the slot dipole antenna with a 2-pole filter (Substrate:MgO, Film: YBa Cu O).

Fig. 5. Frequency dependence of return loss of the slot dipole antenna with a2-pole bandpass filter [(a) Return Loss, (b) Smith Chart].

Fig. 9. Obtained frequency characteristic of the input impedance [(a) experimental result, (b) simulation result].

IV. EXPERIMENT
A. Experimental Procedure

Fig. 6. Directivity pattern of the slot dipole antenna (EM field simulation).

An Experiment was carried out on the YBCO slot dipoleantenna with a 2-pole bandpass filter fabricated by wet etchingprocess, whose layout and photograph are shown in Fig. 3(b)and Fig. 8, respectively. The antenna was placed in a vacuum
cryostat, and was measured by using a coplanar waveguideODA et al.: ELECTRICALLY SMALL SUPERCONDUCTING ANTENNAS WITH BANDPASS FILTERS

881

curve taking account the reduction of -parameters due to overetching. Furthermore, the deterioration of the return loss seems
to be caused by a residual loss from the connection of the probe,
and this is simulated in Fig. 11, where the simulated return loss
taking account of the residual loss of a probe are comparativelyillustrated.
However, the observed two peaks which are characteristic offilters are in reasonable agreement to the theoretical performance. Considering experimental errors, it is demonstrated
that the designed matching circuit operated as expected.
V. CONCLUSION

Fig. 10. Comparison of frequency characteristics of the return loss and the calculations taking account of theover etching of J -inverters.

In this paper, slot dipole antennas with a bandpass filter
have been designed and tested. We designed a circuit whichof ESA tomatches the small radiation resistancethe feeder impedance. As a result, we designed,a small (4.1 mm 1.9 mm) and broadband () planar antenna. Moreover, an YBCO slot dipole
antenna with a 2-pole bandpass filter was fabricated and tested
at a cryogenic temperature, thereby we demonstrated the frequency characteristic as expected.
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Fig. 11. Comparison of simulated return loss taking account of theresidual lossof a probe.

probes. The frequency response of the -parameter was measured with a vector network analyzer (HP-8722C) at 20 K.
B. Experimental Result
Fig. 9(a) shows the experimental result for the input
impedance of the present miniaturized antenna, which is expressed asin Fig. 1, and Fig. 9(b) shows the EM-field
simulation result.
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