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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

Design and Performance of an Electrically Small Slot
Loop Antenna With a Miniaturized SuperconductingMatching Circuit
Y. Tsutsumi, H. Kanaya, and K. Yoshida

Abstract—In order to realize a small superconducting receiver,
we designed an electrically small slot loop antenna with a miniaturized impedance matching circuit. The area of the slot loop antenna
4 mm on MgO substrate at a frequency of 2.4 GHz.
is 4 mm
By applying 2-pole bandpass filter technique, we propose a designtheory of the miniaturized impedance matching circuit that consists of coplanar waveguide (CPW) quarter and half wavelengthtransmission lines of meandering geometry and the impedance inverting circuits (J inverters). By using electromagnetic field simulator, we realized the circuit which matches the small radiation reof the electrically small slot loop antenna
sistance
to the feeder impedance 0, and attained the normalized
bandwidth of 3%. We also made a preliminary experiment on thisminiaturized antenna using a YBCO thin film on MgO substrate.

= 0 776= 50Index Terms—Coplanar waveguide, miniaturized impedance
matching circuit, slot loop antenna.

I. INTRODUCTION

DURING recent years, miniaturized planar antennas are essential to radio communication devices such as wirelessLAN, RF-ID and MIMO in information society, because thesize of antenna often has a great influence on the whole sizeof a wireless system [1]–[5].
The characteristic of the antenna is closely concerned with its
size. Some antennas are designed at the cost of their bandwidth
for miniaturization [1]. On the other hand, many studies havedone for wide bandwidth [2]. Antennas fed by CPW, that havethe advantages such as low radiation loss, flexible configurationand so on, have also studied frequently [3]–[5].
There are also many reports on high-temperature superconducting (HTS) antennas, which have high efficiency due to highQ of superconducting resonators [6]–[11]. A small antenna issensitive to the conductor resistance because its radiation resistance is small. Thus, an HTS antenna, whose electrode resistance is negligibly small, is useful for miniaturization.

Fig. 1.

The equivalent circuit of the matching circuit for the antenna.

In our previous works, we have designed slot dipole antenna
whose length is one wavelength [8] and the antenna connected tothe low noise amplifier with a prototype matching circuit using
CPW [9], [10]. In order to increase the directivity of the antenna,
array antenna was also designed by folding the slot antenna andbranching out the feed line [10], [11].
In this paper, we propose a new design formula of an electrically small antenna (ESA), that is, an antenna whose dimension is much smaller than one wavelength, with a miniaturized
matching circuit. In the present study, we designed a slot loop
antenna with a 2-pole matching circuit using CPW. Furthermore, we tested its performance by experiment as well as electromagnetic field simulator.

II. DESIGN OF IMPEDANCE MATCHING CIRCUIT FOR ESA
Fig. 1 shows the circuit model of the proposed matchingcircuit for the antenna, which was obtained by modifying
the design theory given in [8], [12]. The matching circuit
becomes smaller by using a quarter wavelength transmissionline. In the figure,andare theimpedance of the antenna and the external port, respectively.
The proposed design values for the characteristic impedanceof the transmission lineand J-parameters ( ,) aregiven by

Manuscript received October 5, 2004. This work was supported in part by
a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS) and in part by a grant of Fukuoka project in the Cooperative Link of Unique Science and Technology for Economy Revitalization(CLUSTER) of the Ministry 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, Fukuoka 8128581 (e-mail: yoshida@ed.kyushu-u.ac.jp).
Digital Object Identifier 10.1109/TASC.2005.850187
1051-8223/$20.00 © 2005 IEEE

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TSUTSUMI et al.: ELECTRICALLY SMALL SLOT LOOP ANTENNA WITH MINIATURIZED MATCHING CIRCUIT

Fig. 2.

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Configuration of the slot loop antenna.

withwhere is the reactance slope parameter at the series resonanceof the antenna impedance [8], is the normalized frequency, 1, 2, 3) is the normalized filter elebandwidth, and (andare the unloaded Q of antenna and the externalment,Q, respectively, and (, 2) is the susceptance slope parameter.
It is noted that, because the present design theory of thematching circuit is based on that of the bandpass filter [12], it
is possible not only to match impedance but also to adjust thebandwidth .

Fig. 3. (a) L and W (parameter: s = 15:5 m) dependence of R and x ,(b) s (parameters: L = W = 4000 m) dependence of R and x .

III. SIMULATION OF SLOT LOOP ANTENNA
A. Slot Loop Antenna
The structure of proposed slot loop antenna is shown Fig. 2.
The total length of the slot loop including feed line is half ofeffective wavelength. The backend of the loop is the voltage
short end, thereby the whole operates as an in-series resonator.
As shown in Fig. 3, the radiation resistanceand the reof the slot loop antenna are depenactance slope parameter
dent on both the area of the loop and the slot width at 2.45 GHz.
In order to widen the bandwidth of the antenna, that is, to lower
the Q value of the antenna, the area of the loop portion
needs to be expanded. In order to miniaturize of the antenna,
the value ofis set as 4000
. The frequency depenis
dence of the input impedance of the slot loop antenna
andareshown in Fig. 4. Thereby,.
obtained for the gap spacing

Fig. 4.

Antenna impedance of the slot loop antenna. Parameters: L = W =4000 m, s = 15:5 m.

B. Slot Loop Antenna With Matching Circuit Using CPW
Fig. 5 shows the layout of the slot loop antenna with 2-polematching circuit. J inverters consist of interdigital gap andin-series resonators consist of meandering coplanar waveguide.
Fig. 6 shows the frequency dependence of the return loss.

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

Fig. 5. The layout and design value of the slot loop antenna with 2-polematching circuit.

Fig. 8.

j j

Fig. 6. The frequency dependence of S of the slot loop antenna with 2-polematching circuit ((a) return loss, (b) Smith chart).

Directivity pattern of the slot loop antenna.

Fig. 9. Photograph of the slot loop antenna with 2-pole matching circuit.
.
Substrate: MgO (size 15 mm 15 mm), film: YBa Cu O2IV. EXPERIMENTAL RESULTFig. 7.

The magnetic current of the slot loop antenna.

Simulation result is in fair agreement with the result of transmission line model. The desirable bandwidth (,) and the center of frequency
have been
andobtained, and impedance matching between
is realized in the passband.
The magnetic current and radiation pattern are shown in Fig. 7and Fig. 8, respectively. It is thought that because the magnetic
current at the backend of a loop portion mainly radiates, thedirectivity pattern resembles that of dipole antenna.

Fig. 9 shows the photograph of the prototype YBCO slot loop
antenna with the miniaturized matching circuit fabricated by thewet etching process. This antenna was placed in a vacuum cryostat, and was measured by using coplanar waveguide probes.
Figs. 10(a) and (b) show the experimental results for thereturn loss of the present miniaturized antenna. The frequency
response of -parameter was measured with vector network
analyzer (HP-8722C) at 20 K. In the figures, the results oftransmission line models are also plotted for discussion. As forthe return loss, the value of 3 dB or less was obtained in thepassband. Because of the over-etching and the residual loss
from the connection of the probe, the return loss is inferior tothose of the simulation results. It seems that the shift of the observed center frequency results from the influence of the kinetic
inductance and/or the variation of MgO relative permittivity.
As shown in Fig. 10(a), the center frequency that takes accountof the kinetic inductance was shifted to the lower frequency.
Similarly, as shown in Fig. 10(b) the center frequency simulatedusing larger dielectric constant value was shifted to the lowerfrequency too. These influences were obtained by changing theinductance and capacitance of transmission lines as a primary
approximation in transmission line models.

TSUTSUMI et al.: ELECTRICALLY SMALL SLOT LOOP ANTENNA WITH MINIATURIZED MATCHING CIRCUIT

Fig. 10.

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Obtained frequency characteristic of the return loss and the calculations taking account of (a) kinetic inductance, and (b) permittivity variation.

However, the observed fractional bandwidth and the two
filter are in reasonablepeaks which are characteristic ofagreement to the result of simulation.
V. CONCLUSION
In this paper, a slot loop antenna with a novel matching circuit
based on 2-pole bandpass filter has been designed and tested. We
succeeded to realize the circuit which matches the small radiaof ESA to the feeder impedancetion resistance. As a result, we designed a small (4 mm 8 mm),) planar antenna.
and broadband (Moreover, YBCO slot loop antenna with 2-pole matching circuit was fabricated and tested in the cryogenic temperature.
We will further fabricate the miniaturized RF receiver
front-end by combining the small antenna with RF semiconductor devices described in [9], [10].
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