AbstractWe examined a microstrip line structure to reduce the current concentration at the outer edge of a microstrip line. We developed three kinds of microstrip line; sliced-microstrip line, layered-film microstrip line, and conventional microstrip line. The first microstrip line is a line divided into the narrow line, and the second one is a line made by a layered thin film. Electromagnetic simulations indicated that the current concentration of the outer edge of the sliced microstrip line and layered film microstrip line was lower than that of a conventional microstrip line. We measured the power handling capability of the filters made by sliced-microstrip lines, layered-film microstrip lines, and conventional microstrip lines. The value of the first and the second filters was better than that of a conventional one. The difference in the characteristics is based on the difference in the current concentration at the outer edge of the microstrip lines

Endo, M.

Honma, T.

Nakagawa, K.

Ohshima, S.

Saito, A.

Sato, S.

Takahashi, S.

Takeda, K.

Tanaka, Y.

Elsevier - Publisher Connector 

Improved Power Handling Capability of Superconducting Microstrip Lines for Microwave Devices 

 Physics Procedia  36 ( 2012 )  429 – 434 
1875-3892  © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. 
doi: 10.1016/j.phpro.2012.06.258 
Superconductivity Centennial Conference 
Improved Power Handling Capability of Superconducting 
Microstrip Lines for Microwave Devices 
 
S. Ohshima*, M. Endo, K. Takeda, K. Nakagawa, T. Honma, S. Sato,    
S. Takahashi, Y. Tanaka, A. Saito  
Graduate School of Science and Engineering, Yamagata University, 
4-3-16, Jonan Yonezawa, 992-8510, Japan 
 
Abstract 
We examined a microstrip line structure to reduce the current concentration at the outer edge of a 
microstrip line. We developed three kinds of microstrip line; sliced-microstrip line, layered-film 
microstrip line, and conventional microstrip line. The first microstrip line is a line divided into the narrow 
line, and the second one is a line made by a layered thin film. Electromagnetic simulations indicated that 
the current concentration of the outer edge of the sliced microstrip line and layered film microstrip line 
was lower than that of a conventional microstrip line. We measured the power handling capability of the 
filters made by sliced-microstrip lines, layered-film microstrip lines, and conventional microstrip lines. 
The value of the first and the second filters was better than that of a conventional one. The difference in 
the characteristics is based on the difference in the current concentration at the outer edge of the 
microstrip lines. 
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and 
Peter Kes. 
keywords: Superconducting filter, HTS,YBCO, NbN,  layered film 
1.Introduction                                                                                                                         
Superconducting band pass filters have advantages such as high-frequency selection, small insertion loss, 
and a large out-of-band rejection [1-6]. Consequently, such filters have been put to practical use in 
receiving systems of base stations for wireless communications in the U.S.A [7] and China [8-9]. 
Recently, narrow bandpass filters made by HTS resonators were used for a rader of weather  in Japan [10]. 
Available online at www.sciencedirect.com
© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.
Open access under CC BY-NC-ND license.
Open access under CC BY-NC-ND license.
430   S. Ohshima et al. /  Physics Procedia  36 ( 2012 )  429 – 434 
However, the superconducting band path filters have only been used in receiving systems in which small 
amounts of electric power flow. If the power handling capability of the filter can be improved, it could be 
used as a transmitting filter, and hence its range of use could be expanded. Some papers are reported to 
increase a power handling capabilty of the microstripline filters [11-13]. We think that the limit of power 
handling capability of filters is caused by the current concentration at the outer edge of a microstrip line 
which the resonators of the filter consists of. [14-15]. Therefore, it is very important to reduce the current 
concentration on the edge of a microstrip line. We show three different shapes of the microstrip line; 
sliced-microstrip line, layered-film microstrip line, and conventional microstrip line. We simulated the 
current concentration at outer edge of the microstrip line and discussed the difference of the concentration 
current. In addition, we made filters made by the sliced-microstrip lines, layered film microstrip lines, and 
conventional microstrip lines   How the structure of the resonator shape affects  the electric power-proof 
of the filter is discussed. 
2. Simulation of current concentration at outer edge of microstrip line 
We examined three types of microstrip lines: sliced-microstrip lines, layered-film microstrip lines, and 
conventional microstrip lines. A schematic drawing of conventional, sliced and layered microstrip lines 
are shown in Figure 1. A sliced microstrip line (Fig.1 (b)) is a structure thinly divided along its length. 
The layered microstripline (Fig.1(c)) is a thin film that alternately accumulates the superconducting thin 
film and the insulating layer. It is reported that the current concentration of the layered superconducting 
thin film is about half the value of the conventional microstrip line [16].  A schematic drawing of the 
predicted current concentration at the outer edge of microstrip lines of conventional, as well as the sliced 
and layered film microstrip line is shown in Figure 2. The current concentration of conventional 
microstrip line is quite large at the outer edge: however, the current concentration of a sliced microstrip 
line can be reduced at the outer edge of the same point. The current can flow inside the sliced microstrip 
line edge. The concentrated current in the conventional microstrip line flows at the bottom of microstrip 
line as shown in Figure 2 (c). Therefore, we must reduce the concentrated current to increase the power 
handling capability of microstrip line. To reduce the concentrated current at the bottom of a microstrip 
line, we propose a layered-film microstrip line. As shown in Fig.2 (d), the concentrated current of a 
layered-film microstrip line at the bottom can be reduced 
The configration of the three-pole filter is shown in Figure 3. We designed an optimal configuration of 
the filter using three-type shape resonator by an electromagnetic simulator, Sonnet EM. Table 1 shows the 
maximum current concentration at the outer edge of the conventional, sliced and layered microstrip line 
filter. The maximum current concentration of conventional and sliced microstrip lines was calculated by 
an electromagnetic simulator, Sonnet EM. The value of the layered-film was predicted from the descrived 
in reference 16. The maximum current concentration of a sliced microstrip line was about 25% smaller 
than that of conventional microstrip line, and the layered one was about 50% smaller. Therefore, we will 
obtain a high power handling capability of the filter by using a sliced and a layered microstrip line. 
 
Fig.1.  Schematic drawing of a three kinds of  microstrip lines:  (a) conventional, (b) sliced and  (c) 
layered-film microstrip lines. 
(a) (b) (c)
 S. Ohshima et al. /  Physics Procedia  36 ( 2012 )  429 – 434 431
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig.2. Schematic drawing of current concentration at outer edge, surface, and bottom of microstrip lines 
(a) current concentration of a conventional microstrip line at outer edge of microstrip line (b) current 
concentration of sliced-microstrip line at its outer edge (c) 
current concentration of conventional microstrip line at its 
bottom  (d) current concentration of  layered-film 
microstrip line at bottom. The Imax means the maximum 
current concentration at the outer edge or the surface of the 
microstrip lines in the figure. 
 
 
 
 
Fig.3. Configuration of sliced microstrip line filter and 
layered film microstrip line filter:  (a) sliced microstrip line 
resonator  and (b) layered-film microstrip line resonator. 
II (a) (b)
Imax
Imax
(a)
(b)
0
Imax
Imax
0
z
z
I
I
Superconducting film
Substrate
grand plane
Superconducting film
Substrate
grand plane
insulating film
(c)
(d)
432   S. Ohshima et al. /  Physics Procedia  36 ( 2012 )  429 – 434 
Table I. Current concentration at outer edge of microstrip line; The value of conventional and sliced- 
microstrip lines was obtained by electromagnetic simulation, and that of layered-film was obtained by 
prediction from reference 16. 
 
 
 
 
 
 
 
3. Power handling capability of a conventional, a sliced and a layered microstrip lines filters 
The power dependence of the output power in the filter with conventional and sliced microstrip line filters 
is shown in Figure 4. The both filters were made by a NbN thin film on an MgO substrate. The thickness 
of the NbN thin film was about 200nm. To reduce the heat generation at the feed point, we used a NbN 
thin film. The horizontal axis shows the effective input power.i.e., input power minus reflected power. 
The vertical axis shows the difference between the input and the output power. When the input power is 
smaller than 23 dBm the difference between the input power and the output power is nearly zero, that is 
the loss of the filter is zero. However, when input power becomes larger than 25 dBm, the output power is 
smaller than input power. We are defining the power proof of the filter as that where the output electric 
power minus the input electric power is minus 0.5dBm (dashed line in the figure). As shown in Fig.4, the 
electric power proof of the sliced-microstrip line filter and the conventional microstrip line filter was 27.8 
and 26.0 dBm, 
respectively. Therefore 
we found that the 
electric power proof of 
the sliced-microstrip line 
filter increased by about 
1.8dBm compared with 
that of  the conventional  
microstrip line filter. 
The power dependence 
of the output power in 
the filter with 
conventional and 
layered microstrip line 
filters is shown in Figure 
5. The both filters were 
made by a NbN and 
NbN/AlN/NbN thin film 
on the MgO substrate. 
We prepared NbN and 
AlN thin films by a 
reactive magnetron 
sputtering. As the high 
quality layered YBCO 
Type of 
microstrip line
Conventional
Sliced
Layered
2.4 x 105 A/cm2
1.8 x 105 A/cm2
Current concentration at 
outer edge of microstrip line
1.2 x 105A/cm2 (*)
Fig.4. Power dependence of output power in filter with conventional and 
sliced microstrip line filters. The shape of the sliced microstrip line is 
shown in Fig.3 (a). The electric power-proof value of the filter is shown by 
the arrow in the figure. 
O
ut
pu
t p
ow
er
 –
(In
pu
t p
ow
er
+D
U
T
lo
ss
)
0
-1.0
-3.0
-2.0
0 10 20 30
Effective Input Power (dBm)
sliced microstrip line filter
conventional microstrip line filter
 S. Ohshima et al. /  Physics Procedia  36 ( 2012 )  429 – 434 433
thin films are very difficult to 
obtain in our group, we used  a 
NbN/AlN/NbN layered film for 
filter. The Tc of NbN was about 
15 K and the thickness of the 
NbN film was 200 nm. We are 
defining the input electric power 
value as that where the output 
electric power minus the input 
electric power is minus 0.5dBm 
(dashed line in the figure) as the 
electric power-proof of the filter. 
As shown in Figure 5, we found 
that the power proof of the 
layered microstrip filter 
increased about 1.9 dBm 
compared with that of the 
conventional microstrip line 
filter. 
 
4. Discussion 
To increase power proof of the filter, we propose the use of  two kinds of  resonator, sliced-microstrip line 
resonator and a layered-film microstrip line resonator. From the simulation of the current concentration of 
the microstrip lines, we can estimate the increase of power proof. The concentrated current was about 
2.4x105 A/cm2 in the conventional microstrip line and 1.8 x 105 A/cm2 in the sliced  
microstrip line as shown in Table 1. If this difference directly accounts for the improvement in the power 
handling capability of the filter, we can estimate the difference in proof between the conventional and  
sliced microstrip line filters from the following equation. 
 
25
5
2.4x10( ) 10log 2.5( ) (1)
1.8x10
P dB dB
§ ·  ¨ ¸© ¹
           
This estimate agrees with the experimental results. When similarly calculating from the expression (1) we 
can obtain the difference in proof between the layered-film filter and a conventional-microstrip line one. 
The difference was about 6 dB, which does not agree with the experimental results. To increase the power 
proof of the layered-film filter, we must examine optimal thickness of the superconducting and insulating 
films [16]. However, in this experiment we could not use the optimal thickness. We think that the reason 
for the lack of agreement between the simulation and the experimental result is the thickness of the NbN 
and AlN films. 
 
5.  Conclusion 
To increase the power handling capability of superconducting bandpass filter, we proposed new resonator 
configurations, a sliced and a layered-film microstrip line. The following results were obtained. 
(1) Current flowing along the outer edge of the sliced-microstrip line was less concentrated than in the 
conventional-microstrip line. 
(2) Current flowing at the bottom of the microstrip line of a layered-film was less concentrated than in the 
conventional-microstrip one. 
(3) The electric power proof of the sliced-microstrip line filter and  the layered-film microstrip line filter 
was larger than that of the conventional filter. 
Effective input power (dBm)
0 10 20 30
0
-1.0
-2.0
-3.0
-4.0
-5.0
O
ut
pu
t p
ow
er
 –
(In
pu
t p
ow
er
 +
 D
U
T 
 lo
ss
)
conventional filter
layered-film filter
Fig.5. Power dependence of output power in filter with conventional 
and layered-film microstrip line filters. The shape of the layered-
film microstrip line filter is shown in Fig.3 (b). The electric power-
proof value is shown by the arrow in the figure. 
434   S. Ohshima et al. /  Physics Procedia  36 ( 2012 )  429 – 434 
(4) The electric power proof of the filter can be estimated from the concentrated current along the outer 
edge of the microstrip line. 
 
Acknowledgements 
This research was partially supported by The Ministry of Education, Science Sports and Culture, Grant-in 
Aid for Science Research (C) (22560317) 
 
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Improved Power Handling Capability of Superconducting Microstrip Lines for Microwave Devices

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