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Broadband superconducting microstrip patch antenna using additional gap-coupled resonators
Physica C 445–448 (2006) 994–997 www.elsevier.com/locate/physc
Broadband superconducting microstrip patch antenna using additional gap-coupled resonators N. Sekiya, A. Kubota, A. Kondo, S. Hirano, A. Saito, S. Ohshima
*
Department of Electrical Engineering, Yamagata University, Ohshima Lab, 4-3-16 Johnan, Yonezawa, Yamagata 992-8510, Japan Available online 24 July 2006
Abstract We have designed and fabricated a superconducting microstrip patch antenna with a larger bandwidth. The low surface resistance of superconductor compared to normal conductor corresponds to a large quality factor (Q) and improved performance in passive microwave devices. However, the narrow bandwidth that occurs with large Q is a major obstacle to the wider application of superconducting microstrip antennas. To enlarge the bandwidth of superconducting microstrip antennas, two additional gap-coupled resonators (GCRs) are used for the antenna. An electromagnetic simulator based on the moment method was used for design and analysis. The antenna has a center frequency of 11.85 GHz and was fabricated using YBa2Cu3O7 d (YBCO) thin films on CeO2 buffered r-Al2O3 substrates. Return loss and bandwidth of the GCR antenna were measured using a vector network analyzer. The GCR antenna shows an improvement of the bandwidth over a single-element superconducting patch antenna. 2006 Elsevier B.V. All rights reserved. PACS: 85.25.Am Keywords: Antenna; Broadband; Microstrip
1. Introduction Superconducting passive microwave devices such as antennas, filters, transmission lines, and phase shifters have shown significant superiority over corresponding devices fabricated with normal conductors such as gold, silver, or copper due to the low surface resistance of superconductors. The low surface resistance corresponds to a large quality factor (Q) and improved performance such as higher gain and lower insertion loss in passive microwave devices. In particular, superconducting filters are actively being developed. However, the narrow bandwidth that occurs with large Q is a major obstacle to the wider application of superconducting microstrip antennas. Richard et al. reported that superconducting microstrip antenna bandwidths are typically 0.85%–1.1%, based on definition of bandwidth as the frequency range over which the standing wave ratio (SWR) *
Corresponding author. Tel.: +81 238 26 3286; fax: +81 238 26 3293. E-mail address: [email protected] (S. Ohshima).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.089
is less than two [1]. Bandwidth enhancement has been widely researched, and various methods have been devised to increase the bandwidth for normal conductor microstrip antenna [2–6]. Recent researches of superconducting antenna are antenna matching elements, feed networks, microstrip antennas, and superdirective arrays [7–9]. However, extensive study on the improvement of bandwidth for superconducting antenna has not been reported [9–11]. In this study, we have designed a superconducting microstrip patch antenna with a larger bandwidth. The antenna uses two additional gap-coupled resonators (GCRs). The performance of the antenna was measured in terms of the SWR and return loss. After describing the design of our antenna bandwidth, we will present the simulation and experimental results. 2. Design of GCR antenna The configuration of the GCR antenna is shown in Fig. 1. The radiating patch is fed by direct coupling. This
N. Sekiya et al. / Physica C 445–448 (2006) 994–997
995
4. Results 4.1. Simulation
Fig. 1. Configuration of GCR antenna.
reduces Q by enlarging the size of the antenna that resonates at a certain frequency more than a single-element patch antenna and gives broader bandwidth. The antenna has a very simple configuration and consists of the singleelement rectangular patch antenna (RPA) with two coupled patches. The design method of the single-element superconducting RPA has already been reported [12]. Two couple patches have slightly different resonant lengths and the staggered frequencies of the resonators contribute to the broader bandwidth. We investigated the characteristics of the GCR antenna at 11.85 GHz of center frequency and how wide bandwidth can obtain using high-frequency electromagnetic analysis software based on the moment method (i.e., Sonnet (R) Version 6.0. delta. 1, Sonnet Software, Inc.). The design parameters are shown in Table 1. In this table, we show that dielectric constant of the r-plane sapphire is 10.31, which is determined experimentally.
Widths W1 and W3 of the coupled resonators strongly govern the bandwidth of the GCR antenna. Therefore, changing W1 and W3 improved the bandwidth and determined the size. The bandwidth is widened by making two resonance points in the effective frequency ranges. Fig. 2 shows the simulated SWR of the GCR antenna and a single-element superconducting RPA. The determined dimensions of the GCR antenna are given in Table 2. The simulated bandwidth of the GCR antenna was 311 MHz (2.6%, center frequency f0 = 11.83 GHz). For comparison, a single-element superconducting RPA was simulated for the corresponding substrate specifications, with a resulting bandwidth of 83.3 MHz (0.71%, center frequency f0 = 11.7 GHz). Thus, the bandwidth of the GCR antenna is nearly 3.7 times the bandwidth of RPA. Based on these results, wider bandwidth was obtained by changing widths of resonators to make two resonance points. 4.2. Measurements The experimentally measured SWR for various frequencies at 40 K is shown in Fig. 3. ‘‘Antenna A’’ is simulated SWR of the GCR antenna. ‘‘Antenna B’’ represents the
2.4 RPA GCR antenna
2.2
3. Experimental The GCR antenna was fabricated with YBa2Cu3O7 d (YBCO) thin film deposited by rf-magnetron sputtering on a 20 · 20 · 0.5 mm CeO2 buffered r-sapphire substrate. The film thickness was 300 nm. The ground plane was Au thin film. Conventional photolithography and ECR dry etching techniques were used to pattern the antenna. The GCR antenna was fixed on a copper antenna holder and a microwave connector was attached to the antenna. The antenna was cooled by a G-M-type cryocooler. We measured the return loss and the bandwidth of the antenna with a vector network analyzer (Wiltron 360). The definition of the bandwidth is the frequency range over which the standing wave ratio (SWR) is less than two.
SWR
2.0 1.8 1.6 1.4 1.2 1.0 11.6
11.7
11.8 11.9 Frequency [GHz]
12.0
12.1
Fig. 2. Simulated SWR variation with frequency of RPA and GCR antenna.
Table 2 Simulated dimensions of GCR antenna
Table 1 Design parameters of GCR antenna Parameters
Value
Microstrip line width Wmsl Center frequency Substrate Dielectric constant of the substrate Thickness of the substrate
0.5 mm 11.85 GHz r-Al2O3 10.31 0.5 mm
Parameter
Dimension (mm)
Wmsl W1 W2 W3 L g
0.50 3.55 3.64 3.46 3.64 0.40
N. Sekiya et al. / Physica C 445–448 (2006) 994–997
W1 [mm] (a) (b)
3.50 3.45
W3 [mm]
g [mm]
3.49 3.44
0.45 0.50
Antenna A Antenna B (a) (b)
4.0 3.5
SWR
3.0
-5 -10 -15 -20 -25 -30 RPA GCR antenna
-35 -40
11.4
11.6
11.8 12.0 12.2 Frequency [GHz]
12.4
12.6
Fig. 4. Measured return loss characteristic of RPA and GCR antenna at 60 K.
2.2 2.0 1.8 1.6 1.4 1.2 RPA GCR antenna
1.0 11.4
11.6
11.8 12.0 Frequency [GHz]
12.2
12.4
Fig. 5. Measured SWR of RPA and GCR antenna at 60 K.
(1.2% center frequency f0 = 11.584 GHz). The effective frequency ranges of the GCR antenna were 11.955– 12.156 GHz and 12.232–12.355 GHz. The total effective passband of the GCR antenna was 324 MHz, and the calculated bandwidth was about 2.7%, which is approximately 2.4 times better than that of the single-element superconducting RPA. 5. Conclusion
2.5 2.0 1.5 1.0 11.5
0
R.L. [dB]
SWR of measured GCR antenna with the dimensions shown in Table 2. The measured bandwidth of antenna B is so narrow compared to the simulated one that the SWR of lower resonant frequency at 11.64 GHz shows insufficient decrease. The experimental results do not agree with the simulated ones. This might be caused by the anisotropic dielectric permittivity of sapphire substrate. Therefore, in order to decrease the SWR of lower resonant frequency, adjustment of widths W1 and W3 and gap width g will improve the bandwidth. We used a pulsed excimer laser etching system to adjust widths W1 and W3 and gap width g. The measured SWR of the GCR antenna for two combinations of W1 and W3 and g with W2 = L = 3.64 mm are shown in Fig. 3. We found that when g became wider, lower resonant frequency of the SWR decreased and when width of W1 and W3 became narrower, the upper resonant frequency increased. If we want to obtain the wideband antenna, we must decrease the SWR of lower resonant frequency and shift the resonant frequency of upper one to lower resonant frequency. Based on these results, we determined that when g = 0.50 mm, W1 = 3.55 mm and W3 = 3.46 mm, which are the same dimensions determined by simulation. The measured return loss of a modified GCR antenna and a RPA with W2 = L = 3.82 mm and W1 = W3 = 0 as shown in Fig. 1, at 60 K is shown in Fig. 4. The center frequency in the measured response increased due to overetching of the antenna pattern. The effective frequency ranges were separated due to a narrow W1 and W3 in the coupled resonators. This may also be caused by over-etching of the antenna pattern. However, two response points was appeared into the operational bandwidth. Fig. 5 shows the measured SWR of both antennas at 60 K. The measured bandwidth of the RPA was 137 MHz
SWR
996
11.6
11.7 11.8 11.9 Frequency [GHz]
12.0
12.1
Fig. 3. Variation in experimentally measured SWR with frequency.
We have designed and fabricated a superconducting microstrip patch antenna with a larger bandwidth. The antenna uses two additional gap-coupled resonators (GCRs). The GCR antenna was fabricated with YBa2Cu3O7 d (YBCO) thin film on a 20 · 20 · 0.5 mm CeO2 buffered r-sapphire substrate. The ground plane was Au. Comparisons of the bandwidth of a single-element superconducting rectangular patch antenna (RPA) and a modified GCR antenna show that the bandwidth of the GCR antenna is 2.7% and is approximately 2.4 times better than that of the
N. Sekiya et al. / Physica C 445–448 (2006) 994–997
RPA. Therefore, larger bandwidth antenna is successfully fabricated. Acknowledgements This research was partially supported by the Ministry of Education, Culture, Sports, Science and technology, Grant-in-Aid for Young Scientists (B), 16760019, 2005. A part of this work was carried out in the clean room of Yamagata University. References [1] M.A. Richard, K.B. Bhasin, P.C. Claspy, IEEE Trans. Antennas Propagat. 9 (1993) 967. [2] D.M. Pozar, B. Kaufman, Electron. Lett. 23 (1987) 368.
997
[3] N. Fayyaz, S.S. Naeini, IEEE Antennas Propagat. Symp. Digest (1998) 1100. [4] G. Kumar, K.C. Gupta, IEEE Trans. Antennas Propagat. AP-32 (1984) 1375. [5] G. Kumar, K.C. Gupta, IEEE Trans. Antennas Propagat. AP-33 (1985) 173. [6] G. Kumar, K.C. Gupta, IEEE Trans. Antennas Propagat. AP-33 (1985) 588. [7] H. Chaloupka, H. Piel, A. Pischke, N. Klein, M. Peiniger, G. Splitt, IEEE Trans. Microwave Theory Tech. 39 (1991) 1513. [8] C. Kuester, H. Chaloupka, J. Knauth, Proc. Military Commun. Conf. 1 (1999) 671. [9] K. Yoshida, T. Takahashi, H. Kanaya, T. Uchiyama, Z. Wang, IEEE Trans. Appl. Supercond. 11 (2001) 103. [10] D. Chung, IEEE Trans. Appl. Supercond. 11 (2001) 107. [11] D. Chung, IEEE Trans. Appl. Supercond. 13 (2003) 297. [12] K. Ehata, T. Ogasawara, M. Kusunoki, S. Ohshima, in: S. Nakazima, M. Murakami (Eds.), Advances in Superconductivity IX, Springer, Sapporo, 1997, p. 1273.
Broadband superconducting microstrip patch antenna using additional gap-coupled resonators N. Sekiya, A. Kubota, A. Kondo, S. Hirano, A. Saito, S. Ohshima
*
Department of Electrical Engineering, Yamagata University, Ohshima Lab, 4-3-16 Johnan, Yonezawa, Yamagata 992-8510, Japan Available online 24 July 2006
Abstract We have designed and fabricated a superconducting microstrip patch antenna with a larger bandwidth. The low surface resistance of superconductor compared to normal conductor corresponds to a large quality factor (Q) and improved performance in passive microwave devices. However, the narrow bandwidth that occurs with large Q is a major obstacle to the wider application of superconducting microstrip antennas. To enlarge the bandwidth of superconducting microstrip antennas, two additional gap-coupled resonators (GCRs) are used for the antenna. An electromagnetic simulator based on the moment method was used for design and analysis. The antenna has a center frequency of 11.85 GHz and was fabricated using YBa2Cu3O7 d (YBCO) thin films on CeO2 buffered r-Al2O3 substrates. Return loss and bandwidth of the GCR antenna were measured using a vector network analyzer. The GCR antenna shows an improvement of the bandwidth over a single-element superconducting patch antenna. 2006 Elsevier B.V. All rights reserved. PACS: 85.25.Am Keywords: Antenna; Broadband; Microstrip
1. Introduction Superconducting passive microwave devices such as antennas, filters, transmission lines, and phase shifters have shown significant superiority over corresponding devices fabricated with normal conductors such as gold, silver, or copper due to the low surface resistance of superconductors. The low surface resistance corresponds to a large quality factor (Q) and improved performance such as higher gain and lower insertion loss in passive microwave devices. In particular, superconducting filters are actively being developed. However, the narrow bandwidth that occurs with large Q is a major obstacle to the wider application of superconducting microstrip antennas. Richard et al. reported that superconducting microstrip antenna bandwidths are typically 0.85%–1.1%, based on definition of bandwidth as the frequency range over which the standing wave ratio (SWR) *
Corresponding author. Tel.: +81 238 26 3286; fax: +81 238 26 3293. E-mail address: [email protected] (S. Ohshima).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.089
is less than two [1]. Bandwidth enhancement has been widely researched, and various methods have been devised to increase the bandwidth for normal conductor microstrip antenna [2–6]. Recent researches of superconducting antenna are antenna matching elements, feed networks, microstrip antennas, and superdirective arrays [7–9]. However, extensive study on the improvement of bandwidth for superconducting antenna has not been reported [9–11]. In this study, we have designed a superconducting microstrip patch antenna with a larger bandwidth. The antenna uses two additional gap-coupled resonators (GCRs). The performance of the antenna was measured in terms of the SWR and return loss. After describing the design of our antenna bandwidth, we will present the simulation and experimental results. 2. Design of GCR antenna The configuration of the GCR antenna is shown in Fig. 1. The radiating patch is fed by direct coupling. This
N. Sekiya et al. / Physica C 445–448 (2006) 994–997
995
4. Results 4.1. Simulation
Fig. 1. Configuration of GCR antenna.
reduces Q by enlarging the size of the antenna that resonates at a certain frequency more than a single-element patch antenna and gives broader bandwidth. The antenna has a very simple configuration and consists of the singleelement rectangular patch antenna (RPA) with two coupled patches. The design method of the single-element superconducting RPA has already been reported [12]. Two couple patches have slightly different resonant lengths and the staggered frequencies of the resonators contribute to the broader bandwidth. We investigated the characteristics of the GCR antenna at 11.85 GHz of center frequency and how wide bandwidth can obtain using high-frequency electromagnetic analysis software based on the moment method (i.e., Sonnet (R) Version 6.0. delta. 1, Sonnet Software, Inc.). The design parameters are shown in Table 1. In this table, we show that dielectric constant of the r-plane sapphire is 10.31, which is determined experimentally.
Widths W1 and W3 of the coupled resonators strongly govern the bandwidth of the GCR antenna. Therefore, changing W1 and W3 improved the bandwidth and determined the size. The bandwidth is widened by making two resonance points in the effective frequency ranges. Fig. 2 shows the simulated SWR of the GCR antenna and a single-element superconducting RPA. The determined dimensions of the GCR antenna are given in Table 2. The simulated bandwidth of the GCR antenna was 311 MHz (2.6%, center frequency f0 = 11.83 GHz). For comparison, a single-element superconducting RPA was simulated for the corresponding substrate specifications, with a resulting bandwidth of 83.3 MHz (0.71%, center frequency f0 = 11.7 GHz). Thus, the bandwidth of the GCR antenna is nearly 3.7 times the bandwidth of RPA. Based on these results, wider bandwidth was obtained by changing widths of resonators to make two resonance points. 4.2. Measurements The experimentally measured SWR for various frequencies at 40 K is shown in Fig. 3. ‘‘Antenna A’’ is simulated SWR of the GCR antenna. ‘‘Antenna B’’ represents the
2.4 RPA GCR antenna
2.2
3. Experimental The GCR antenna was fabricated with YBa2Cu3O7 d (YBCO) thin film deposited by rf-magnetron sputtering on a 20 · 20 · 0.5 mm CeO2 buffered r-sapphire substrate. The film thickness was 300 nm. The ground plane was Au thin film. Conventional photolithography and ECR dry etching techniques were used to pattern the antenna. The GCR antenna was fixed on a copper antenna holder and a microwave connector was attached to the antenna. The antenna was cooled by a G-M-type cryocooler. We measured the return loss and the bandwidth of the antenna with a vector network analyzer (Wiltron 360). The definition of the bandwidth is the frequency range over which the standing wave ratio (SWR) is less than two.
SWR
2.0 1.8 1.6 1.4 1.2 1.0 11.6
11.7
11.8 11.9 Frequency [GHz]
12.0
12.1
Fig. 2. Simulated SWR variation with frequency of RPA and GCR antenna.
Table 2 Simulated dimensions of GCR antenna
Table 1 Design parameters of GCR antenna Parameters
Value
Microstrip line width Wmsl Center frequency Substrate Dielectric constant of the substrate Thickness of the substrate
0.5 mm 11.85 GHz r-Al2O3 10.31 0.5 mm
Parameter
Dimension (mm)
Wmsl W1 W2 W3 L g
0.50 3.55 3.64 3.46 3.64 0.40
N. Sekiya et al. / Physica C 445–448 (2006) 994–997
W1 [mm] (a) (b)
3.50 3.45
W3 [mm]
g [mm]
3.49 3.44
0.45 0.50
Antenna A Antenna B (a) (b)
4.0 3.5
SWR
3.0
-5 -10 -15 -20 -25 -30 RPA GCR antenna
-35 -40
11.4
11.6
11.8 12.0 12.2 Frequency [GHz]
12.4
12.6
Fig. 4. Measured return loss characteristic of RPA and GCR antenna at 60 K.
2.2 2.0 1.8 1.6 1.4 1.2 RPA GCR antenna
1.0 11.4
11.6
11.8 12.0 Frequency [GHz]
12.2
12.4
Fig. 5. Measured SWR of RPA and GCR antenna at 60 K.
(1.2% center frequency f0 = 11.584 GHz). The effective frequency ranges of the GCR antenna were 11.955– 12.156 GHz and 12.232–12.355 GHz. The total effective passband of the GCR antenna was 324 MHz, and the calculated bandwidth was about 2.7%, which is approximately 2.4 times better than that of the single-element superconducting RPA. 5. Conclusion
2.5 2.0 1.5 1.0 11.5
0
R.L. [dB]
SWR of measured GCR antenna with the dimensions shown in Table 2. The measured bandwidth of antenna B is so narrow compared to the simulated one that the SWR of lower resonant frequency at 11.64 GHz shows insufficient decrease. The experimental results do not agree with the simulated ones. This might be caused by the anisotropic dielectric permittivity of sapphire substrate. Therefore, in order to decrease the SWR of lower resonant frequency, adjustment of widths W1 and W3 and gap width g will improve the bandwidth. We used a pulsed excimer laser etching system to adjust widths W1 and W3 and gap width g. The measured SWR of the GCR antenna for two combinations of W1 and W3 and g with W2 = L = 3.64 mm are shown in Fig. 3. We found that when g became wider, lower resonant frequency of the SWR decreased and when width of W1 and W3 became narrower, the upper resonant frequency increased. If we want to obtain the wideband antenna, we must decrease the SWR of lower resonant frequency and shift the resonant frequency of upper one to lower resonant frequency. Based on these results, we determined that when g = 0.50 mm, W1 = 3.55 mm and W3 = 3.46 mm, which are the same dimensions determined by simulation. The measured return loss of a modified GCR antenna and a RPA with W2 = L = 3.82 mm and W1 = W3 = 0 as shown in Fig. 1, at 60 K is shown in Fig. 4. The center frequency in the measured response increased due to overetching of the antenna pattern. The effective frequency ranges were separated due to a narrow W1 and W3 in the coupled resonators. This may also be caused by over-etching of the antenna pattern. However, two response points was appeared into the operational bandwidth. Fig. 5 shows the measured SWR of both antennas at 60 K. The measured bandwidth of the RPA was 137 MHz
SWR
996
11.6
11.7 11.8 11.9 Frequency [GHz]
12.0
12.1
Fig. 3. Variation in experimentally measured SWR with frequency.
We have designed and fabricated a superconducting microstrip patch antenna with a larger bandwidth. The antenna uses two additional gap-coupled resonators (GCRs). The GCR antenna was fabricated with YBa2Cu3O7 d (YBCO) thin film on a 20 · 20 · 0.5 mm CeO2 buffered r-sapphire substrate. The ground plane was Au. Comparisons of the bandwidth of a single-element superconducting rectangular patch antenna (RPA) and a modified GCR antenna show that the bandwidth of the GCR antenna is 2.7% and is approximately 2.4 times better than that of the
N. Sekiya et al. / Physica C 445–448 (2006) 994–997
RPA. Therefore, larger bandwidth antenna is successfully fabricated. Acknowledgements This research was partially supported by the Ministry of Education, Culture, Sports, Science and technology, Grant-in-Aid for Young Scientists (B), 16760019, 2005. A part of this work was carried out in the clean room of Yamagata University. References [1] M.A. Richard, K.B. Bhasin, P.C. Claspy, IEEE Trans. Antennas Propagat. 9 (1993) 967. [2] D.M. Pozar, B. Kaufman, Electron. Lett. 23 (1987) 368.
997
[3] N. Fayyaz, S.S. Naeini, IEEE Antennas Propagat. Symp. Digest (1998) 1100. [4] G. Kumar, K.C. Gupta, IEEE Trans. Antennas Propagat. AP-32 (1984) 1375. [5] G. Kumar, K.C. Gupta, IEEE Trans. Antennas Propagat. AP-33 (1985) 173. [6] G. Kumar, K.C. Gupta, IEEE Trans. Antennas Propagat. AP-33 (1985) 588. [7] H. Chaloupka, H. Piel, A. Pischke, N. Klein, M. Peiniger, G. Splitt, IEEE Trans. Microwave Theory Tech. 39 (1991) 1513. [8] C. Kuester, H. Chaloupka, J. Knauth, Proc. Military Commun. Conf. 1 (1999) 671. [9] K. Yoshida, T. Takahashi, H. Kanaya, T. Uchiyama, Z. Wang, IEEE Trans. Appl. Supercond. 11 (2001) 103. [10] D. Chung, IEEE Trans. Appl. Supercond. 11 (2001) 107. [11] D. Chung, IEEE Trans. Appl. Supercond. 13 (2003) 297. [12] K. Ehata, T. Ogasawara, M. Kusunoki, S. Ohshima, in: S. Nakazima, M. Murakami (Eds.), Advances in Superconductivity IX, Springer, Sapporo, 1997, p. 1273.