X-band microstrip antenna bandwidth enhancement using multi-walled carbon nanotubes Marbey M. Mosso(1), Rogerio N. Rebello Filho(1), Fernando H. R. Monteiro(2), Fernando L. F. Junior(2)
Gelza M. Barbosa Diretoria de Sistemas de Armas da Marinha Brazilian Navy Rio de Janeiro, Brazil
[email protected]
Abstract—This paper presents a comparison between conventional X-band microstrip patch antennas and the same antenna design with insertion of multi-walled carbon nanotubes (MWCNT) in two different configurations: buckypaper and epoxy-MWCNT composite. The results show that insertion of epoxy-MWCNT composite layer over the microstrip patch antenna increases its operation bandwidth without considerable degradation in other parameters.
(1) Centro de Estudos em Telecomunicações/PUC-Rio (2) Instituto de Física/PUC-Rio Rio de Janeiro, Brazil
thickness 0.787 mm, the antenna was defined with the values W=11.8mm; L=10mm; yo=2.9mm and Wo= 3mm. Fig. 1 shows the designed standard antenna. Afterwards, the antenna was modeled and simulated using Ansoft’s HFSS threedimensional full-wave electromagnetic field software [5]. Simulations presented minimum Input Reflection Coefficient (S11) equal to -17.90 dB at 9.9 GHz, and maximum gain 8.92 dB, as it can be shown in Figs. 2 and 3.
Keywords: Antenna; carbon nanotubes; microstrip; radiation pattern; microwave.
INTRODUCTION
In order to investigate the patch antennas radiation patterns change in microwave band due to insertion of Multi-Walled Carbon Nanotubes (MWCNT), several experiments were performed using different types and concentrations of that material on the antennas. In this paper, microwave radiation of patch antennas with insertion of a composite using MWCNT and silver epoxy and also with buckypaper are presented and compared to the standard microstrip antenna.
II.
Figure 1 – Standard recessed microstrip-line antenna Input Reflection Coefficient (S11) [dB]
I.
A patch antenna consists of a flat rectangular metalized sheet mounted over a substrate with a ground plane. The configuration is lightweight, inexpensive, simple and can be applied in wireless communications, traffic radar, GPS, military and aerospace systems. Typical dimensions and configurations designs are available in literature [1]. Carbon Nanotubes have attracted significant interest in electromagnetic field due to interaction between their mechanical oscillating frequencies in the GHz domain [2] and other special properties like EMI shielding [3]-[4], high conductivity, mechanical strength and high aspect ratio.
XY Plot 7
ANSOFT
-0.50
-3.00
-5.50
-8.00
-10.50
-13.00
-15.50
-18.00 9.50
9.75
10.00
10.25
10.50
Frequency (GHz) Figure 2 - Standard antenna S11 [dB] simulated in HFSS
MICROSTRIP PATCH ANTENNA CONSTRUCTION
A. Design and simulation of the standard antenna First of all, the microstrip standard antenna is designed to meet some basic requirements. In this work, the requirements were: 10 GHz operation frequency, 50 Ohms feed line, minimum return loss of 14 dB and minimum gain 5 dB. Using the design method presented in [1] and choosing the recessed microstrip-line feed configuration, substrate Rogers 5880 with
978-1-4577-1664-5/11/$26.00 ©2011 IEEE
891
Radiation Pattern 15
ANSOFT
0
-30
30 3.00
patch antenna, a patch antenna with buckypaper, and a patch antenna with MWCNT-epoxy composite.
-4.00 -60
60 -11.00 -18.00
-90
90
-120
120
Figure 6 – From left to right: standard patch antenna; patch antenna with buckypaper; patch antenna with MWCNT-epoxy composite. 150 -180
Figure 3 – HFSS simulated Radiation Pattern: Gain (dB)
B. Antenna fabrication Several standard antennas were fabricated at CETUC/PUC-Rio. The MWCNT were fabricated at PUC-Rio Physics Institute. The Spray Pyrolysis of 2,0wt% ferrocene [Fe(C5H5)2] in a toluene solution [C7H8], at 850oC, produced MWCNT with diameters around 50nm. Figs. 4 and 5 show the images of the fabricated MWCNT. The commercial buckypaper was produced by Nanolab, Inc.
It is relevant to note that the cover with buckypaper and with MWCNT composite had to be adapted to the patch design, with special attention to the inset feed. Otherwise, the resonant frequency could be greatly shifted. In order to evaluate the epoxy contribution to the antenna performance, another antenna was covered with only silver epoxy over the micrsotrip. III.
MICROSTRIP PATCH ANTENNA MEASUREMENTS
The input reflection coefficient (S11) was measured in HP8720C Network Analyser and the radiation pattern in LabVolt 9553 antenna setup. Instead of the 10 GHz LabVolt RF generator, a HP 8620C was used in order to cover any frequency in the X-band. The measurement results for the microstrip standard antenna are shown in Figs. 7 and 8. Input Reflection Coefficient (S11) [dB]
-150
Figure 4 – Images of fabricated MWCNT in the Field Emission Scanning Electron Microscope JEOL JSM 6701F
Frequency (GHz) Figure 7 – Measured S11 [dB] of standard patch antenna Figure 5 – Raman spectroscopy (laser 473 nm) of fabricated MWCNT
One of the antennas was chosen to be the reference one (standard); another was chosen to have buckypaper added upon the metallic patch, and two other antennas were chosen to have the MWCNT-epoxy composites added upon the metallic patch. For the antennas with MWCNT composites, different concentrations of the same sample of MWCNT were used: approximately 30% and 50%, mixed with silver epoxy, in order to compare the antenna performance. Fig. 6 shows a standard
Fig. 7 shows that S11 is minimum (-18.6 dB) at 10.34 GHz. Therefore, 10.34 GHz is the resonant frequency of the antenna under test (AUT). Fig. 8 shows the relative signal levels detected by the AUT in the E-plane, with 3 dB attenuation inserted in the diagram for better visualization. A standard large aperture horn antenna was used to irradiate the RF signal supplied by a HP 8620C generator. The horn antenna gain is 16.7 dB at 10 GHz. The radiation pattern (not shown here) of the horn antenna was also measured for gain comparison in each measurement throughout this work. The measurements
This work was sponsored by PUC-Rio and Brazilian Navy.
978-1-4577-1664-5/11/$26.00 ©2011 IEEE
892
Input Reflection Coefficient (S11) [dB]
were performed for several frequencies between 9.5 and 10.6 GHz. It is important to note that, for each test frequency, it is necessary to adjust the transmitted power output from the RF generator. In this work, the power output from the RF generator was adjusted to 0 dBm for all frequencies. This is the reference signal level.
Frequency (GHz) Figure 9 – Measured amplitude of input reflection coefficient (S11) of the microstrip antenna: covered with silver epoxy (continuous line) and covered with MWCNT buckypaper (dashed line).
V.
Figure 8 – Measured radiation pattern (E-plane) of standard patch antenna at S11 resonant frequency (outer curve) and at 9.5 GHz (inner cuve).
MICROSTRIP-BUCKYPAPER PATCH ANTENNA MEASUREMENTS
The S11 of the microstrip antenna covered with MWCNT buckypaper was measured and the result is presented in Fig. 9 (dashed line). The minimum S11 is -27.2 dB at 10.45 GHz.
Fig. 8 presents the comparison between the radiation patterns of the patch antenna in the resonant frequency (10.34 GHz) and at 9.5 GHz. The squinted pattern (9.5 GHz) is due to the antenna impedance mismatch far from the resonant frequency.
IV.
MICROSTRIP-EPOXY PATCH ANTENNA MEASUREMENTS
The S11 of the microstrip antenna covered with silver epoxy was measured and the result is presented in Fig. 9 (continuous line). The minimum S11 is -9.7 dB at 10.35 GHz. The antenna radiation pattern was also measured, which resulted in the diagram presented in Fig. 10, red color (inner curve). It can be observed that the antenna covered with epoxy shows significant (6dB) gain reduction compared to the standard antenna.
Figure 10 – Radiation pattern (E-plane) of the microstrip antenna at resonant frequency: covered with silver epoxy (red color, inner curve); covered with MWCNT buckypaper (blue color, middle curve); standard - without additional coverage (brown color, outer curve).
Then, the “buckypaper” antenna radiation pattern was measured at 10.45 GHz, which resulted in the diagram presented in Fig. 10 (middle curve, blue color). It can be observed that the antenna covered with buckypaper shows 3dB gain reduction compared to the standard antenna.
978-1-4577-1664-5/11/$26.00 ©2011 IEEE
893
VI.
MICROSTRIP-MWCNT-EPOXY PATCH ANTENNA MEASUREMENTS
Input Reflection Coefficient (S11) [dB]
The S11 of the microstrip antenna covered with 50% MWCNT-silver epoxy composite was measured and the result is presented in Fig. 11, which includes the standard antenna S11 measurement for comparison.
Figure 13 - Measured radiation pattern for microstrip antennas at 9.5 GHz: standard (brown color); 30% MWCNT mixed with 70% silver epoxy (blue color); 50% MWCNT mixed with 50% silver epoxy (red color). Frequency (GHz)
Figure 11 – Measured S11 of the microstrip patch antenna: covered with 50% MWCNT/silver epoxy composite (continuous line) and standard antenna (dashed line).
Gain (dB)
The minimum S11 is -16 dB at 10.18 GHz for the MWCNT composite antenna, indicating some decrease in the resonant frequency (1.5%). Then, the MWCNT antenna radiation patterns were measured at their resonant frequencies, which resulted in the diagram presented in Fig. 12.
After compensation of the transmitter horn antenna gain in the band of operation, the gain of the microstrip antennas were compared, as shown in Fig. 14. Considering 5 dB gain as the reference level, in Fig. 14, the bandwidth (BW) of the antennas covered with MWCNT-epoxy composites is greater than standard antennas BW in the X-band, without considerable degradation in gain. The antenna with 50% concentration of MWCNT shows 50.7% BW enhancement and the antenna with 30% MWCNT presents 23.3% BW enhancement, compared to the standard antenna BW.
Frequency (GHz) Figure 12 – Measured radiation pattern for microstrip antennas at resonant frequency: standard (red color); 30% MWCNT mixed with 70% silver epoxy (brown); 50% MWCNT mixed with 50% silver epoxy (blue).
Afterwards, the antennas radiation pattern measurements were performed from 9.5 GHz to 10.6 GHz, for bandwidth comparison. The 9.5 GHz radiation pattern diagram is presented in Fig. 13, showing greater gains for the MWCNT composite antennas when compared to the standard antenna gain.
Figure 14 - Bandwidth (BW) enhancement of MWCNT composite antennas compared to standard antenna. For 30%MWCNT composite, BW is 23.3% larger (dashed line); for 50%¨MWCNT composite, the BW is 50.7% larger (continuous line) than the standard microstrip antenna (pointed line).
VII. CONCLUSION This work presents X-band microstrip patch antennas in standard configuration as well as different uses of carbon nanotubes over it, in buckypaper and epoxy composites. The measurements showed that the antennas covered by MWCNTepoxy composites present enhanced bandwidth than their
978-1-4577-1664-5/11/$26.00 ©2011 IEEE
894
standard counterpart, without overall gain loss. Future work will present aligned MWCNT over microstrip and over silicon substrate in patch antennas.
REFERENCES [1] [2]
ACKNOWLEDGMENT The authors thank M.S. Canabarro for the support in the radiation pattern measurements, R. S. Pereira and B. S. Cruz for the antenna adapter fabrication, and Vice-Admiral E. T. Öberg for his encouragement to this work. The authors also acknowledge the use of the Ansoft 3D Full-Wave Electromagnetic Field Simulation software platform support for CETUC/PUC-Rio.
[3]
[4]
[5]
978-1-4577-1664-5/11/$26.00 ©2011 IEEE
C. A. Balanis, “Antenna Theory – Analysis and Design”, John Wiley & Sons, 2005. D. Dragoman and M. Dragoman, “Electromagnetic wave propagation in dense carbon nanotube arrays”. Journal Appl. Phys. 99, 2006. R.Che, L-M Peng, X. Duan, Q.Chen and X. Liang, “Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated carbon nanotubes”, Advanced materials, 2004, 16 No. 5. Li et al, “Electromagnetic Interference (EMI) Shielding of SingleWalled Carbon Nanotube Epoxy Composites”, Nano Letters, 2006, vol. 6, No. 6, pp 1141-1145. HFSS 13 – Ansoft 3D full-wave electromagnetic field software. http://www.ansoft.com/products/hf/hfss.
895
