|Year : 2009 | Volume
| Issue : 5 | Page : 196-200
Bandwidth Increasing Technique Using Modified Ground Plane with Diagonal Edges
Niruth Prombutr1, Phumin Kirawanich1, Prayoot Akkaraekthalin2
1 Department of Electrical Engineering, Mahidol University, Nakorn Pathom, 73170, Thailand
2 King Mongkut's Institute of Technology North Bangkok, Bangkok 10800, Thailand
|Date of Web Publication||5-Nov-2009|
Department of Electrical Engineering, Mahidol University, Nakorn Pathom, 73170
| Abstract|| |
This paper presents a bandwidth increasing technique using a modified ground plane with diagonal edges for the design of compact antennas. The proposed low-cost, compact-size circular patch antenna on 3 cm x 5.1 cm printed circuit board (FR-4) is designed and validated through simulations and experiments. Results show that with diagonal cuts at corners of the ground plane, the enhanced bandwidth can be tunable depending on the removed area. Return losses of minus 17 and minus 30 dBs for the first and second resonant frequencies, respectively, can be achieved when the depth of the diagonal cut is at optimum value of five mm, providing an 18.13% wider bandwidth (2.95-12.615 GHz) than the Federal Communication Commission (FCC) standard.
Keywords: Bandwidth enhancement, Diagonal edge, Low profile antenna, Microstrip antenna, Monopole antenna, Ultra-wideband.
|How to cite this article:|
Prombutr N, Kirawanich P, Akkaraekthalin P. Bandwidth Increasing Technique Using Modified Ground Plane with Diagonal Edges. IETE J Res 2009;55:196-200
| 1.Introduction|| |
Ultra-wideband (UWB) technology has been regarded as one of the most promising wireless technologies with a capability to revolutionize high data rate transmission. Since release of UWB wireless communication bandwidth of 7.5 GHz (3.1-10.6 GHz), by the FCC, a number of new techniques to support high data rate wireless communication for the next generation technologies have been rapidly increasing. The maximum achievable data rate or capacity for the ideal band-limited additive white Gaussian noise (AWGN) channel is related to bandwidth and signal-to-noise ratio through
where C denotes the maximum transmit data rate, B stands for the channel bandwidth, and SNR is the signal-to-noise ratio by Shannon-Nyquist criterion , .
From this principle, the transmit data rate can be enhanced by increasing either the bandwidth occupation or transmission power. However, transmission power cannot be readily increased since many portable devices are battery powered and potential interference should also be avoided. Thus, a large frequency bandwidth seems to be the proper solution to achieve a high data rate.
To outline the aforementioned antennas, a comparison between the main characteristics and features observed in all the antennas described in the present article is shown in [Table 1]  .
We report the technique to enhance a bandwidth using a microstrip-fed planar circular disc monopole  . The circular disc monopole with a 50-Ω microstrip feed line is fabricated on the FR4 substrate. We modified the ground plane with the use of diagonal edges to increase the bandwidth. Applications of corner cut technique have been previously employed to improve the impedance bandwidth for microstrip patch antennas , . The preliminary simulation analysis of our proposed antenna is compared with measurement counterparts. The study is organized as follows:
Details of antenna design and preliminary results from simulations are described in section 2. Section 3 discusses experimental results and the work concludes in section 4.
| 2. Antenna Design|| |
The geometries of the antennas in our study are shown in [Figure 1]. We use the microstrip structure which is compact, inexpensive and light weight. However, a lower bandwidth is a shortcoming for this structure. Our objectives are to modify the structure and incorporate the techniques to improve bandwidth. In what follows, a brief analysis of the parametric studies to achieve optimum values of return loss and bandwidth is discussed.
The antenna configuration in [Figure 1]a is first used for the parametric study. The planar circular disc monopole is fabricated on a 3 cm x 5.1 cm x 0.16 cm FR-4 board (εr is equal to 4.4) with a feed line and a finite ground plane. The radius R of the circular monopole disc is the first parameter to optimize for the lowest return loss and widest bandwidth while the other parameters are kept constant. The width wg of the microstrip feed line is designed at three mm for the impedance of 50 Ω. The results in [Figure 2]a show that the increase of the radius R will result in reductions of the return loss and the bandwidth. Even though the disc radius of 11 mm provides the lowest return loss, we chose the radius of 10 mm for the better bandwidth.
The next tuning parameter is the gap between the top section of the ground plane and the bottom portion of the disc. The parameter h is positive when the bottom of the disc is at the higher lever than the top of the ground plane. The same can be said for the negative value of h in the opposite direction. The results of the return loss and bandwidth as a function of the parameter h are shown in [Figure 2]b. Compared with the negative value of h, the positive value gives the higher return loss at high frequency while providing the lower return loss at low frequency. We chose the parameter h to be zero (the bottom of the disc is at the same level as the top of the ground plane) due to a compromise between the observed results.
To accomplish a compact size design, the minimum size of the ground plane is desirable. The parameter to study is the length L of the ground plane. From the simulation we show the results in [Figure 3]a, the length of the ground plane will have only slight effect on the bandwidth. We select the length of 15 mm for a wider bandwidth and moderately small size. Next, the parameter W is the width of the ground plane and the results as a function of W is shown in [Figure 3]b. Increase in width gives the lower return loss at low frequency. Additionally, the second resonant frequency is shifting to the lower frequency region while increasing the width. We preferred the width of 30 mm due to appropriately low return loss and the second resonance occurs at around 6.4 GHz. From the above parameter selection, the result of the return loss is shown in [Figure 4] where the antenna is applicable from 2.957 GHz to 11.892 GHz, related to the bandwidth of 8.935 GHz.
To further improve the bandwidth of the antenna, we remove the top corners of the ground plane, resulting in symmetrical diagonal edges. The resultant antenna is shown in [Figure 1]b with the parameter G associated with the cut area. The return losses in [Figure 5] show that the parameter G only has a slight effect at low frequency while it has a significant effect at high frequency. The parameter G of five mm seems to offer relatively low return loss and appropriately wide bandwidth. This antenna can use from 2.957 GHz to 12.615 GHz, related to the bandwidth of 9.658 GHz. Compared with the result in [Figure 4], the antenna with diagonal edges on the ground plane can increase the bandwidth of approximately 0.7 GHz.
| 3. Experimental Result and Discussion|| |
The photograph of the fabricated proposed antenna with diagonal edges on the ground plane is shown in [Figure 6]. The comparison between the simulated results using commercial high frequency structure simulator (IE3D) and results from measurement of the fabricated antenna using an Agilent PNA-L series N5230A vector network analyzer is shown in [Figure 7]. The measured result is relatively close to that obtained from simulation. The discrepancy of the return loss at the first resonant frequency would be caused by the size difference of the circular discs between the simulation model and the fabrication as mentioned in the previous section. The measured radiation patterns of the antenna on the E-plane and H-plane at the first resonant frequency of 3.2 GHz are shown in [Figure 8]. Results show that reasonable omni directional radiation pattern can be observed along the H-plane. The radiation pattern similar to that of the short monopole can be observed on the E-plane. Consistency in pattern can also be observed across the operating frequencies. Important observations from results of the return loss are detailed as follows:
First, the diameter of the circular disc basically corresponds to the quarter wavelength of the associated resonant frequency. A variation of resonant frequencies as a function of the disc radius in [Figure 2]a shows that the circular disc is capable of supporting multiple resonant modes through 2R is equal to nλ/4, where n is the mode number and λ is the wavelength. In addition, the parameter h results in not only the resonant frequency shifting, but also the return loss level between the first and second resonant frequencies.
For the second observation, the first resonance is barely changed for all different ground plane sizes as shown in [Figure 3]a and b. When the ground plane is reduced in either length or width, the first resonant frequency is shifted slightly at more or less three GHz  . These two observations imply that the resonant frequency is typically determined by the circular disc component and slightly detuned by the size of the ground plane.
The last observation is that, as shown in [Figure 5], the first resonant frequency is dependent on the size of the circular disc as mentioned above while the second resonant frequency and the bandwidth obey the dimension of the cut area at the ground plane corners. A possible explanation on the bandwidth increasing with the presence of the diagonal edges could be that there is a high concentrate of the current distribution around the diagonal edge area.
| 4. Conclusion|| |
A compact antenna with a technique to increase a bandwidth has been proposed and implemented. The proposed low-cost, compact-size circular patch antenna on 3 cm x 5.1 cm printed circuit board (FR- 4) is designed and validated through simulations and experimental counterparts. Results show that the bandwidth can be tunable depending mainly on the circular disc size and the vertical gap between the disc and the ground plane. With the presence of the diagonal cut areas at the corners of the ground plane, the bandwidth can be further improved. Return losses of minus 17 and minus 30 dBs for the first and second resonant frequencies, respectively, can be achieved when the depth of the diagonal cut is at optimum value of five mm, providing an 18.13% wider bandwidth (2.95-12.615 GHz) than the FCC recommended standard of 3.1-10.6 GHz. Finally, the size of the ground plane, which has an insignificant effect, can be further reduced to around 30 cm x 1.5 cm to meet a compact size design.
| Authors|| |
Niruth Prombutr received his B. Eng. in Electrical Engineering from Mahidol University and M.Eng. in Telecommunication engineering from Kasetsart University in Thailand, in 1998 and 2001 respectively. He is currently working towards his Ph.D. degree in Electrical Engineering at King Mongkut's institute of North Bangkok. His current research interests include antenna, electromagnetic model and microwave circuits.
Phumin Kirawanich received B.Eng. from Prince of Songkhla University, Thailand, M.Eng. and Ph.D. degrees in Electrical Engineering from Electrical Engineering, University of Missouri-Columbia, USA. He is currently an Assistant Professor at the Department of Electrical Engineering, Mahidol University. His research interests include photoconductive antennas, terahertz (THz) generation, pulsed power technology for bioelectrics and biomedical applications, power electronics, harmonics analysis and compensation in power systems and electromagnetic and semiconductor device physics computation.
Prayoot Akkaraekthalin received B.Eng. and M.Eng. degrees in Electrical Engineering from King Mongkut's institute of Technology, North Bangkok (KMITNB) Thailand, in 1986 and 1990, respectively, and the Ph.D. degree from the University of Delaware, New York, USA, in 1998. He is currently an Associate Professor at the Department of Electrical Engineering, KMITNB. His research interests include microwave circuits, antennas, and optoelectronics.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]