CPW-fed capacitive H-shaped narrow
slot antenna
Y.-F. Lin, P.-C. Liao, P.-S. Cheng, H.-M. Chen,
C.T.P. Song and P.S. Hall
A miniature printed H-shaped narrow slot antenna fed by a coplanar waveguide (CPW) is proposed for WLAN applications in the 2.4 GHz band. The antenna with a simplified open-end tuning stub is also proposed. The prototype has been designed and fabricated and found to have a bandwidth of 13.5%, and the radiation pattern measured at resonance is very close to omnidirectional in the H-plane. The characteristics of the proposed antenna has been investigated using simulation software HFSS and experimental results. The measured and simulated results show excellent agreement.
Introduction: Recently, coplanar waveguide (CPW) transmission lines have been widely used as feeding networks with slot antennas. CPW lines have many useful design characteristics such as low radiation leakage, less dispersion, little dependence of the character-istic impedance on substrate height, and uniplanar configuration. They also allow easy mounting and integration with other microwave integrated circuits and RF frequency devices. Many slot antenna elements suitable for a CPW-fed configuration have been proposed in the literature. The main issue with a CPW-fed slot antenna is to provide an easy impedance matching and to increase the antenna bandwidth. So far, several impedance matching techniques based on the change of the slot shape, coupling mechanism or both have been reported. Shorted-end CPW-fed slot antennas have been studied experimentally [1, 2]. These CPW-fed slot antennas are one-wavelength centre-fed slot antennas. There are two CPW-fed slot antennas with varied shapes via inductive coupling, one is the L-shaped slot in[1]and the other is the U-shaped slot in[2]. Antennas of this type have impedance bandwidth less than 7% and are intended for various applications. An alternative to this design is an open-end CPW-fed, which can be modified to a half-wavelength capacitive coupled slot antenna giving an impedance bandwidth of above 60%
[3, 4]. In[3, 4], the rectangular wide slot antennas are fed by CPW lines with an open-end tuning stub. The tuning stub length was found to be effective in controlling the coupling of the electromagnetic energy from the CPW line to the slot and enhance the bandwidth of the slot antenna. This Letter presents a new CPW-fed capacitive H-shaped narrow slot antenna. It is noted that the electric field inside the rectangular slot has a sinusoidal distribution with a maximum at the centre, and a null at the end of the slot. Therefore the electric field distribution in the slot significantly affects the value of the input impedance and the radiation pattern. To improve the radiation characteristics of the slot antenna and to reduce its size, an H-shaped slot antenna is proposed. In particular, the electric field distribution in the H-slot is much more uniform compared with that in a rectangular slot[5]. The proposed antenna can be easily excited by a 50 O CPW line with an open-end tuning stub, and good impedance matching can be obtained for operating frequency within the 2.4 GHz band.
Antenna design: The structure of the CPW-fed capacitive H-shaped slot antenna with a tuning stub is shown inFig. 1, and is printed on an FR4 substrate of thickness h ¼ 1.6 mm and relative permittivity er¼4.2. This slot antenna has a simple structure with one layer of dielectric and metal. The H-shaped slot is etched at the centre of a 25 mm (0.21 l0) 35 mm (0.29 l0) ground plane, where l0is the free space wavelength at the centre frequency 2450 MHz. The main horizontal slot has a length 2WsþW þ 2G and width, W2, and the side arm of the H-slot has dimensions of LsW1. The 50 O CPW feeding structure is composed of a centre strip width, W ¼ 3.7 mm and gap width G ¼ 0.4 mm. In addition, a tuning stub of length of S and width W terminates the CPW feed. To achieve better impedance matching, the tuning stub should be lengthened and that causes the horizontal slot section of the H-slot to be etched outwards with a length of Sdand width of W þ 2G. By carefully adjusting the tuning stub, the simple printed slot antenna can operate in the different bands and good impedance matching for the operating frequency can easily be obtained. HFSS simulation software was used and measurements were conducted to verify the simulation results.
35 W2 Ws W1 Sd S Ls G W CPW feed er h ground plane 25 mm
Fig. 1 Geometry of CPW-fed capacitive H-shaped slot antenna
Fig. 2 Photograph of prototype slot antenna
Measured results: A 2.4 GHz prototype antenna was fabricated on an FR4 substrate. The design parameters are: Ws¼7 mm, W2¼3 mm, Ls¼11 mm, W1¼1.2 mm, Sd¼5 mm and S ¼ 6 mm. The width of the strip and slot of the 50 O CPW feed line, W and G, are chosen to be 3.7 and 0.4 mm, respectively. The photograph of the prototype antenna is shown inFig. 2.Fig. 3shows the measured and simulated return loss. A resonance with good impedance matching can be seen. The measured data, in general, agree with the HFSS simulated results. A measured impedance bandwidth of 13.5% (for S11< 10 dB), corresponding to the frequency ranged from 2250 to 2575 MHz, is obtained. It can be seen that the impedance bandwidth is sufficient to cover the 2.4 GHz band for WLAN operation. In addition, the total length of the perimeter of the H-shaped slot is about 1.02 ls (101.8 mm) where ls is about 99.5 mm and ls is the guided wavelength in the slot determined to be about 0.8 l0 in [6] by considering the presence of different dielectric substrates on the two sides of the slot.Fig. 4shows the measured return loss for different stub lengths (S). It can be found that the total length of the perimeter of the H-slot is approximately equal to a guided wavelength lsof the slotline at resonance for S ¼ 5, 6 and 7 mm, and the resonant frequency decreases with an increase in the stub length. For the cases S ¼ 0 mm (without stub) and S ¼ 8 mm, these types of structure are referred to as a CPW-fed capacitive coupling and inductive coupling, respectively. Fig. 5 shows the measured and simulated radiation patterns at 2450 MHz. It is observed that the radiation patterns are similar to a half wavelength dipole antenna in the two radiating planes. The patterns are in good agreement with the simulated ones. The patterns in the H-plane are more omnidirectional when compared to the conventional slot antenna because the H-slot is narrower than the conventional, and because of different electric field distribution in the H-shape slot. The measured peak gain in the 2.4 GHz band is about 2.5 dBi, with small gain variations across the band. The gain was obtained using the gain transfer method where a standard gain horn antenna was used as a reference.
Fig. 3 Measured and simulated return loss of prototype antenna
Fig. 4 Measured return loss for proposed antenna with different stub lengths (S)
Fig. 5 Measured and simulated radiation patterns of prototype antenna at 2450 MHz
Conclusions: A novel simplified miniature CPW-fed capacitive H-shaped slot antenna has been proposed and tested at 2.4 GHz. The prototype has been designed and fabricated and found to have the bandwidth and antenna gain of 13.5% and 2.5 dBi, respectively, and the radiation pattern measured at resonance is very close to omni-directional in the H-plane. The proposed antenna has a simple and effective feeding structure, has adequate operational bandwidth, and has radiation patterns such that it is suitable for use in WLAN applications.
#IEE 2005 29 April 2005
Electronics Letters online no: 20051571 doi: 10.1049/el:20051571
Y.-F. Lin and H.-M. Chen (Institute of Photonics and Communi-cations, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan)
E-mail: [email protected]
P.-C. Liao and P.-S. Cheng (Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan)
C.T.P. Song (Sendo International Limited, Hatchford Way, Birmingham B26 3RZ, United Kingdom)
P.S. Hall (Department of Electronic, Electrical and Computer Engineering, School of Engineering, University of Birmingham, Edgbaston Park Road, Birmingham B15 2TT, United Kingdom)
References
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