Feed for dual-band printed dipole antenna
H.-M. Chen, J.-M. Chen, P.-S. Cheng and Y.-F. Lin
A dual-band printed dipole antenna is proposed for WLAN applica-tions in the 2.4 and 5.2 GHz bands. A spur line was etched on the arms of the printed dipole to achieve the dual-band operation. The printed dipole antenna with a simplified matching network is proposed. The novel simplified feed structure consists of a pair of parallel metal strips printed on the opposite sides of the dielectric substrate and connected to a 50 O microstrip line with a truncated ground plane. This feeding network does not require additional transition devices, such as a T-junction, power divider, tuning stub or microstrip-to-coplanar stripline balun.
Introduction: In recent years there have been rapid developments in wireless local area network (WLAN) applications. To satisfy the 2.4 GHz band of IEEE 802.11b and the 5.2 GHz band of 802.11a WLAN standards, dual-band operations in the 2.4 GHz (2412– 2484 MHz) and 5.2 GHz (5150–5350 MHz) bands are demanded in practical WLAN applications. A single antenna is highly desirable if it can operate at these two bands. The antenna should be in planar form, lightweight and compact, so that it can easily be embedded in the cover of communication devices. In addition, a simplified feeding circuit is also an important component, because it can reduce the transmission line length and the radiation losses. For this purpose, some dual-band printed antennas have been reported[1–4]. The antenna in[1] uses a microstrip-fed double-sided printed dipole antenna, and impedance matching is performed by adjusting the width of the micro-strip line and the gap of the micro-stripline. In the design of[2], the antenna is fed by a standard 50 O microstrip line using a broadband microstrip-to-coplanar stripline transition. The transition network is realised by a matched T-junction and a narrowband delay line, which not only increases the design complexity but also requires long transmission lines. In[3, 4], the antenna consists of a printed monopole and a 50 O microstrip line with an open-circuited tuning stub. The tuning stub length was found to be effective in controlling the coupling of the electro-magnetic energy from the microstrip line to the monopole. In this Letter, a new microstrip-fed dual-band printed dipole antenna is presented. With a spur line embedded into both arms of the dipole, a dual-band operation is generated in a single antenna. The proposed design does not require separate dipole arms to achieve two separate operating bands, and it has a shorter length of arms than the design in[1]. The proposed antenna can easily be excited by a 50 O microstrip line and a pair of parallel metal strips between the dipole and the microstrip feed line; good impedance matching can be obtained for operating frequencies within both 2.4 and 5.2 GHz bands. 50 mm L L W Ls W Ws h er Wf top side 22 units: mm bottom side z y 7.5 1 7.5 2 2 ground plane (bottom side)
Fig. 1 Geometry of dual-band printed dipole antenna with spur lines
Antenna design: The structure of the simplified feed for a printed dipole antenna with a spur line is shown inFig. 1, which is printed on an FR4 substrate of thickness h ¼ 1.6 mm and relative permittivity er¼4.2. The top side consists of a microstrip feed line, one of
the parallel metal strips and an arm of the printed dipole antenna. The bottom side consists of a truncated ground plane (50 22 mm), the other parallel metal strip, and the second arm of the dipole antenna printed on the opposite side of the substrate. The arms of the dipole have an area of L W mm2, and are printed on opposite sides of a
dielectric substrate. The feeding structure is composed of a 50 O microstrip line and two parallel metal strips. The parallel strips have a uniform width of Wsand a length of LsþW (Ls¼3 mm in this study).
Both the microstrip line and parallel metal strips must be carefully
designed with a characteristic impedance of 50 O. It is noted that the width Wsof the parallel metal strips is different from the width Wfof
the microstrip feed line. Although both lines are designed for the 50 O impedance, the fringing field of the lines shared by the air (er¼1) and
the substrate make a significant difference. The width Wfof 3.16 mm
for the 50 O characteristics impedance is easily obtained. To reduce experimental cut-and-try design cycles, the simulation software IE3D is used to guide fabrication.Fig. 2shows the simulated real part and imaginary part of the characteristics impedance (Zc) for the parallel
strips. It can be seen that the width Wsof the 50 O parallel strips is
chosen to be 4.0 mm in the frequency range 2–6 GHz.
Fig. 2 Simulated characteristics impedance (Zc) for different parallel
strips width (Ws)
a Real part b Imaginary part
At first, by carefully adjusting the printed dipole arms of length L and width W, the simple conventional printed dipole antenna (without spur line) can operate in the different bands. In the proposed design, a spur line is etched on each of the two rectangular dipole arms (detailed dimensions are given inFig. 1). With the presence of the spur lines, two additional dipole arms are obtained, which form a second dipole antenna with a smaller length and width and which are used to generate a higher resonant mode for the 5.2 GHz band operation. It is clear that the longer dipole arm in this design operates in the 2.4 GHz band. Using this feeding structure, good impedance matching for both operating frequencies can easily be obtained.
Fig. 3 Measured and simulated return loss for proposed antenna
Measured results: Fig. 3shows the measured and simulated return loss for the proposed dipole antenna. Each resonant mode with good impedance matching can be seen. The measured data in general agree with the simulated results obtained from simulation software HFSS. For the proposed antenna, the areas of 16 10 mm and 7.5 2.0 mm for the long and the short dipole arms are designed for 2.4 and 5.2 GHz bands, respectively. For the lower band, an impedance bandwidth of 9.3% (for S11< 10 dB), corresponding to the
frequency range 2360–2590 MHz, was obtained. For the higher band, an impedance bandwidth of 5.1% (for S11< 10 dB),
corre-sponding to the frequency range 5150–5420 MHz, was also obtained. It can be seen that the impedance bandwidth of the lower and the higher bands covers the 2.4 and 5.2 GHz bands for WLAN operation. In addition, the overall length (2L þ Ws) of the proposed antenna is
about 0.294 l0(36 mm), where l0is about 122.5 mm.Figs. 4and5
show the measured and simulated radiation patterns at 2450 and 5250 MHz, respectively. It can be observed that the radiation patterns are dipole-like for the two operating bands. For the two frequencies, the patterns in the x–y plane (E-plane) still act as a dipole, and the patterns in the x–z plane (H-plane) are affected by the ground plane.
