A CPW-Fed Rectangular Ring Monopole Antenna for WLAN Applications

We present a simple coplanar waveguide(CPW-) fed rectangular ring monopole antenna designed for dual-band wireless local area network (WLAN) applications. The antenna is based on a simple structure composed of a CPW feed line and a rectangular ring. Dual-band WLAN operation can be achieved by controlling the distance between the rectangular ring and the ground plane of the CPW feed line, as well as the horizontal vertical lengths of the rectangular ring. Simulated and measured data show that the antenna has a compact size of 21.4 × 59.4mm, an impedance bandwidths of 2.21–2.70GHz and 5.04–6.03GHz, and a reflection coefficient of less than −10 dB.The antenna also exhibits an almost omnidirectional radiation pattern.This simple compact antenna with favorable frequency characteristics therefore is attractive for applications in dual-band WLAN.


Introduction
Wireless local area networks (WLANs), which enable user mobility, are considered to be a cost-effective solution for high-speed data connectivity.The growing demand for WLAN technology has led to interest in integrating the 2.412-2.482GHz (IEEE802.11b/g)and 5.15-5.825GHz (IEEE 802.11a) frequency bands into a single device that requires a dual-band antenna [1].Numerous designs for dual-band antennas have been reported, including a dual-monopole antenna with different lengths [2] or sleeves [3], a bent folded monopole [4][5][6], a combination of monopole and helical antennas [7], and a slot antenna [8][9][10].An antenna should exhibit impedance matching to the feed line, be low in cost and compact, and have a low profile, a high efficiency, and an omnidirectional radiation pattern.Broadband and omnidirectional characteristics of ring antennas have been reported [11][12][13][14][15][16][17]; however, to realize dual-band operation, additional structures must be employed on the bottom layer, such as a Tshape, S-shape, vertical strip resonator, or parasitic elements.Coplanar waveguide-(CPW-) fed antennas have received much attention because of their wide operating bandwidth, low profile, low cost, simple fabrication, potential for integration with monolithic microwave integrated circuits, and simple configuration using a single metallic layer [2,16].
In this paper, we describe a design for rectangular ringshaped monopole antenna that satisfies the 2.4-GHz and 5.2/5.8-GHzWLAN applications (i.e., it operates at frequencies in the range of 2.4-2.4835GHz and 5.15-5.825GHz).The antenna does not require any additional structures for dualband WLAN operation.Through numerical simulations, we show that the distance between the rectangular ring and the ground plane, as well as the horizontal vertical lengths of the rectangular ring, can be optimized to control the resonant frequencies and hence achieve dual-band operation.Therefore, the proposed antenna achieved the small size, simple structure, and good frequency response with the omnidirectional radiation pattern.

Antenna Design
The geometry of the rectangular ring-shaped CPW-fed antenna is shown in Figure 1, together with a photograph of Frequency (GHz)   the fabricated optimized structure.The antenna had overall dimensions of 21.4 × 59.4 × 1.6 mm 3 and was fabricated on an FR4 substrate with a relative permittivity of   = 4.4.A 50-Ω CPW transmission line with a signal strip width of 1.8 mm and a gap distance of 0.3 mm between the single strip and the coplanar ground plane was used to feed the antenna.The size of the rectangular ring was 11 × 20.5 mm 2 , and the two ground planes with dimensions of 9.5 × 14 mm 2 were located symmetrically on each side of the CPW line.The antenna was symmetrical with respect to the longitudinal direction (i.e., the -axis).
To investigate the performance of the proposed antenna for dual-band operation, the commercially available simulation software HFSS was used.T-shaped or vertical strip resonators that correspond to radiation around 5 GHz are typically added to square ring structures to achieve dualband operation [11][12][13][14]; however, with the antenna described here, we can achieve dual-band operation by controlling the distance between the rectangular ring and the ground plane, as well as the length and the width of the rectangular ring.
Figure 2(a) shows simulated data that reveal the effect of varying the distance between the rectangular ring and the ground plane  1 on the return losses of the antenna.The parameter  1 could be used to control the lower frequency and the upper band matching.and upper resonant frequencies were red-shifted.Figure 2(c) shows the effect of varying  3 when the width was fixed at  2 = 20.5 mm.The parameter  3 primarily affected the upper band; therefore,  3 can be varied to tune the properties of the upper frequency band.The optimized antenna dimensions for operation at 2.4 GHz and 5.2/5.8GHz are listed in Table 1.
Figure 3 shows the simulated current distributions in the optimized structure at the different frequencies.The current distributions varied significantly depending on the resonant frequency.It is evident from these current distributions that, at a frequency of 2.4 GHz, the radiation mainly resulted from the vertical strip line of the rectangular ring.The current distributions at a frequency of 2.4 GHz are similar to resonant Path 1, as shown in Figure 1(a).The horizontal strip line of the rectangular ring contributed mainly to radiation at frequencies of 5.2 and 5.8 GHz.The resonant currents at frequencies of 5.2 and 5.8 GHz are distributed on the horizontal strip line which is similar to resonant Path 2, as shown in Figure 1(a).Hence, by appropriately adjusting the lengths of the vertical strip line and the horizontal strip line of the rectangular ring, the suitable resonant current paths can be created to radiate the electromagnetic energy at the different resonant frequencies.Good agreement was evident between the simulated and measured data.At the lower resonant frequency of 2.4 GHz, the measured return loss was approximately −20 dB with a bandwidth of 500 MHz.At the upper resonant frequencies of 5.2 and 5.8 GHz, the return losses were less than −20 dB and the bandwidth was 1 GHz.

Experiment Result
The measured x-y plane and z-x plane radiation patterns are shown in Figure 5.The electric field patterns exhibited some lobes in the radiation patterns, whereas the magnetic field distributions were almost completely omnidirectional.The measured peak gains in the 2.4-GHz and 5.2/5.8-GHzbands are shown in Figure 6; the peak antenna gain in the 2.4-GHz band was 2.0 dBi, and that in the 5.2/5.8-GHzband was 5.0 dBi.

Conclusion
We have described a CPW-fed rectangular ring monopole antenna for WLAN applications.The antenna design is compact, with dimensions of only 21.4 × 59.4 × 1.6 mm 3 .The antenna had a simple geometry and is relatively easy to fabricate.Because the bandwidths of the lower and upper bands of the proposed antenna were sufficient for operation at the 2.4-GHz and 5.2/5.8-GHzbands, the antenna is suitable for dual-band WLAN applications.Furthermore, the antenna provides omnidirectional radiation patterns and stable gains within the bands, making this CPW-fed rectangular ring antenna design suitable for dual-band WLAN applications.

2 InternationalFigure 1 :
Figure 1: Geometry of the printed rectangular ring antenna: (a) top view and (b) side view.(c) Photograph of the fabricated antenna.

Figure 5 :
Figure 5: Simulated and measured radiation patterns of the proposed antenna at (a) 2.4 GHz, (b) 5.2 GHz, and (c) 5.8 GHz.

Figure 6 :
Figure 6: Measured peak gain of the proposed antenna at operational frequency bands.

Figure 4
Figure 4 compares the simulated return losses with those measured from the fabricated antenna shown in Figure 1(c).