Rectangular Ring Antenna Excited by Circular Disc Monopole for WiMAX System

This research presents a rectangular ring antenna excited by a circular disc monopole (CDM)mounted in front of a square reflector. The proposed antenna is designed to cover a frequency range of 2.300–5.825GHz and thereby is suitable for WiMAX applications. Multiple parametric studies were carried out using the CST Microwave Studio simulation program. A prototype antenna was fabricated and experimented. The measurements were taken and compared with the simulation results, which indicates good agreement between both results.Theprototype antenna produces an impedance bandwidth (|S 11 | < −10 dB) that covers theWiMAX frequency range and a constant unidirectional radiation pattern (θ = 0∘ and 0 = 90). The minimum and maximum gains are 3.7 and 8.7 dBi, respectively. The proposed antenna is of compact size and has good unidirectional radiation performance. Thus, it is very suitable for a multitude of WiMAX applications.


Introduction
Besides an integral part of wireless communications, a wideband antenna is a crucial technology in the short-range, highspeed, and indoor wireless communications.According to [1][2][3][4], the WiMAX frequency bands are classified into three frequency bands of 2.500-2.690GHz, 3.400-3.690GHz, and 5.250-5.850GHz (2.5/3.5/5.5 GHz).Over the past decades, a greater number of countries have utilized cellular base stations for installation of WiMAX antennas.Moreover, advances in mobile communications technology and rapid urbanization help promote the growth of indoor base stations, for example, in buildings, underground train systems, and tunnels.One distinct characteristic of indoor base stations is their omnidirectional radiation pattern, which is however their main drawback since an omnidirectional antenna can cover a limited circular area.This renders the omnidirectional antenna unsuitable for applications in the environment characterized by long and confined spaces in which a unidirectional antenna is more appropriate.In addition, unidirectional antennas are applicable to point-topoint communications.In these environments, for example, streets, highways, tunnels, and corridors, a unidirectional or bidirectional antenna performs better than an omnidirectional antenna [5][6][7][8][9][10][11].To generate a unidirectional beam, a planar reflector [12,13], corner reflector [14,15], parabolic reflector [16], or conical reflector [17] was used with an omnidirectional monopole antenna.The unidirectional bandwidth is narrow and could be enhanced by replacing the linear monopole with a surface monopole, for example, circular, triangular, square, or rectangular monopole [18][19][20][21][22].
To achieve the unidirectional pattern along the WiMAX frequency range of 2.300-5.825GHz, this research proposes a rectangular ring antenna excited by a CDM mounted in It is however found that the beam direction beyond a higher frequency of 5.5 GHz tilts upward as a result of the asymmetrical areas between the upper and lower portions of the rectangular ring chamber.The organization of this paper is as follows.Section 1 is the introduction.Section 2 details the structure and parameters of the proposed antenna.Section 3 presents the design principle and the parametric study, while Section 4 compares the simulation and measured results.The concluding remarks are provided in Section 5.

Geometry of the Proposed Antenna
Figure 1(a) illustrates the structure of the proposed antenna, which consists of a CDM and a rectangular ring with width , height , length , and thickness .The rectangular ring center is at the -plane origin.The ring is excited by the CDM on the -axis via a 50 Ω SMA connector to generate vertical polarization.The CDM radius   and the delta gap  are 1.10 cm and 1.00 mm, respectively.The structure is mounted in front of an  ×  square reflector.The space between the rectangular ring and the reflector is denoted by ℎ, as illustrated in Figure 1(b).The antenna radiation pattern is unidirectional with the beam peak pointing in the  direction.

Design Principle and Parametric Study
The antenna is designed to operate in the WiMAX frequency range of 2.300-5.825GHz.The design principle starts with exciting the rectangular ring with a circular-disc monopole (CDM), as shown in Figure 1.The initial ring width  of the 2.3 GHz frequency is slightly greater than /2 (i.e., 6.60 cm).The ring height  is proportional to the ring width ; that is,  = /2.The ring length  and thickness  are 3.25 cm (/4) and 2.00 mm, respectively.The rectangular ring is excited by a circular-disc monopole with an   radius 1.3 cm (0.1).The selected square reflector is 16 × 16 cm ( × ) in dimension with the space (ℎ) between the rectangular ring and the reflector of 3.90 cm (0.3).Both the rectangular ring and the reflector are made of aluminum.Ideally, the width  and height  of a rectangular ring should be minimal to obtain an antenna of a size as compact as possible.
The width  of the proposed rectangular ring follows the following rule, where  (i.e., 13.00 cm) is the wavelength at the 2.3 GHz frequency: To enhance the bandwidth, the rectangular ring is thus excited by a circular-disc monopole with an   radius.The initial circular monopole radius (  ) is determined by ( 2) with the predetermined feed gap () of 1.00 mm and 0.21 being referenced from [15]: Parametric studies were carried out to determine the optimal ring dimensions that produce an optimal combination of resonant frequency, radiation pattern, and gain and also are of compact size.Figure 2 illustrates the fractional bandwidths of resonant frequency (closest to the 2.3 GHz frequency) by varying  and  of the rectangular ring antenna which is excited by a CDM.Both the ring antenna and the CDM are together mounted in front of the square reflector.As shown in Figure 2, the fractional bandwidth of resonant frequencies at 2.3 GHz is achieved with  of 7.20 cm (0.553),  of 3.60 cm, and  of 2.60 cm, which are selected as the design parameters.It is noted that the ring width  of 7.20 cm (0.553) is selected because of the wide bandwidth, compact antenna size, and available material in the market suitable for the mass production.
To determine an optimal matching condition with a wide bandwidth, the space between the CDM and the square reflector ℎ is varied from 1.65 (0.21) to 3.90 cm (0.30).The simulated unidirectional beam is achieved at ℎ of 0.10-0.30and of 0.60-0.70,while that with the widest bandwidth is achieved at ℎ < 0.30.The beam splits occur when ℎ is between 0.30 and 0.60.According to [13], the unidirectional beam can also be realized when ℎ is ≥0.70.It is found that the radiation pattern becomes split for ℎ between 0.30 and 0.60.The simulation results are shown for ℎ of 1.65-2.90cm because while ℎ is >2.90 cm the | 11 | cannot cover the frequency bandwidth (e.g., ℎ = 3.15 cm, the | 11 | > −10 dB from 4.92 to 5.91 GHz).Based on the simulation results (Figure 3), ℎ of 2.40 cm (0.184) is selected as it produces a unidirectional beam with wide bandwidth.
Figures 4(a the ring height.The radius of CDM (  ) is varied between 1.00 and 1.40 cm to determine a ring height (/) that gives good resonance at the 2.3 GHz frequency.To produce the rectangular ring of a size as compact as possible,   = 1.10 cm for  = 0.48 or 3.40 cm is selected.
Figure 5 illustrates the impedance bandwidth | 11 | of the resonant frequency closest to 2.3 GHz as a function of / of the rectangular ring antenna, assuming a constant   of 1.10 cm.To achieve the goal of a compact ring antenna with the widest bandwidth, the ring width () and ring height () of 7.20 and 3.40 cm are chosen for experiment.The inclusion of the CDM into the rectangular ring antenna is to increase the antenna's frequency bandwidth to cover the WiMAX band of 2.300-5.825GHz. Figure 6 illustrates the frequency response curves as a function of | 11 | and the frequency for the CDM radii of 1.00 to 1.40 cm and constant feed gap () of 1.00 mm.Even though the | 11 | curves for five CDM radii follow a similar pattern, an   of 1.10 cm is selected as it gives the highest overall efficiency in terms of impedance bandwidth, radiation pattern, and gain.
In addition, the effect of the feeding gap () is examined and the results are depicted in Figure 7.It is found that varying the feeding gap () impacts the impedance bandwidth.That is, a reduction in  from 1.50 mm to 0.50 mm causes the impedance bandwidth to become narrower and thereby fails to cover the entire WiMAX frequency band.Based on Figure 7, the feeding gap () of 1.00 mm is chosen for the widest impedance bandwidth which covers the entire WiMAX band.Table 1 presents the optimal parametric values from the simulation of the proposed antenna.

Measured Results
To validate the simulation results, a prototype antenna was fabricated based on the optimal parameters in Table 1. Figure 8 is a photograph image of the prototype antenna.

Impedance Bandwidth.
The measurements of impedance bandwidth, radiation pattern, and gain were taken using an HP8720C Network Analyzer.A comparison of the simulated and measured | 11 |, represented, respectively, by a solid line and a dashed line, is presented in Figure 9.The simulated impedance bandwidth (| 11 | < −10 dB) of 88% was achieved in a frequency range of 2.28-5.91GHz (central frequency of 4.095 GHz), while that of the prototype antenna of 93% was in a frequency of 2.16-5.96GHz (central frequency of 4.06 GHz).It is found that the simulation and measured results are in reasonable agreement.In addition, the frequency ranges of both simulation and experiment satisfy the WiMAX requirement.
Figure 10 illustrates a comparison of the gain along the 2.300-5.825GHz frequency range of the prototype antenna and that of the simulation, in which the former is represented by a solid line and the latter by a dashed line.The minimum International Journal of Antennas and Propagation   in the -plane at 5.5 GHz slightly tilts upward from the axis as a result of the CDM installation on the rectangular ring base.The CDM contributes to the asymmetrical areas between the upper and lower portions of the ring chamber.However, the radiation pattern in the -plane is symmetrical because of the symmetrical areas between the left and right portions of the ring chamber.The simulation and measured radiation patterns show good agreement.At 2.5 GHz, the   The measured front-to-back ratio (/) in both zand planes is greater than 20 dB.Thus, the proposed antenna produces a good radiation pattern and is a very good unidirectional antenna.

Conclusions
The unidirectional antenna suitable for WiMAX is developed using the CDM-excited rectangular ring mounted in front

2 InternationalFigure 1 :
Figure 1: The structure of the proposed antenna with CDM.

4. 2 .
Radiation Pattern.Figures 11 and 12  illustrate the simulation and measured radiation patterns in the and planes, respectively, at 2.5, 3.5, and 5.5 GHz.In Figure11, the beam peaks direct in the 3 + direction.The radiation pattern

Figure 7 :
Figure 7: | 11 | relative to frequency as a function of .

Figure 8 :
Figure 8: Photograph image of the prototype antenna.

Figure 12 :
Figure 12: The comparison between the simulated and measured radiation patterns in the -plane.

Table 1 :
The optimal parametric values of the proposed antenna.