A UWB Bidirectional Rectangular Ring Antenna Fed by CDM with a Rod and Ridges for Constant Beam Direction

Abstract

This paper presents an ultra-wideband (UWB) rectangular ring antenna excited by a circular disc monopole (CDM) with a conducting rod and two double ridges to radiate bidirectional pattern with constant beam direction along the entire UWB frequency range of 3.1–10.6 GHz. The conducting rod and double ridges at the upper wall of the ring are added to solve the tilted beam problem at the higher edge frequency whereas the double ridges at the lower wall are used to enhance the impedance bandwidth. The dimensions of the rectangular ring and the CDM are initially considered to achieve the bidirectional pattern with the suitable resonant frequencies and bandwidth. Then, the parameters of copper rod and two double ridges are determined by parametric study using CST Microwave Studio simulation software. The prototype antenna was fabricated, and the measured results show good agreement with the simulated ones. The obtained bandwidth of |S₁₁|<−10 dB can cover the UWB frequency range as well as the bidirectional beam radiation with constant beam direction (𝜃=0^∘,180^∘ and 𝜙=90^∘). The minimum and maximum measured gains are 3.1 dBi to 5.3 dBi, respectively. The proposed antenna possesses compact size with good radiation performance that can be a promising candidate for UWB applications.

1. Introduction

Nowadays, the wireless communication systems become the significant part of daily-life essence. The ultra-wideband (UWB) is a key technology for future short-range, high-speed, and indoor-data wireless communication. UWB technology is extensively and continuously spread since the regulation of Federal Communication Commission (FCC) defined the frequency spectrum from 3.1 GHz to 10.6 GHz for this system [1]. The demand for the multipurpose and multifunction of this system imposes inevitably the use of antenna to cover very wideband frequency range. For the typical cellular system, the service area is decomposed into small cells for the urban area. The cell shape is approximated by a circle where omnidirectional antenna plays a vital role to employ at the base station [2]. However, for some specific application that requires the long-range service area such as point-to-point communication between the buildings, when the mobiles move along the confined paths such as street, highways, tunnels, and corridors, for these environments, a unidirectional or bidirectional antennas are more suitable than the omnidirectional one to serve these demands [3–6]. The omnidirectional monopole antenna is modified by adding different shapes of reflectors such as planar reflector [7], corner reflectors [8, 9], curved reflectors [10], ring reflectors [11], and others to obtain the unidirectional or bidirectional beams. To enhance the bandwidth, the linear monopole is extended to surface monopole [12] or three-dimensional (3D) monopole [13–15]. To realize the bidirectional pattern along the UWB frequency range, this paper presents the circular disc monopole (CDM) excited rectangular ring antenna. However, the beam direction at the higher edge frequency is tilted due to unsymmetrical structure of the antenna along the upper and lower parts of the ring. The conducting rod and two double ridges are added at the upper wall of the ring to induce the current along the upper part of the ring that makes the constant beam direction at the higher edge frequency. This tilted beam problem can be solved at the expense of slightly narrower impedance bandwidth. To enhance the impedance bandwidth to meet the frequency requirement of UWB system along 3.1–10.6 GHz, the double ridges at the lower wall of the ring are added. The design of this antenna is easy and straightforward. The ring dimension and radius of CDM are firstly considered to operate at the desired resonant frequency with very wide bandwidth. Subsequently, the appropriate parameters of conducting rod and two double ridges are determined from the parametric study. The antenna characteristics such as |S₁₁|, radiation pattern, and gain along the UWB frequency are reported.

The content of this paper is organized as follows. The proposed antenna structure and its parameters are illustrated in Section 2. Section 3 describes the design principle and parametric study. To verify the simulated results, Section 4 presents the measured results. The conclusions are drawn in Section 5.

2. Antenna Description

The proposed antenna structure consists of the rectangular ring excited by CDM as shown in Figure 1. The ring is used to confine the CDM radiating omnidirectional pattern to become bidirectional beam. The ring is made of metallic material with the width of a, the height of b, and the length of c. The thickness of the ring is denoted by t. This structure is excited by the CDM of the radius 𝑟_𝑐 via 50 Ω SMA connector with delta gap 𝛿. The CDM is located at the center of the lower wall of the ring. The copper rod of the length 𝑙_𝑝 is located at the center of the upper wall of the ring. The double ridge of the height ℎ_𝑢 is located with 𝑠_𝑢 apart from the ring center on the upper wall of the ring. At the lower wall of the ring, there are double ridges of the height ℎ_𝑙 located at the distance 𝑠_𝑙 from the center. It is noted that the length of the ridges is identical to the length of the ring (c).

Figure 1 

Antenna structure.

3. Design Principle and Parametric Study

The principle of the antenna design starts with consideration of the conventional linear monopole antenna of the length 𝑙_𝑓. This structure radiates the omnidirectional beam. To obtain the bidirectional pattern, this structure is surrounded by the rectangular ring to focus the main beam into two opposite directions along +z and −𝑧 axes as shown in Figure 2. The initial parameter of the ring width a of 5 cm (𝜆/2) is chosen with TE₁₀ mode operation at 3 GHz that is around the lower edge frequency of UWB. It is noted that the ring in this paper is made of aluminum with thickness t of 8 mm for the reason of available fabricated material.

Figure 2 

An initial antenna structure.

The length 𝑙_𝑓 of linear monopole is 𝜆/4 at 3 GHz (𝑙_𝑓=2.5 cm). Figure 3 shows the bandwidth of the resonant frequency closest to the frequency of 3 GHz as a function of the ratio between the height and the width (𝑏/𝑎) of the ring for different ring lengths (c). The CST Microwave Studio [16] is used as the simulation tool.

Figure 3 

Bandwidth of the lower resonant frequency as the function of 𝑏/𝑎 (𝑎=5 cm and 𝑙𝑓=2.5 cm).

From Figure 3, the ring dimensions of a, b, and c can be selected from the compact size with the widest bandwidth. Therefore, the ring width (a) of 5 cm, ring height (b) of 3.5 cm (𝑏/𝑎=0.7), and ring length (c) of 1.5 cm are chosen. The next step is to find the suitable feeding structure and its dimension to cover the UWB frequency range of 3.1–10.6 GHz. The linear monopole is the simplest structure for consideration. Figures 4 and 5 show the lower and upper resonant frequencies of the rectangular ring with the dimension mentioned above, and it is excited by the linear electric probe of the length 𝑙_𝑓. The dimension of the probe length is expressed in term of 𝑙_𝑓/𝑏 ratio. To achieve the lower and upper resonant frequencies of 3.1 and 10.6 GHz, respectively, the length of the probe can be selected from different 𝑏/𝑎. For example, if 𝑏/𝑎 is 0.7, the probe length should be 0.62b. In the same fashion, the suitable 𝑙_𝑓/𝑏 for 𝑏/𝑎 = 0.8 and 𝑏/𝑎=0.9 are 0.52 and 1.0, respectively. For compact size, 𝑙_𝑓=0.62𝑏 for b = 0.7a is chosen in this paper. Then, the effect of the ring width is studied to determine the possible smallest size. Figure 6 shows the lower and upper resonant frequencies as a function of the ring width. It is apparent that the suitable ring width is 4 cm. Therefore, the dimension of the ring of a = 4 cm, b = 0.7a = 2.8 cm, c = 1.5 cm is used hereafter.

Figure 4 

The lower resonant frequency for various 𝑙𝑓/𝑏 (a = 5 cm and c = 1.5 cm).

Figure 5 

The upper resonant frequency for various 𝑙𝑓/𝑏 (a = 5 cm and c = 1.5 cm).

Figure 6 

Resonant frequency for various a (𝑏/𝑎=0.7, 𝑙𝑓=0.62𝑏).

To extend the bandwidth of the antenna, the one-dimensional feeding structure of linear monopole will be modified to two-dimensional monopole, that is CDM. The CDM is introduced to increase the bandwidth to 7.5 GHz (UWB: 3.1–10.6 GHz). The radius 𝑟_𝑐 of CDM is varied to determine the appropriate dimension. Figure 7 shows the |S₁₁| as the function of the frequency for various diameters (2𝑟_𝑐) of CDM. It is obvious that 2𝑟_𝑐 of 0.55b or 1.5 cm is chosen to cover UWB frequency range. At this step, the bidirectional pattern along the frequency range of 3.0–10.2 GHz is obtained.

Figure 7 

|S11| versus frequency as a function of 2𝑟𝑐.

Figures 8 and 9 show the |S₁₁| and the radiation pattern for different rod lengths compared with case of without conducting rod. The bandwidth can be further enhanced by adding conduction rod. It can be seen from Figure 9 that the beam direction in yz-plane without conducting rod at the higher edge frequency is shifted from the desired direction (𝜃=0^∘, 180° and 𝜙=90^∘). Therefore, the conducting rod of the length 𝑙_𝑝 is inserted at the upper wall of the ring to solve the tilted-beam problem. It is found that 𝑙_𝑝 of 0.21b (6 mm) is appropriately chosen to obtain the widest bandwidth with minimized beam tilt in yz-plane at 10.6 GHz.

Figure 8 

|S11| versus frequency as a function of 𝑙𝑝.

Figure 9 

The radiation pattern in yz and xz planes as a function of 𝑙𝑝 at 10.6 GHz.

Although the conducting rod can mitigate the beam tilt problem at the higher edge frequency, the radiation pattern can be further improved. From the simulated results, it is found that the radiation pattern is still slightly tilted. Thus, the double ridges at the upper wall of the ring with the height ℎ_𝑢 and the position 𝑠_𝑢 from the center are added to improve the radiation patterns at the higher edge frequency. To study the influence of the double ridges, the height ℎ_𝑢 is varied. From the results of parametric study, it is found that the height of double ridges equal or higher than 0.18b can improve level of |S₁₁| but the frequency band cannot cover UWB as shown in Figure 10. Instead, the double ridges of the length 0.11b (3 mm) can cover the UWB. From Figure 11, it is observed that the radiation pattern is slightly tilted when the frequency become higher. Moreover, to change the main beam to the desired direction, the positions 𝑠_𝑢 are varied from 0.05a to 0.25a. From the results, it is obvious that variation of 𝑠_𝑢 does not affect to |S₁₁| at the lower edge frequency as shown in Figure 12. When the frequency becomes higher than 9 GHz, the beam possesses minimum beam tilt when 𝑠_𝑢 equals to 0.1a (0.4 cm) as shown in Figure 13. As a result, 𝑠_𝑢 of 0.1a is selected as the design parameter.

Figure 10 

|S11| versus frequency as a function of ℎ𝑢.

Figure 11 

The radiation pattern in yz and xz planes as a function of ℎ𝑢 at 10.6 GHz.

Figure 12 

|S11| versus frequency as a function of 𝑠𝑢.

Figure 13 

The radiation pattern in yz and xz planes as a function of 𝑠𝑢 at 10.6 GHz.

To further determine the suitable parameter for the widest bandwidth and compact size, the height of double ridges ℎ_𝑙 and the position of ridge 𝑠_𝑙 at the lower wall of the ring are varied. The height ℎ_𝑙 and the position of the ridge 𝑠_𝑙 are varied at 0–0.25b and 0.175a–0.375a, respectively. From Figures 14 and 15, the impedance bandwidth and radiation pattern are depended on the height ℎ_𝑙 at the lower wall of the ring. From the results, it is found that |𝑆₁₁| is higher than −10 dB making the bandwidth cannot cover UWB when the height of double ridges ℎ_𝑙 is increased higher than 0.11b. The radiation patterns at the higher edge frequency will be worse with the increased height of double ridges at the lower wall of ring as illustrated in Figure 15. Moreover, the |S₁₁| and radiation pattern at the higher edge frequency can be improved by varying the position of ridge 𝑠_𝑙 at the lower wall of the ring. From the results, it is obvious that |S₁₁| can be reduced to lower than −10 dB with minimized beam tilt in yz-plane at 10.6 GHz when the position of ridge is increased as illustrated in Figures 16 and 17, respectively. Hence, the height of double ridges ℎ_𝑙 of 0.11b (3 mm) and the position of ridges 𝑠_𝑙 of 0.275a (1.1 cm) are chosen as the design parameter.

Figure 14 

|S11| versus frequency as a function of ℎ𝑙.

Figure 15 

The radiation pattern in yz and xz planes as a function of ℎ𝑙 at 10.6 GHz.

Figure 16 

|S11| versus frequency as a function of 𝑠𝑙.

Figure 17 

The radiation pattern in yz and xz planes as a function of 𝑠𝑙 at 10.6 GHz.

Figures 18 and 19 show the compared |S₁₁| of the proposed antenna by using CDM excited rectangular ring with rod and double ridges and without rod and double ridges. It is found that |S₁₁| of the rectangular ring antenna fed by CDM without rod and double ridges is higher than −10 dB at the higher edge frequency that makes this structure cannot cover the frequency band from 3.1 to 10.6 GHz. The bandwidth can be extended to cover the UWB frequency range by adding the rod and double ridges. Moreover, the radiation pattern at the higher edge frequency can also be improved with the rod and double ridges.

Figure 18 

Comparison of |S11| between with and without rod and double ridges.

Figure 19 

Comparison of radiation pattern with and without rod and double ridges in yz and xz planes at 10.60 GHz.

Ultimately, from the simulated results of the proposed antenna, the suitable parameters are obtained as tabulated in Table 1.

Table 1 

Suitable antenna parameters.

4. Measured Results

To confirm the simulated results, the prototype antenna with suitable parameters as tabulated in Table 1 was fabricated as depicted in Figure 20.

Figure 20 

Photograph of the prototype antenna.

4.1. Impedance Bandwidth

The impedance characteristics and radiation pattern are measured by using an HP8720C Network Analyzer inside anechoic chamber. The compared |S₁₁| between the simulated and measured impedance bandwidth is plotted in Figure 21. Apparently, the simulated and measured impedance bandwidths of |S₁₁|<−10 dB can cover the frequency range of 3–11 GHz. In addition, Figure 22 shows the simulated and measured gain at the desired direction (𝜃=0^∘ and 𝜙=90^∘) along the frequency range of 3.1–10.6 GHz. The minimum and maximum gains from the simulation and measurement are 3.5–6.9 dBi and 3.1–5.3 dBi, respectively.

Figure 21 

Compared |S11| between simulation and measurement.

Figure 22 

Simulated and measured gain.

4.2. Radiation Pattern

The simulated and measured radiation patterns in the yz- and xz-planes at 3.1, 6.8, and 10.6 GHz are shown in Figure 23. It is obvious that the beam peak directs to the +𝑧 and −𝑧 directions. Furthermore, it is found that the radiation pattern in the yz-plane is slightly tilted from z axis at the higher frequency because of the present CDM located at the lower wall of rectangular ring making unsymmetrical structure the upper and lower parts of the ring. Due to the symmetrical structure along left and right part of the ring, the radiation pattern is symmetry in xz-plane. The radiation patterns from simulation and measurement in the yz- and xz-planes are in good agreement.

(a)

(b)

(c)

Figure 23 

The comparison between the simulated and measured radiation pattern in yz and xz planes.

5. Conclusions

The rectangular ring antenna fed by CDM with conducting rod and double ridges is proposed for an antenna at the base station of ultra-wideband applications. This antenna radiates bidirectional pattern with constant beam direction over the UWB frequency. The effect of antenna parameters to |S₁₁| characteristics and the radiation patterns is investigated by parametric study. This antenna possesses the maximum gain from the measurement of 5.3 dBi at 9.8 GHz and higher than 3 dBi for the frequency range from 3.1 to 10.6 GHz. The impedance bandwidth of |S₁₁|<−10 dB can cover the frequency range of UWB. Since the proposed antenna possesses the compact size and good radiation performance, it can be a potential candidate for ultra-wideband applications.

Acknowledgment

This work is supported by AUN/Seed-Net Program.