Improved Design of a Broadband Dielectric Resonator Antenna with Wide Beam

A wide-beam and broadband dielectric resonator antenna (DRA) with broadside pattern is proposed and analyzed. The excellent performance of wide beam is achieved by designing the metal ground. In addition, the antenna has broad bandwidth by introducing an air gap between the dielectric resonator (DR) and the ground. From simulated results, the impedance bandwidth ( S11 <−10 dB) of the proposed antenna is about 35%, from 2.8 to 4GHz, and the maximum 3 dB beam width in H-plane is greater than 210°. A prototype of the proposed DRA is produced and tested. Good agreement between the simulated and measured results is achieved.


Introduction
In modern wireless communication systems, such as safeguard systems, active phased arrays, and intelligent transport systems, beam width of the antenna is a significant performance.For example, in communication base station, wide-beam antenna can help to reduce the number of the antenna array to synthesize omnidirectional radiation.In detection system, wide-beam width antenna will increase the coverage area and reduce detection time.Recently, a lot of work has been done to improve the beam width of the antenna.The dipole antenna with the back cavity is useful to get wide-beam characteristic [1,2].However, the size is not easy to be compact.Beyond that, the patch antenna is another alternative.In [3], a low profile patch is proposed with wide-beam performance.However, at the same time, it is difficult to get broadband characteristics for this type of antennas.
Dielectric resonator antenna (DRA) has got more and more attentions for many superiorities compared with metal microstrip antennas such as low conductor loss, high radiation efficiency, and compact size [4].Much useful work has been done to improve the performance of DRAs, such as wide band [5], multipolarization [6], and high gains [7].However, there are only a few studies on the wide beam of DRA in the published literature [8,9], and in these papers, except for the good performance of the beam width, other performances such as impedance bandwidth and the size of the antenna are unsatisfactory.In [10], a wide-band widebeam DRA is proposed by me.However, there are only preliminary simulation results in the paper, no physical test results, design principles, and performance analysis in it.
In this paper, a wide-beam and broadband DRA is proposed.It has a broadside radiation pattern.By comparing the proposed antenna and the previous works in Table 1, the proposed antenna has improved performance.

Antenna Configuration and Design Process
In Figures 1 and 2, the geometry and the fabricated prototype of the proposed antenna are shown, respectively.The proposed antenna consists of a rectangular DR and a decreased metal ground.A SMA probe is used to excite TE δ11 mode in the DR.By optimizing the length and diameter of the probe, impedance matching of the port has been realized.The DR is made of a ceramic material with the dielectric constant ε r = 10.The metal ground is under the DR, and the size is close to the bottom of it.All the configuration parameters for the proposed antenna are presented in Figure 1.
The design process of the proposed antenna includes three steps to realize broadband and broadbeam characteristics.First, a rectangular DR with ε r1 = 10 is placed on a big ground, which works in TE δ11 mode, as ant.1 shown in Figure 3(a).The resonant frequency of TE δ11 mode can be estimated using (1), where l, w, and h are length, width, and height of DR, respectively [11], so the size of DR can be approximately calculated.Second, the metal ground is decreased to widen the radiation beam on H-plane, as ant. 2 shown in Figure 3(b).The principle will be analyzed in detail in Section 3 of this paper.Finally, an air gap is inserted between the bottom of the DR and the ground to decrease the value of Q, which can achieve wide-band feature [12].The proposed antenna is shown in Figure 3(c).The numerical analysis of the proposed antenna is performed using the commercial 3D full-wave electromagnetic (EM) simulation software Ansoft HFSS, based on the finite element method.

Analysis and Discussion
Figure 4 shows the simulated reflection coefficient of two reference and proposed antennas.It can be seen that the reference antenna 1 has two resonant frequencies, which are 3.2 GHz and 4.5 GHz, respectively.The first resonant frequency corresponds to TE δ11 mode, which has broadside radiation characteristic.However, the bandwidth does not meet the broadband requirement.What is more, decreased metal ground in reference antenna 2 makes the reflection coefficient further deteriorate.In order to increase the bandwidth, an optimized air-filled gap is inserted between the dielectric resonator and the metal ground, which can decrease the value of Q of the DRA and broaden the bandwidth.This is the final proposed antenna.It can be seen that the proposed antenna achieves good impedance matching in a wide band, with the −10 dB impedance bandwidth being 2.8-4 GHz and the relative bandwidth achieving 35%.   2 International Journal of Antennas and Propagation In Figure 5, the simulated electric field vector of TE δ11 mode on XOZ plane of the proposed antenna at 3.5 GHz is shown.It can be seen that the electric field vector of TE δ11 is similar to a horizontal magnetic dipole, in view of the mirror effect of the mental ground, so it has the broadside radiation pattern.
The wide-beam feature of the proposed antenna is achieved by decreasing the metal ground.In Figure 6(a), simulated current field distribution on metal ground of ant. 1 at 3.2 GHz is presented.From it, the current on the ground is along the y-axis, and current distribution of the antenna 1 is relatively uniform.The current intensity on the two edges parallel to the y-axis is relatively large, while it is very small parallel to the x-axis.Therefore, the change of ground size in Y direction has little influence on the current on the ground and the radiation pattern of the DRA. Figure 6(b) shows the simulated current field distribution on metal ground of the proposed antenna at 3.5 GHz.In this figure, the ground size is reduced in the X direction.The current intensity increases sharply on the two edges; therefore, the radiation intensity of the current increases.The radiation energy will leak below the metal ground along the edge parallel to the y-axis, so the beam width of the antenna on the XOZ plane (H-plane) is broadened.
The ground effect on the beam can also be reflected in another picture.Figures 7(a The electric field energy of the antenna 1 on H-plane is mostly concentrated above the dielectric resonator and has less distribution at the edges and back of the metal ground.When the width of the metal ground is reduced, the back radiation of the antenna is significantly enhanced and the electric field distribution in the space is more uniform.Therefore, the width of the antenna can be effectively broadened by reducing the size of the metal ground.
Figure 8 presents the simulated radiation patterns on H-plane with different values of wg.It is evident that the width of metal ground plays a crucial role in the farfield radiation patterns.When wg = 13 mm, the beam on the H-plane is widened obviously.The simulated radiation patterns on E-plane with different values of wg are shown in Figure 9; it can be seen that the value of wg has little effect on the width of the E-plane beam.
Figure 10 presents the reflection coefficient of the proposed antenna with different values of wg.It is shown that as wg increases from 13 mm to 33 mm, the operating band is almost no change.For maximum beam width, wg = 13 m m is the best choice.
In Figure 11, simulated radiation patterns on the H-plane with different frequencies of the proposed antenna are shown.When the frequency is 2.8 GHz, the 3 dB beam width is about 214 °.When the frequency is increased to 4.0 GHz, the 3 dB beam width is about 140 °.Therefore, the beam width of the proposed antenna within the impedance bandwidth is from 140 °to 214 °.
Figure 12 shows the simulated curve of front-to-back ratio versus wg at 3.5 GHz.It can be seen that front-to-back ratio increases first and then decreases when wg increases.When wg = 23 mm, the value of front-to-back ratio reaches the maximum of 34 dB.When wg = 13 mm, the value of front-to-back ratio is about 20 dB.  Figure 13 presents the simulated gain versus wg at 3.5 GHz.As can be seen from it, the gain gradually increases as the value of wg increases.By comparing the results of Figure 8, we can draw a conclusion that when the beam width of H-plane increases, the gain reduces, because the energy distribution is not concentrated.

Experimental Results and Discussions
To prove the theoretical analysis, the proposed antenna has been fabricated.It was measured by Agilent N5242A vector network analyzer.In Figure 14 The proposed wide-band wide-beam DRA has also been measured in an anechoic chamber.Figure 15 shows far-field radiation patterns (both copolar and cross-polar) at 3.5 GHz on the E-plane (XOZ plane) and H-plane (YOZ plane).From that, the 3 dB beam width on H-plane of the proposed antenna is greater than 180 °at 3.5 GHz.The measured crosspolarization levels in the maximum radiation direction achieve −20 dB and −35 dB on the H-plane and E-plane, respectively.
In Figure 16, simulated gain and measured gain versus frequency of the proposed antenna are illustrated.It can be seen that the measured gain shows a good agreement with the simulated one.The maximum measured gain reaches 5.1 dB.The partial parameters are presented in Table 1.It can be seen that the proposed antenna has better comprehensive performance compared with others.

Conclusion
A wide-band wide-beam dielectric resonator antenna has been developed in this paper.Its measured impedance bandwidth (|S 11 | < −10 dB) is from 2.8 Hz to 4 GHz, which is about 35%.Furthermore, wide-beam characteristic on the H-plane (XOY plane) is obtained.It may be serviceable in wireless communication systems which seek wide-band and wide-beam radiation features.

Figure 2 :
Figure 2: Fabricated prototype of the proposed antenna.
) and Figures 7(b) show the simulated electric field energy distributions on H-plane at 3.5 GHz of ant. 1 and the proposed antenna, respectively.

3
International Journal of Antennas and Propagation , the measured and simulated reflection coefficient of the proposed antenna is shown.It can be seen that the measured curve of |S 11 | is very close to the simulated one, covering the frequency range from 2.8 to 4 GHz (|S 11 | < −10 dB), indicating a relative impedance bandwidth of 35%.

Figure 5 :
Figure 5: Simulated electric field vector of TE δ11 mode on XOZ plane of the proposed antenna.

Figure 8 :Figure 9 : 4 Figure 11 :Figure 10 :
Figure 8: Simulated radiation patterns on the H-plane with different values of wg.

Figure 15 :
Figure 15: Measured and simulated radiation patterns of the proposed antenna in E-plane (a) and H-plane (b) at f = 3 5 GHz.

Figure 14 :
Figure 14: Measured and simulated reflection coefficient of the proposed antenna.

Figure 12 :Figure 13 :
Figure 12: Simulated curve of front-to-back ratio versus wg at 3.5 GHz.

Figure 16 :
Figure 16: Simulated and measured gains versus frequency of the proposed antenna.

Table 1 :
Comparisons of different wide-beam antennas.