Dual Band Hybrid CPW Fed Planar Monopole/Dielectric Resonator Antenna

Yih-Chien Chen1, Shi-Li Yao2, Kuei-Chien Chen3 Department of Electrical Engineering, Lunghwa University of Science and Technology,

Gueishan Shiang, Taoyuan County, Taiwan, 1E-mail: ycchenncku@yahoo.com.tw 2E-mail: newyear0325@yahoo.com.tw 3E-mail: kcchen@mail.lhu.edu.tw

Abstract— A dual band hybrid DR antenna consists of a CPW fed planar monopole and a circular-disk shape high permittivity dielectric resonator has been presented in this paper. The -10 dB S11 bandwidth of the dual band hybrid antenna is 28 % and 11 % in the ISM band and UNNI band, respectively. The radiation in the ISM and UNNI band are due to the CPW fed planar monopole and circular-disk shape dielectric resonator, respectively. The radiation patterns in the x-z and y-z plane in the ISM and UNNI band are symmetrical about the broadside. The peak gains are about 5.17 dBi and 7.88 dBi in the ISM band and UNNI band, respectively. Keywords— Monopole, DR antenna, return loss, radiation pattern, gain

I. INTRODUCTION Recently, dual band antenna has been implemented for

applications in WLAN (wireless local area network, 2.4–2.484 GHz), ISM (Industrial, Scientific, Medical), and Bluetooth at low band. Additionally, the dual band antenna can be applied at high band, such as HIPERLAN (high-performance radio local area network, 5.15–5.35 GHz) and Unlicensed National Information Infrastructure (UNNI) applied. There are many commercial applications, such as mobile radio and wireless communications that use microstrip antennas. Microstrip antennas however have limitations in size, bandwidth, and efficiency. On the other hand, the dielectric resonator (DR) antenna is attractive due to its compact size, high radiation efficient, compatibility with MMICs, and ease of obtaining different radiation patterns by exciting different modes.[1-3] Three dielectric properties of materials must be considered for DR antenna used: a high permittivity, a high quality factor, and a near zero temperature coefficient of resonant frequency. The size of the DR antenna decreases with increasing the permittivity of the DR. The quality factor is representative of the antenna losses. Typically there are radiation, conduction, dielectric, and surface wave losses. Therefore the total quality factor is influenced by these losses. The DR antenna offers very high radiation efficiency due to its low dielectric loss and it has no metallic loss.

In traditionally, the DR with relatively small permittivity around 10 is chosen for DR antenna to enhance the radiation capability.[4-10] Low profile DR antenna with relatively low resonant frequency can be achieved by using high permittivity. However, DR antenna using high permittivity will induce a

small bandwidth. Many researches about bandwidth enhancement of DR antenna have been made by using geometry deforming,[11] and stacking DR antennas.[12] In recently, hybrid DR antennas have attracted extensive interest with reference to dual-band or wideband operations without increasing antenna volume. The hybrid DR antenna can be considered as combination of two different radiating resonators. These two radiating resonators are stacked tightly and resonate at different frequencies. An ultra wideband antenna using a hybrid antenna presented, consisting of an annular DR antenna combined with a quarter-wave monopole to simultaneously act as a radiator and a loading element.[13] However, the monopole and annular DR antenna mounted on the ground plane induces the system complexity. Planar monopole antennas fed by a coplanar waveguide (CPW) are increasing popular due to their wide bandwidth, simple structure, and easy construction. In this paper, we present the design of a dual band hybrid antenna consists of a CPW fed planar monopole and a circular-disk shape high permittivity DR for operating at the ISM band and UNNI band. Details of the proposed antenna and experimental results are present. The characteristics of dual band hybrid antenna, such as return loss, input impedance, radiation pattern, and gain have been measured and discussed.

II. ANTENNA DESIGN The configuration of the proposed dual band hybrid DR

antenna consists of a CPW fed planar monopole and a circular-disk shape high permittivity DR is as shown in Fig. 1. The CPW fed planar monopole not only acts as an effective radiator but also as a feed for circular-disk shape high permittivity DR. The rectangular RF4 substrate has dimensions of 60.0 × 30.0 mm2 and thickness of 1.6 mm. A CPW transmission line is used to feed the planar monopole antenna. Dimensions of the CPW transmission line on FR4 substrate were calculated by close-form formulas given in[14], assuming infinite ground plane and finite dielectric thickness. The CPW transmission line dimensions were confirmed by AWR Microwave Office. The CPW transmission line dimensions were compatible with a subminiature version A (SMA) connector. The diameter of dielectric core of conventional SMA connector was about 4.5 mm. The CPW transmission line has signal strip width of 2.6 mm, and signal strip length of 20.5 mm, gap width between the ground plane

Proceedings of the 2009 IEEE 9th Malaysia International Conference on Communications 15 -17 December 2009 Kuala Lumpur Malaysia

978-1-4244-5532-4/09/$26.00 ©2009 IEEE 37

and signal strip of Wf of 1.0 mm,. Two finite ground planes with size Lg × Wg of 20.5 × 12.7 mm2 are arranged symmetrically on each side of the CPW transmission line.

The CPW fed printed strip monopole on the FR4 substrate resonates at approximately quarter guided wavelength. The resonance mode at 2.5 GHz is due to the CPW fed planar monopole whose length is 18.5 mm, approximately λg/4 at 2.5 GHz, where λg is the guided wavelength. The resonating frequency decreases with the increasing of the monopole length due to the increasing of resonating length.

The DR with a circular-disk shape is loaded on the CPW fed planar monopole antenna. The hybrid DR antenna is a cascaded resonating circuit with two resonating frequencies. The DR has a high permittivity rε =19, Radius a= 25.1 mm, and High b= 4.7 mm. The DR is from a low loss ceramic material composed of La(Mg0.49Zn0.01Sn)0.5O3 synthesized by conventional solid state method. The resonance mode at UNNI band is due to the circular disk DR antenna. The electromagnetic energy is coupled from the CPW fed planar monopole to circular-disk shape DR. The parameters that affect the overall performance of the dual band hybrid DR antenna, includes the length of the planar monopole, the permittivity of the DR, and the height of the DR. Some degree of impedance matching can be achieved by offsetting the circular-disk shape DR form the CPW fed planar monopole antenna. The position of circular-disk shape DR is adjusted to achieve maximum bandwidth at the UNNI band.

Fig. 1 Configuration of dual band hybrid DR antenna.

Simulation has been carried out by using a commercial electromagnetic simulator Agilent ADS. In simulation, the conducting grounds and the substrates were assumed to be finite in transverse plane. The CPW fed line was fabricated by using wet etching process. A coaxial connector was soldered to the CPW fed line to output signal from the dual band hybrid antenna to the port 1 of network analyzer. Reflection coefficient was measured on a PNA-L network analyzer (N5230A). Radiation pattern measurement was measured in a

chamber. A standard double ridged horn antenna was used as a transmitting antenna. The dual band hybrid antenna fed with CPW transmission line is mounted on a position which is controlled by a computer.

III. RESULTS The measurement return loss of the non-loaded and

circular-disk shape DR loaded CPW fed planar monopole is as shown in Figure 2. The measurement frequency range is from 1.0 GHz to 6.0 GHz. The non-loaded CPW fed planar monopole resonating at 3.74 GHz offers a bandwidth of 18%. When DR is loaded on the planar monopole, the dual band hybrid antenna shows two bands with three resonant frequencies. The measurement resonant frequencies are 2.52, 5.30, and 5.75 GHz. Using this cascaded arrangement, hybrid antenna with dual bands can be achieved. CPW fed planar monopole energizes the circular-disk shape DR antenna resulting in two distinct resonant modes. The resonant frequency of the CPW fed planar monopole is shifted to low frequency range due to the loading of circular-disk shape DR. The measurement resonant frequency of 2.52 GHz is at the ISM band. This resonant frequency of 2.52 GHz is due to the CPW fed printed strip monopole. The measurement resonant frequencies of 5.30 and 5.75 GHz are at the UNNI band. These two resonant frequencies of 5.30 and 5.75 GHz are due to the DR antenna. The return losses are -17, -22, and -29 dB at 2.52, 5.30 and 5.75 GHz, respectively. As seen from the measurement results, dual band hybrid antenna has a 10 dB return loss with bandwidth 697 MHz (2102-2799 MHz) and 625 MHz (5191-5816 MHz) at the ISM band and UNNI band, respectively. Alternatively, the antenna has a -10 dB S11 bandwidth of 28 % and 11 % at the ISM band and UNNI band, respectively. Although the dual band hybrid antenna is composed of high permittivity DR in our study. The bandwidth at the UNNI band due to the circular-disk shape DR antenna is similar to the typical value of 6 ~ 12% using conventional DR antennas with permittivity around 10.[3-8] Additionally, the achieved bandwidth is enough for many practical applications.

Fig. 2 The measurement and simulation return loss of the dual band hybrid antenna.

38

(a)

(b)

Fig. 3 The measured radiation pattern of the dual band hybrid antenna at resonant frequency of 2.52, 5.30 and 5.74 GHz in the (a) x-z plane and (b) y-z plane.

Fig. 4 The measured antenna gain of the dual band hybrid antenna.

The measured radiation pattern of the dual band hybrid antenna at resonant frequency of 2.52, 5.30 and 5.74 GHz in the x-z and y-z plane is shown in Fig. 3. The radiation patterns were measured within the elevation angle: -180o to 180o. The elevation angle θ is measured from the positive z axis, as seen in Fig. 1. The radiation patterns are observed to be stable across the return loss ≤ -10 dB bands. Both the radiation patterns in the x-z and y-z plane at resonant frequency of 2.52, 5.29 and 5.74 GHz are symmetrical about the broadside. As seen from the Fig. 3, the radiation patterns at 2.52 GHz are due to the CPW fed planar monopole. It is observed that the radiation patterns are similar to a half-wavelength dipole antenna in the two radiating planes. The radiating patterns in the x-z plane are near omni-directional when compared to the conventional dipole antenna due to the asymmetric circular disk DR loading on the CPW fed planar monopole antenna. As seen from the Fig. 3, the radiation patterns at 5.30 and 5.74 GHz due to the circular disk dielectric resonator are near omni-directional both in the x-z and y-z plane. With reference to the figure, the radiation patterns on the DR side are very similar to those of the previous configuration.[15-16] However, the radiation patterns on the substrate side are quite different. The front-to-back radiation ratio is near zero, which is smaller than that of the previous configuration. The difference is due to that a backlobe radiation is produced in our study whereas the previous one dose not. Because of ground on the back side of substrate was built in previous configuration. Large cross-polarization is observed in both x-z and y-z plane in our study. However, large cross-polarization can become an advantage for practical WLAN applications. The wave usually propagates with multiple reflections between the transmitter and receiver, especially in indoor applications. The measured dual band hybrid antenna gain in the broadside direction is shown in Fig. 4. As observed from Fig. 4, the peak gains are about 5.17 dBi at 2.7 GHz and 7.88 dBi at 5.77 GHz in the ISM band and UNNI band, respectively. The gain variations are 4.20 and 2.72 dBi in the ISM band and UNNI band, respectively.

IV. CONCLUSIONS Successful design of dual band hybrid DR antenna consists

of a CPW fed planar monopole and a circular-disk shape high permittivity dielectric resonator has been presented. The dual band hybrid antenna has a -10 dB S11 bandwidth of 28 % and 11 % in the ISM band and UNNI band, respectively. The radiation patterns in the ISM band are due to the CPW fed planar monopole. The radiation patterns in the UNNI band due to the circular-disk shape dielectric resonator are near omni-directional. Large cross-polarization is observed in both x-z and y-z plane in our study. The peak gains are about 5.17 dBi and 7.88 dBi in the ISM band and UNNI band, respectively.

39

ACKNOWLEDGMENT This work was supported by the National Science Council

of the Republic of China under Grant NSC 98-2622-E-262-003-CC3.

REFERENCES [1] S. A. Long, M. McAllister, L. C. Shen, “The Resonant Cylindrical

Dielectric Cavity Antenna,” IEEE Transactions on Antennas and Propagation, vol. 31, pp. 406-412, 1983.

[2] A. Petosa, A. Ittipiboon, Y. M. M. Antar, P. Bhartia, M. Cuchaci, “Recent Advances in Dielectric Resonator Antenna Technology,” IEEE Transactions on Antennas and Propagation, vol. 40, pp. 35-48, 1998.

[3] R. K. Mongia, A. Ittipiboon, M. Cuhaci, “Low Profile Dielectric Resonator Antennas Using a Very High Permittivity Material,” Electronics Letter, vol. 30, pp. 1362-1363, 1994.

[4] Roger A. Kranenburg, Stuart A. Long, “Coplanar Waveguide Excitation of Dielectric Resonator Antennas,” IEEE Transactions on Antennas and Propagation, vol. 39, pp. 119-122, 1991.

[5] J. T. H. ST. Martin, Y. M. M. Antar, A. A. Kishk, A. Ittipiboon, M. Cuhaci, “Dielectric Resonator Antenna Using Aperture Coupling,” Electronics Letter, vol. 26, pp. 2015-2016, 1990.

[6] R. A. Kranenburg, S. A. Long, “Microstrip Transmission Line Excitation of Dielectric Resonator Antennas,” Electronics Letters, vol. 24, pp. 1156-1157, 1988.

[7] K. M. Luk, M. T. Lee, K. W. Leung, E. K. N. Yung, “Technique for Improving Coupling Between Microstripline and Dielectric Resonator Antenna,” Electronics Letters, vol. 35, pp. 357-358, 1999.

[8] Y. X. Guo, K. M. Luk, “On Improving Coupling Between a Coplanar Waveguide Feed and a Dielectric Resonator Antenna,” IEEE Transactions on Antennas and Propagation, vol. 51, pp. 2144-2146, 2003.

[9] K. P. Esselle, T. S. Brid, “A Hybrid-Resonator Antenna: Experimental Results,” IEEE Trans Antenna and Propagation, vol. 53, pp. 870-871, 2005.

[10] K. W. Leung, K. M. Luk, K. Y. A. Lai, D. Lin, “Theory and Experiment of an Aperture-couped Hemispherical Dielectric Resonator Antenna,” IEEE Transactions on Antenna and Propagation, vol. 43, pp. 1192-1198, 1995.

[11] A. A. Kishk, “Wideband Dielectric Resonator Antenna in a Truncated Tetrahedron from Excited by a coaxial Probe,” IEEE Transactions on Antenna and Propagation, vol. 51, pp. 2907-2912, 2003.

[12] A. A. Kishk, B. Ahan, and D. Kajfez, “Broadband Stacked Dielectric Resonator Antenna,” Electronics Letter, vol. 25, pp. 1232-1233, 1989.

[13] M. Lapierre, Y. M. M. Antar, A. Ittipiboon, and A. Petosa, “Ultra Wideband Monopole/Dielectric Resonator Antenna,” IEEE Microwave Wireless Component Letter, vol. 15, pp. 7-9, 2005.

[14] G. P. Junker, A. A. Kishk, A. W. Glison, “Input Impedance of Aperture-coipled Dielectric Resonator Antennas,” IEEE Trans Antenna and Propagation, vol. 44, pp. 600-607, 1996.

[15] M. S. Al Salameh, Yahia M. M. Antar, Guy Seguim, “Coplanar-Waveguide-Fed Slot-Coupled Rectangular Dielectric Resonator Antennas,” IEEE Trans Antenna and Propagation, vol. 50, pp. 1415-1419, 2002.

[16] K .W. Leung, K. M. Chow, K. M. Luk, “Low-Profile High-Permittivity Dielectric Resonator Antenna Excited by a Disk-Loaded Coaxial Aperture,” IEEE Trans Antenna and Wireless Propagation Letters, vol. 2, pp. 212-214, 2003.

40