Introduction
The dielectric resonator antenna (DRA) was shown to be an efficient radiator that offers a compact size, high radiation efficiency, and wide impedance bandwidth. Moreover, it provides several design degrees of freedom, which enhances its performance and enables it to serve as an multifunctional device. For instance, the dimensions of the DRA could be tuned to excite multiple modes to create amultiple-input multiple-output (MIMO) antenna [Reference Roslan, Kamarudin, Khalily and Jamaluddin1,Reference Abdalrazik, El-Hameed and Abdel-Rahman2]. Also, the DRA has been used to generate circularly polarized wave by exciting two orthogonal modes inside it [Reference Chen and Wong3,Reference Zou and Fumeaux4]. Furthermore, the DRA could serve as a frequency-reconfigurable antenna where switches are used to select multiple frequency bands [Reference Danesh, Rahim, Abedian and Hamid5]. In the aforementioned work, the authors proposed four identical rectangular dielectric resonator blocks with a permittivity of 10. PIN diode switches are integrated into the feed lines between the dielectric resonator blocks to generate four operating bands, where different modes are excited inside the DRAs to generate the four operating bands.
Moreover, generating multiple modes inside DRAs are used to create antennas with polarization diversity patterns [Reference Li and Leung6–Reference Sun and Leung9]. In [Reference Li and Leung6], cylindrical DRA with a top-loaded Alford loop is investigated. Two ports are utilized to excite two different modes inside the DRA, such that the two modes radiate like vertical and horizontal magnetic dipoles. As the electric field polarizations of the two modes are perpendicular to each other, the two ports exciting these two modes are able to receive signals independently, and considered as two diversity branches. In [Reference Chen, Shoaib, Yao, Yu, Chen and Parini7], another DRA structure was proposed, such that orthogonal modes were excited in a single dielectric resonator structure. The feeding network of the antenna has been designed such that two orthogonal modes 
$TE^x_{111}$ and 
$TE^y_{111}$ are excited through the two ports of the antenna, which are considered as receive diversity branches.
The DRA was also shown to be able to create recieve/transmit diversity patterns in [Reference Zhong, Hong and Zhou10], where a cylindrical DRA with switchable beams was proposed. An annular slot was placed underneath the DRA, with eight switches soldered across it. By turning on one of the eight PIN diodes, the proposed antenna can rotate the beam to the opposite direction of the switch. The bandwidth of the antenna is 3.64
$\percnt $. Other several pattern diversity antennas were also studied in the literature [Reference Gu, Gao, Liu, Luo, Loh, Sobhy, Li, Wei, Xu, Qin and Sanz-Izquierdo11–Reference Sharma, Sarkar, Saurav and Srivastava16]. In [Reference Gu, Gao, Liu, Luo, Loh, Sobhy, Li, Wei, Xu, Qin and Sanz-Izquierdo11], a hybrid antenna with switchable diversity patterns was proposed. The hybrid antenna consists of several parasitic elements that are closely-placed around a driven antenna, where each parasitic element is composed of a printed dipole with a PIN diode. Using different combinations of PIN diode ON/OFF states, the radiation pattern can be switched toward different directions. The polarization of the antenna is controlled by changing the feeding point of the antenna. The bandwidths of the antenna ports are 0.06, 0.08, and 0.04
$\percnt $. In [Reference Masood, Person and Sauleau12], two patch antennas were proposed to radiate multiple patterns as desired for both on- and off-body applications. An omni-azimuthal radiation pattern directed along the body and a broadside radiation directed away from the body respectively. This was achieved by exciting the fundamental TM 11 mode ina classical circular patch antenna for off-body radiation and the TM 01 mode in another circular patch antenna. However, the antenna fractional BW was only 0.036 and 0.06
$\percnt $. In [Reference Sharma, Sarkar, Saurav and Srivastava16], a MIMO system that consists of a chamfered edge square patch antenna with an offset feed and two printed dipole antennas, was proposed. Three diversity patterns were generated with less than −18 dB coupling. The antenna fractional BW was only 16.2, 16.2, and 4.14
$\percnt $.
In this paper, a reduced sized DRA that generates two switchable diversity patterns is proposed. The two modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ are excited through ground slots, where the electric field distributions of the two modes are symmetrical over the xz plane. Thus, a perfect electric conductor is used to reduce the antenna size to half. PIN diodes are placed over the slots so as to allow only one slot to generate a mode inside the DRA, which simplifies the input structure of the antenna. The envelope correlation coefficient (ECC) is found to be 0.4 which is an acceptable value [Reference Vaughan and Andersen17]. Compared to previous publications [Reference Gu, Gao, Liu, Luo, Loh, Sobhy, Li, Wei, Xu, Qin and Sanz-Izquierdo11–Reference Sharma, Sarkar, Saurav and Srivastava16], this simple and novel design consists of only a single element DRA, and is fed by only one port while being able to create two diversity patterns. Moreover, the size of the proposed DRA antenna with switchable patterns is reduced to half of its original size through placing a copper sheet over the DRA. Furthermore, the antenna achieves wide fractional bandwidths of 17.1, and 13.8
$\percnt $ in cases of exciting 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ modes, respectively. The antenna also attains a high gain of 7.1 and 3.2 dB at the center frequency although its size is reduced to half of the original size.
The proposed DRA design
Antenna design
The antenna consists of a rectangular DRA with dimensions of 7.2 mm × 8.1 mm × 1.9 mm as shown in Fig. 1. Linear and ring slots are placed on the ground plane to generate two switchable modes inside the DRA. Also, PIN diodes are placed over the slots so as to allow only one of the slots to couple power inside the DRA. The DRA material is Rogers 3010 with εr = 10.1. The feeding structure is a microstrip antenna with dielectric made from Rogers 5880 with εr = 2.2. As will be explained in the next subsection, the symmetry of the antenna enables to place a copper sheet upon one side of the DRA, which serves as a mirror for internal fields of the DRA, and reduces the antenna size to half, as shown in Fig. 2.
Fig. 1. Antenna geometry of the proposed DRA.
Fig. 2. Proposed antenna's size is reduced to half. Two groups of diodes G1 and G2 are placed over the slots. The space between each two diodes is 0.5 mm.
Modes excitation analysis
By choosing appropriate dimensions for the proposed DRA, the two modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ could be excited inside it. Based on the waveguide model of the DRA [Reference Aldo18], a resonance frequency of an TE x mode is related to the DRA's dimensions according to the following transcendental equation
$$k_x \tan\lpar k_x d/2\rpar =\sqrt{\lpar \epsilon-1\rpar k^{2}_0 -k^{2}_x}$$where 
$k^{2}_0=\lpar k^{2}_x+k^{2}_y+k^{2}_z\rpar /\epsilon $, k 0 = 2πf 0/c, k x = δπ/d, and k y = nπ/w, k z = pπ/b; f 0 is the mode resonance frequency and c is the speed of light; d, w and b are the DRA dimensions in the x, y, and z directions, respectively; n and p are positive integers, and δ is a positive fraction of unity.
Thus, a DRA with dimensions of 7.2 mm × 8.1 mm × 1.9 mm generates the two modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ with resonance frequencies of 19 and 22 GHz, respectively. To verify this, the previous transcendental equation (1) is rewritten as
$$D_{TE^x}\lpar f\rpar =k_x \tan\lpar k_x d/2\rpar -\sqrt{\lpar \epsilon-1\rpar k^{2}_0 -k^{2}_x}$$where by plotting the 
$D_{TE^x}\lpar f\rpar $ function as shown in Fig. 3, every zero-crossing represents a resonance frequency for a generated mode. Thus, the modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ are obtained at frequencies of 19 and 22 GHz, respectively. Furthermore, the proposed antenna structure is simulated using CST Microwave Studio commercial software, where simulated and theoretical electric field distributions for modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ at frequencies of 19and 22 GHz are shown in Fig. 4. It is noted that the simulated electric field distributions (Figs. 4(a) and 4(c)) are very close to the theoretical field distributions of modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ (Figs. 4(b) and 4(d)). Also, it is noted that simulated modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ at frequencies of 19 and 22 GHz, respectively, agree with the theoretical resonance frequencies obtained from Fig. 3.
Fig. 3. Plot of D function. The modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ are obtained at frequencies of 19 and 22 GHz.
Fig. 4. (a) Simulated electric field distribution of 
$TE_{\delta 11}^x$ in the yz cross section of the DRA at a frequency of 19 GHz. (b) Theoretical electric field distribution of 
$TE_{\delta 11}^x$ in the yz cross section of the DRA, (c) simulated electric field distribution of 
$TE_{\delta 12}^x$ in the yz cross section of the DRA at a frequency of 22 GHz, and (d) theoretical electric field distribution of 
$TE_{\delta 12}^x$ in the yz cross section of the DRA.
Interestingly, it is seen from Fig. 4 that the electric field vectors of both modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ are normal to the plane AA′. Thus, a copper sheet can be placed over the plane AA′, while field distributions of both modes are not effected. This copper sheet serves as a mirror for fields generated inside the DRA, and keeps the resonance frequencies of the modes unchanged, while reducing the antenna size to half as shown in Fig. 2.
Two groups of slots are etched into the ground plane to couple power from the microstrip antenna to the DRA. The first group consists of linear slots that generate linear equivalent magnetic current, as shown in Fig. 5(b); and the second group consists of a circular slot that generates circular equivalent magnetic current as shown in Fig. 5(d). PIN diodes are placed over the slots of the antenna, where the groups of PIN diodes over the linear slot are referred to as G1, and those over the ring slot are referred to as G2.
Fig. 5. (a) Simulated magnetic field distribution of 
$TE_{\delta 11}^x$ in the ground plane at a frequency of 19 GHz, (b) equivalent magnetic current excited in linear slots when G2 are turned off, (c) simulated magnetic field distribution of 
$TE_{\delta 12}^x$ in the ground plane at a frequency 22 GHz, and (d) equivalent magnetic current excited in circular slots when G1 are turned off.
The magnetic field distributions for the modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ are shown in Figs. 5(a) and 5(c), respectively. When G1 diodes are turned off and G2 diodes are turned on, magnetic currents are excited through the linear slots and couple power to the DRA (Fig. 5(b)), which generates the mode 
$TE_{\delta 11}^x$. On the other hand, when G2 diodes are turned off and G1 diodes are turned on, magnetic currents are excited through the ring slot and couple power to the DRA (Fig. 5(d)), which generates the mode 
$TE_{\delta 12}^x$. This technique simplifies the feeding structure of the antenna since only one feeding line is needed to excite the antenna. The two modes excited by these slot groups generate two switchable patterns with low ECC as it will be shown in the next section, which creates diversity patterns for the proposed antenna.
Furthermore, the stub length affects the matching of the antenna in the two cases of switch states. S 11 of the proposed antenna for the two cases of the switch states is shown in Fig. 6. The simulation is done using CST Microwave Studio commercial software. Clearly, the optimum value for the stub length is 2.4 mm when G2 is turned on, while the optimum value for the stub length is 1.4 mm when G1 is turned on. A PIN diode is placed over the stub to switch its length between 2.4 and 1.4 mm.
Fig. 6. Effect of stub length changes on the S 11 value of the proposed antenna for the two cases where G1 and G2 are off.
Simulation and experimental results
The proposed antenna is simulated and fabricated as shown Fig. 7; measurements are taken using a Vector Network Analyzer. Simulated and measured S-parameters of the proposed antenna are shown in Fig. 8. The simulated bandwidths are 17.6–20.9 GHz (17.1
$\percnt $), and 18.3–21.6 GHz (13.8
$\percnt $) in cases of turning off G1 diodes and G2 diodes, respectively. The measured bandwidths are 18–20.3 GHz (12
$\percnt $) and 18.8–22.2 GHz (16.6
$\percnt $) in cases of turning off G1 diodes and G2 diodes, respectively. Thus, the proposed antenna attains very good bandwidth compared to previous designs, as shown in Table 1. Also, the two patterns generated by the antenna are shown in Fig. 9, where the first pattern is generated when G1 diodes are turned off and the second pattern is generated when G2 diodes are turned off. It is noted that the gains are 7.1 and 3.2 dB, respectively, and both beam patterns are in the broadside direction. Moreover, the gain over frequency is shown in Fig. 10. It is noted that the gain ranges from 7 to 3 dB over the operating band.
Fig. 7. Prototype of the proposed antenna.
Fig. 8. S 11 value for the proposed antenna at the optimized lengths.
Fig. 9. Cut planes of the power patterns of the proposed antennas for the two cases where (a) G1 diodes are off and (b) G2 didoes are off.
Fig. 10. Gain frequency response plot for the proposed antenna.
Table 1. Comparison of fractional bandwidth for the proposed antenna and previous designs.
Finally, to calculate the correlation coefficient between the two patterns, we use [Reference Sharma, Sarkar, Saurav and Srivastava16]
$$\rho={\int_v \beta^1.\beta^2 d{v}\over \sqrt{ \int_v \beta^1 d{v} \int_v \beta^2 d{v} } }$$where βi is the ith power pattern. The value of ρ is 0.4 which is less than the maximum acceptable limit of 0.7 [Reference Vaughan and Andersen17].
Conclusion
A wideband DRA with switchable diversity patterns is proposed in this paper. Ring- and linear-shaped slots are placed on the ground to generate two modes 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ inside the DRA. Also, PIN diodes are placed over these slots such that only one slot couples power to the DRA, and the corresponding pattern is generated. The ECC between the two switchable patterns is calculated and found to be low. The simulated bandwidths are 17.6–20.9 GHz (17.1
$\percnt $) and 18.3–21.6 GHz (13.8
$\percnt $) in cases of exciting 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ modes, respectively. The measured bandwidths are 18–20.3 GHz (12
$\percnt $) and 18.8–22.2 GHz (16.6
$\percnt $) in cases of exciting 
$TE_{\delta 11}^x$ and 
$TE_{\delta 12}^x$ modes, respectively. Moreover, compared to previous publications [Reference Gu, Gao, Liu, Luo, Loh, Sobhy, Li, Wei, Xu, Qin and Sanz-Izquierdo11–Reference Sharma, Sarkar, Saurav and Srivastava16], this simple and novel design consists of only a single element DRA, and is fed by only one port while being able to create two diversity patterns. Furthermore, the antenna size is reduced to half by placing a perfect electric conductor over a symmetry plane of the antenna structure. This reduction in the DRA size saves space that could be utilized in a future design to add a second reduced-sized DRA on the same substrate, which forms an antenna array. Also, the antenna attainsa high gain of 7.1 and 3.2 dB at the center frequency, despite its size reduction to half of the original size.
Ahmad Abdalrazik received his B.Sc. degree in Electrical Engineering from Port Said University, Egypt in 2012. He received his M.Sc. degree from the same university in 2016. Currently, he is a Ph.D. student at Egypt-Japan University of Science and Technology (E-JUST), Egypt since 2017. His research interests include antenna design, microwave devices, wave propagation modeling, metamaterial, and optimization.
Adel B. Abdel-Rahman is currently a Professor at the Department of Electronics and Communications Engineering, Egypt-Japan University of Science and Technology, Alexandria, Egypt. He received his B.S. and M.S. in Electrical Engineering, Communication, and Electronics from Assiut University, Egypt, and his Dr.-Ing. degree in Communication Engineering from Otto von Guericke University, Germany in 2005. Since October 2006, he has been an Assistant Professor at the Electrical Engineering Department, South Valley University, Qena, Egypt. He has published more than 120 refereed journal and conference papers and has two patents. He was the Executive Director for Information and Communication Technology, South Valley University, from 2010-2012. Since October 2012, he joined the School of Electronics, Communications and Computer Engineering, Egypt-Japan University of Science and Technology (E-JUST), Alexandria, Egypt, and has been the Dean of the Faculty of Computers and Information, South Valley University from 2016-2018. His research interests include the design and analysis of antennas, filters, millimeter-wave devices, WPT, and metamaterials and their application in wireless communication, as well as optimization techniques with applications to microwave devices and antenna arrays.
Ahmed Allam is currently an Associate Professor at the Department of Electronics and Communications Engineering, Egypt-Japan University of Science and Technology, Alexandria, Egypt. He received his B.Sc. in Electrical Engineering from Alexandria University, Egypt, and his M.Eng. and Ph.D. from the University of Alberta, Canada. From April 1994 to January 1998, he worked as an instrument engineer with Schlumberger. From May 2000 to September 2001, he was with Murandi Communications Ltd., Calgary, Alberta, where he worked on RF transceivers design. From April 2007 to April 2008, he worked on RF CMOS transceivers design at Scanimetrics Inc., Edmonton, Alberta. His research interests include the design of RF circuits and systems.
Mohammed Abo-Zahhad received the B.Sc. and M.Sc. degrees in electrical engineering from the University of Assiut, Egypt, in 1979 and 1983, respectively, and the Ph.D. degree from the University of Kent, Canterbury, U.K., and Assiut University (channel system), in 1988. He was a member of the European Society of Circuit Theory and Applications in 1998, a member of the National Communication and Electronics Promotion Committee, and a Reviewer of the National Quality Assurance and Accreditation Authority, NAQQA, Egypt, since 2011. Since 1999, he has been a Professor of electronics and communication engineering with Assiut University. He is currently the Dean of the School of Electronics, Communication and Computer Engineering and has been a Professor of communication and electronics engineering with the Egypt-Japan University of Science and Technology (E-JUST) since 2017. He is also the General Director of the E-JUST Information and Communication Technology Centre. His research interests include biomedical and genomic signal processing, speech processing, optical and digital filters, switched-capacitor, data compression, wavelet-transforms, genetic algorithms, immune algorithms, wireless sensor networks, microwave, millimeter wave wireless communications, energy harvesting, and electronic circuits. He was a recipient of the Encouragement State Award in Engineering, from the Egyptian Research and Technology Academy, Ministry of Higher Education, Egypt, in 2005..
