# Higher-Order-Mode Triple Band Circularly Polarized Rectangular Dielectric Resonator Antenna
## 1 Introduction  
Over the past few decades, the dielectric resonator antenna (DRA) has been the center of attention among antenna researchers. This is due to the prominent features of DRA over the microstrip patch such as high radiation efficiency, high gain, less metallic loss, wide bandwidth, lightweight, and tunable far-field pattern due to several modes in DRA [1, 2, 3]. The DRAs can be designed in various shapes; however, basic shapes such as rectangular and cylindrical shapes are preferred because of their low cost and ease of fabrication. According to the forms, the rectangular DRA is more favored than cylindrical and hemispherical DRAs owing to flexibility in the option of the acceptable aspect ratio [2, 4] considering that it will affect the bandwidth of the DRA antenna. Additionally, the mode degeneracy existing in spherical DRA and cylindrical DRA can be avoided by using a proper rectangular DRA dimension. The mode degeneracy is disfavourable as it increases the cross-pol levels of an antenna and hence limits the antenna performance [5]. On the other hand, considerable attention had focused on the DRAs with circular polarization (CP) as they offered a better quality of signals, alleviate fading and multipath, insensitive to polarization mismatch, which makes them more suitable for wireless communication systems than the linearly polarized (LP) antenna [6, 7, 8, 9, 10, 3, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53,  
modifications on basic DR shapes by drilling and cutting the DRs [3; 6; 11], multi-dielectric layering [16], and a diode for the reconfigurable band [17]. A strip-fed excitation method to the rectangular DRA is proposed in [2], where adding a parasitic patch at the corner of the DRA structure resulted from a CP DRA. The antenna resonates at 3.4 GHz with 14% bandwidth and 3-dB AR bandwidth of 2.7%. Triband CP DR antenna was proposed in [3; 10], where a triple-band dual-sense circularly polarized hybrid DRA is submitted by a modified hexagonal DR is top-loaded by a square microstrip ring (SMR) in [3]. The antenna is fed by a vertical-tapered-strip and resonates at 1.9 GHz, 2.73 GHz, and 3.7 GHz, which exhibit left-handed circular polarization (LHCP) in the lower band. In contrast, right-handed circular polarization (RHCP) is obtained in the upper CP band. While authors in [10] proposed a top-loaded modified Alford loop CP DRA, where the CP occurs at 1.937, 2.45, and 3.51 GHz. On the other hand, a dual-band CP antenna is demonstrated in [6; 7; 12; 18; 19]. A singly fed dual-band CP rectangular DRA, where quasi-TE111 and -TE113 modes are excited to provide dual-frequency at 1.58 GHz and 2.44 GHz [6]. Two opposite corners of the DRA are removed at 45\({}^{\circ}\) with the diagonal groove on the top DRA surface to obtain CP fields. A dual-band CP DR antenna is proposed by a stacked rectangular dielectric resonator (DR) of three layers excited by an asymmetrical cross-slot [7]. As reported in [7], the lower CP band is formed by a quasi-TE111 mode, and the upper CP band is produced by quasi-TE113 and quasi-TE115 mode. Excited by a triangular ring-shaped aperture and an additional parasitic strip on the side of DRA, the LHCP and RHCP of the proposed antenna can be controlled by making a mirror image of the feed [18]. Incorporating a dielectric coating layer of different relative permittivity to the single DRA generates a higher-order-mode circularly polarized [16]. Compared to a single DRA layer, the double-layer DRA fed by a single microstrip feedline successfully enhanced the impedance and AR bandwidth, the far-field characteristic, as well as the antenna gain. There are various techniques that have been proposed recently in the antenna design, for example by introducing frequency selective surface (FSS) [20; 21; 22], near-field transformation [23] and electromagnetic band gap (EBG) structure [24].