# A Circularly Polarized mmWave Dielectric-Resonator-Antenna Array for Off-Body Communications
###### Abstract  
This paper presents a novel 28 GHz circularly polarized rectangular dielectric-resonator antenna (DRA) array for millimeter-wave (mmWave) off-body applications. A feed network incorporating cross slots was utilized in the rectangular DRA design to realize circular polarization (CP). In terms of the free-space wavelength, \(\lambda_{\rm o}\), the DRA dimensions were (\(0.48\lambda_{\rm o}\times 0.48\lambda_{\rm o}\times 0.27\lambda_{\rm o}\)) at 28 GHz. The antenna array was simulated by incorporating dielectric layers with parameters that are equivalent to those of the human body at the desired frequency of 28 GHz for off-body communications. Moreover, the precise alignment and assembly of the DRA, which pose major difficulties at mmWave frequencies, were achieved by outlining the DRA positions on the ground plane using a three-dimensional (3D) printer. The array configuration was fabricated and measured with excellent performance, realizing a measured impedance bandwidth of 29% in conjunction with an axial-ration (AR) bandwidth of 13% and a broadside gain of 13.7 dBic at 28 GHz.  
millieter waves; antenna array; dielectric-resonator antenna; off-body communications; circular polarization +
Footnote †: journal: _Scienced Reviews of Applied Physics_  
0000-0002-3199-2230]M.T.T.  
A number of research studies have utilized mmWave DRA arrays. For example, a 2 \(\times\) 2 DRA array was fabricated using substrate-integrated technology with a gain of 12.7 dBic, together with impedance and CP bandwidths of 16.4% and 1.1%, respectively [5]. An aperture-coupled 2 \(\times\) 2 DRA array was reported in which each cylindrical DRA element operated in the HEM\({}_{118}\) resonance mode, which led to a gain of 11.43 dBi, as well as an impedance bandwidth of 26% [17]. In addition, a monolithic polymer-based DRA array was proposed that used four elements with a realized broadside gain of 10.5 dBi and stable radiation patterns at 60 GHz, along with an impedance bandwidth of 12% [18]. Another study proposed a 1 \(\times\) 8-element-grid dielectric resonator antenna array with a measured impedance bandwidth of 18.3% and a broadside gain of 12 dBi [19]. Moreover, a 2 \(\times\) 2 circularly polarized DRA array with an operating frequency of 30 GHz and a maximum gain of 9.5 dBic with impedance and axial-ratio bandwidths of 33% and 5%, respectively, was reported [20]. In order to easily align the individual array elements, a grooved and grounded superstrate was utilized for a 16-cylinder DRA array with a measured impedance bandwidth of 9.8% and a maximum gain of 15.68 dBi at 28.72 GHz [21]. Furthermore, it was demonstrated that when the DRA elements are joined using dielectric arms, for alignment purposes, the achieved impedance bandwidth and gain are 31% and 9.8 dBi, respectively [22]. From the literature, it can be noted that the highest reported gain of a mmWave DRA array is 15.68 dBi; this was achieved by using 16 linearly polarized DRA elements [21]. However, the increased number of elements is associated with a larger antenna size, as well as a complex feed network. Therefore, arrays of four circularly polarized DRA elements address these limitations, albeit with a lower gain [5; 17; 20]. As a result, a compact array with the advantages of higher gain and wider CP bandwidth would be beneficial and was not reported earlier at the considered frequency range.  
In this study, a compact rectangular DRA element that operates at higher-order resonance modes was utilized to design a 2 \(\times\) 2-element array. Compared to the research described above, the proposed DRA array offers the advantages of compact size, a wide impedance bandwidth of 29% combined with a wide AR bandwidth of 13% and an enhanced gain of 13.7 dBic at 28 GHz. Moreover, in order to overcome the misalignment issues, an automated process was followed to precisely outline the DRA positions on the utilized ground plane. The simulations were conducted using CST Microwave Studio, with close agreement between the measured and simulated results.