# Wideband Millimeter-Wave Perforated Hemispherical Dielectric Resonator Antenna
## 4 Conclusions  
This study presented a theoretical and practical exploration of a layered hemispherical DRA emitting circular polarization, which is stimulated by a spiral slot feed within the millimeter-wave frequency range. Results demonstrate that incorporating a dielectric coat enhances impedance and axial ratio bandwidths while also boosting gain. This improvement stems from exciting multiple resonance modes beyond the initial dual modes of a single-layer HDRA. Specifically, two-layer HDRA configurations exhibit increased  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ref.** & **Antenna Type** & **Solid Layers** & **Feeding Mechanism** & \(\mathbf{1}\mathbf{S}_{\mathbf{1}}\mathbf{1}\) **Bandwidth** & **Gain** & **AR Bandwidth** \\ \hline
[6] & HDRA & 3 & Cross Slot & 35.8\% & 9.5 dBi & - \\
[9] & HDRA & 2 & Annular Slot & 55\% & - & - \\
[10] & HDRA & 3 & Probe & 65\% & 3.5 dBi & - \\
[11] & HDRA & 3 & Waveguide & 9.6\% & 6.4 dBi & - \\
[12] & HDRA & 3 & Conformal strip & 34\% & - & 4.39\% \\
[13] & CDRA & 3 & Probe & 12.2\% & 4.69 dBi & 12.6\% \\
[14] & HDRA & 3 & Probe & 65.7\% & 2.3 dBi & - \\
[15] & HDRA & 3 & Probe & 29\% & 4.4 dBi & - \\
[16] & HDRA & 2 & Probe & 32\% & 2.5 dBi & - \\
[19] & CDRA & 1 & Cross Slot & 53.3\% & 13 dBic & 17\% \\ This work & HDRA & 1 & Spiral Slot & 58\% & 9.53 dBic & 17.7\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Performance comparison of multi-layered hemispherical DRAs.  
Figure 23: The simulated and measured radiation patterns of an HDRA with a perforated coat at 28 GHz. (**a**) \(\phi=0^{0}\), (**b**) \(\phi=90^{0}\).  
impedance and AR bandwidths of 57% and 18.15%, respectively, coupled with an enhanced gain of 7.5 dBic. Moreover, the successful construction and agreement between simulated and measured results of a two-solid layer HDRA prototype were achieved.  
Furthermore, it was demonstrated that an alumina HDRA with a perforated coat can be fabricated via MDF 3D printing, consolidating the two layers into a physical single layer. This streamlines automated assembly and bonding of the layered DRA, eliminating potential air gaps between dielectric layers. This engineered alumina structure, featuring a solid DRA core and a coat characterized by varied air-filling ratios, is designed to achieve the necessary effective dielectric constants. The implementation of an HDRA with a perforated dielectric coat revealed a significant correlation between the two configurations. Additionally, the 3D-printed model exhibited close agreement between simulated and measured results, with impedance bandwidths covering 59.5% and 58%, and CP bandwidths spanning 18% and 17.7%, respectively, within the frequency range of 24.74 to 29.66 GHz and 24.7 to 29.5 GHz. At 28 GHz, simulated and measured gains were 9.53 dBic and 8.6 dBic, respectively.  
Considering these attributes, the proposed antenna proves suitable for millimeter-wave satellite communication and radar systems. Furthermore, the superiority of the 3D-printed configuration over its two-solid layers counterpart becomes more pronounced at higher frequencies, such as 60 GHz and sub-terahertz, owing to automated bonding and assembly processes.  
W.A.: conceptional design, simulation, measurements, writing; R.A.: investigation and writing; Y.K.: expertise; S.K.K.: supervision and writing. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Data Availability Statement: Data are contained within the article.  
The authors would like to acknowledge the use of the National mmWave Facility.  
The authors declare no conflicts of interest.