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Thermal analysis of Dielectric Resonator Antennas (DRA) for 5G applications

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Introduction

The world is experiencing high advancement in telecommunication technologies especially within wireless communication systems. It is one of the most emerging and dynamic areas supporting the radical change in people’s lifestyle in terms of improved communication and business networks.

The development of cellular wireless technologies has involved into the 5th Generation through spending efforts to improve the bandwidth for a better long-distance communication. The aim of every wireless technology is to collect data which is not possible without antenna device. It is playing a basic role and can affect the whole system functions in terms of bandwidth, gain, efficiency…

Highly effective antenna designs (with high gain performance, increased thermal stability and high-level radiation efficiency) are needed for 5G applications. However, the performance of conventional microstrip patch and Vivaldi antennas is limited by their narrow bandwidth and high conductor loss level which may decrease their gain and efficiency. To overcome these issues, high gain Dielectric Resonator Antenna (DRA) technology was adopted due to its numerous advantages and attractive features: light weight, low cost, wider impedance bandwidth, high power and temperature handling, low conduction losses…

This study illustrates the HFWorks capabilities as an FEM-based tool to perform the electromagnetic and thermal behavior of a rectangular dielectric resonator antenna DRA for 5G applications. Two different analyses for radiation modes are presented based on the resonator dimension: Fundamental and high order mode.

The experimental prototypes used by the reference [1] are shown below:

Figure1 - The fabricated DRA prototypes used by the Ref [1]

Problem description

The basic structure of DRA mainly consists of a ceramic radiator, a substrate of dielectric material, conducting ground, and a metallic 50 $capital omega$ feed line. The studied DRA topologies operate at the frequency range of [13 GHz-17GHz] and the detailed dimensions for each mode are summarized by Table1.

3D CAD design of the a)-Fundamental and b)- high order mode of the used DRA and illustration of c)-top and d)-bottom view

Figure2 - 3D CAD design of the a)-Fundamental and b)- high order mode of the used DRA and illustration of c)-top and d)-bottom view

Table 1 - Dimensions of the studied antenna

Part	Dimension (mm)
Substrate	Height:20	Width:20		Length:0.25
Dielectric resonator -Fundamental mode	Height:7.5	Width:7.5		Length:1.8
Dielectric resonator -High order mode	Height:4	Width:4		Length:11.5
Feed line-Fundamental mode	Width: 0.79		Length:11.92
Feed line-High order mode	Width: 0.73		Length:11.92
Aperture slot-Fundamental mode	Width: 0.65		Length:3.34
Aperture slot-High order mode	Width:0.4		Length:3.34
Slot position from feed line-Fundamental mode	2
Slot position from feed line-High order mode	1.4

Table 2 - Material properties

Material	Relative permittivity	Dielectric loss tangent	Electrical conductivity (S/m)	Thermal conductivity (W/m. K)
Air	1.00058986	0	0	0.024
Arlon AR1000	10	0.0035	0	0.645
Epoxy FR4	2.2	0.001	0	0.36

Electromagnetic boundary conditions

Wave port: The wave port boundary is applied to the substrate input face next to the feeding line beginning.

Perfect Electric conductor: A PEC boundary condition is applied to the ground and feed line faces.

Radiation: A radiation boundary condition is applied to the outer air box faces.

Study 1: Excitation of fundamental mode

An initial analysis is investigating the first DRA topology for the Fundamental excited mode for the working frequency range of [13 GHz-17GHz]: the antenna simulation revealed the next results:

The first figure is showing the animated plot of the electric field inside the dielectric box versus phase for an excitation power set to 1Watt:

Electric field animation for 15GHz

Figure 3 - Electric field animation for 15GHz

The return loss 2D plot is superposed to the measurement results mentioned by the ref [1] to show a good agreement between them. The measured bandwidth for $vertical line S 11 vertical line less or equal than 10 d B$ achieved 1.7GHz.

Return loss 2D plot versus frequency

Figure 4 - Return loss 2D plot versus frequency

Since the new DRA topologies are mainly characterized by the use of dielectric resonators, the loss analysis of the studied DRA allows to show the volume loss mapping for the working frequency 15GHz. Dielectric losses are significant for a low power level like shown below:

Sectional views of the volume loss densities within a)-the dielectric resonator and b)-substrate parts for 15GHz

Figure 5 - Sectional views of the volume loss densities within a)-the dielectric resonator and b)-substrate parts for 15GHz

A steady state thermal analysis coupled to antenna study permitted to predict the temperature profile of the studied DRA example. A convection boundary condition is applied to the outer surrounding air at ambient temperature with a convection coefficient set to 10 W/m²K. the obtained results show the temperature distribution for the working frequency 15GHz. It achieves a maximum value of 31°C across the rectangular resonator box.

: Temperature distribution across a)- the whole model and b)- the dielectric substrate.

Figure 6 - Temperature distribution across a)- the whole model and b)- the dielectric substrate

Study 2: Excitation of High order mode

The second analysis is investigating the high order radiation mode for the second DRA topology. The same frequency range is used with keeping all mentioned electromagnetic boundary conditions in study1.

The next sectional vector plot results are illustrating the electric and magnetic field distribution across the resonator box.

: Sectional vector plot views of the a)- Electric and b)-Magnetic field inside the rectangular resonator

Figure 7 - Sectional vector plot views of the a)- Electric and b)-Magnetic field inside the rectangular resonator

The 2D plot of return loss of higher order DRA is illustrated below compared to measurement. The bandwidth of this second design is improved compared to the fundamental mode design as the height of the rectangular dielectric resonator is increased.

Return loss 2D plot versus frequency

Figure 8 - Return loss 2D plot versus frequency

Sectional views of the volume loss densities within a)-the dielectric resonator and b)-substrate parts for 15GHz

Figure 9 - Sectional views of the volume loss densities within a)-the dielectric resonator and b)-substrate parts for 15GHz

The simulated gain result compared to both experimental and Ref simulation results, is summarized by the next table. It shows a good compromise between HFWorks and experimentally measured results.

Table 3 - Comparative table of gain results

Results	Measurement	Simulation-HFWorks	Simulation-Ref [1]
Gain (dB)	9.76	9.65	9.95

Finally, the thermal coupling analysis is performed under the same thermal boundary conditions used by the first study. For an excitation power set to 1 Watt, the steady state temperature of the studied model achieves 35°C which is due to the dielectric losses induced by the used materials.

Temperature distribution across a)- the whole model and b)- the dielectric substrate

Figure 10 - Temperature distribution across a)- the whole model and b)- the dielectric substrate.

Conclusion

Dielectric resonator antenna has been widely employed for new 5G communication applications due to their: wide bandwidth, high radiation efficiency, increased gain and better temperature handling ability compared to conventional microstrip antenna.

HFWorks allowed to perform the electromagnetic and thermal behavior analysis of DRA for two radiation modes differentiated by the resonator dimensions. Further investigations could be done using HFWorks to include other DR shapes and thermal stress analysis of such device for upcoming works.

References

[1]- Shahadan, Nor Hidayu, et al. "Higher-order mode rectangular dielectric resonator antenna for 5G applications." Indonesian Journal of Electrical Engineering and Computer Science 5.3 (2017): 584-592.