Wide-band stacked annular-ring dielectric resonator antenna

Introduction:

The FEM/MoM is often overlooked as an efficient technique for modelling dielectrics. This is even more true when the model consists of more than one dielectric layer. The traditional MoM method, the Surface Equivalence Principle (SEP) requires that the boundary of each dielectric be meshed into triangles. These triangles contribute a significant part to the computational resources. However, for the FEM/MoM only the outermost surface of the dielectric need to be meshed into triangles while the inside of the dielectric can be meshed into tetrahedrals.

The geometry of the antenna is shown below.

Fig. 1: Geometry of the wide-band stacked annular-ring dielectric resonator antenna [1]

 

 

The geometry is constructed in CADFEKO using mainly cylinders of different sizes subtracted from each other as well as circular ellipsoids. As the FEM/MoM is used, a dielectric "air" layer is created that surrounds the antenna. The permittivity of this dielectric is set to 1 and serves to reduce the computational resources as a result of the coarser surface triangles as compared to those of the dielectrics in the intended model.

A FEM modal source is used as the excitation. The end of a coaxial connector is constructed using dimensions consistent with an air dielectric in 50 Ohm coax.

 

The mesh size was set to Standard and the Refinement factor and Minimum element size were modified to obtain an accurate mesh representation of of the model.

The FEM modal source and frequency loop using continuous sampling is set in CADFEKO and the model is saved after meshing.  Magnetic symmetry is added on planes X=0 and Y=0.

The results for S11 for a FEM modal port  and reference [1] are given below. For comparison purposes, the results for the model with a FEM current source, are also included.

 

Fig. 3: Final FEM/MoM model (cut-plane view) and comparative results
(a) Geometry of FEM/MoM model in POSTFEKO (b) S11 results comparing the FEM/MoM with reference [1]

 SEP model:

As a matter of interest the SEP model is briefly discussed.

The following points are important for setting up the SEP model:

1. A waveguide port is used on the coaxial connector.
   
2. The surrounding air dielectric must be deleted as it only increases the computational resources for the SEP.

 

The final SEP model with a waveguide port is shown below. The comparative results between the SEP and reference [1] are also shown.

Fig. 5: Final SEP model (cut-plane view) and comparative results
(a) Geometry of SEP model in POSTFEKO (b) S11 results comparing the SEP with reference [1]

 

The computational resources for the FEM/MoM and SEP is compared in the table below. Results were obtained on a Intel(R) Core(TM)2 Quad CPU Q9550 @ 2.83 GHz with 8 GB RAM. Simulation was run parallel using three cores.

Table1: Comparing computational resources for the FEM/MoM vs SEP
Technique	Runtime	Memory
FEM/MoM (FEM current source)	10 minutes	189 MByte
FEM/MoM (FEM modal port)	11 minutes	183 MByte
SEP (wire port)	13 minutes	153 MByte
SEP (waveguide port)	12 minutes	165 MByte

It is seen that the FEM/MoM is equally efficient, yet equally accurate technique for solving this problem.

References:

[1] Wide-band Stacked Double Annular-Ring Dielectric Resonator Antenna at the End-Fire Mode Operation, Guo, Ruan, Shi, IEEE Trans. AP, vol. 53, no.10, Oct 2005

[2] Stacked Annular Ring Dielectric Resonator Antenna Excited by Axi-Symmetric Coaxial Probe, Shum, Luk, IEEE Trans. AP, Vol. 43, No. 8, August 1995