SELTIQ_3D
- Dielectric AntennasSELTIQ_3D
- Dielectric Antennas
This section considers the validation of the program, SELTIQ_3D, in dealing with the predicted performance of structures and antennas containing dielectric. The examples include
This antenna has a fairly basic geometry but highlights some important aspects of the EFIE and CFIE formulations and the associated coding that need careful testing. In particular the modelling of wire-to-pec triangles and also the modelling of thin dielectrics, such as substrates are covered in this test case.
The compact-wideband monopole, as proposed by E. Lee et al (2) for use in mobile terminals, has a wire (coaxial) feed though a finite ground plane to a planar dielectric-loaded monopole positioned at various heights above a finite ground plane. Both the monopole and the finite ground plane are modelled with pec-triangles A wire shorting-pin between the edge of the planar monopole and the finite ground, just adds to the number of wire-to-pec triangle junctions associated with the problem.
The schematic of the antenna is shown in Figure 1. There are 4 wire-to-pec junctions, and these junctions have to be modelled well in order to model the characteristics of the antenna properly. The planar pec-surfaces of the monopole and the finite ground plane were meshed with a triangulated mesh similar to that shown in Figure 2, where, for clarity, the mesh of the ground plane has not been included.
Results were generated for various gap-heights of the monopole above the finite ground plane, both with and without the shorting pin. An example of the measured results (2) is given in Figure 3, which shows the return loss measured at the wire feed junction with the ground plane compared with the predictions using SELTIQ_3D.
 Figure 1. A schematic of the planar monopole over a finite ground plane. |
 Figure 2. Part of the triangulated surface mesh used to model the planar monopole, showing the wire-to-pec triangle junctions at the feed and the short-circuit pin. |
 Figure 3. A comparison of predicted (SELTIQ_3D) and measured (2) return loss of a planar square monopole of sides 40x40mm. |
A polyrod is a dielectric (originally polythene and hence the name) insert at the throat of a (usually) circular waveguide. The polyrod dielectric-loaded antenna is seen as a replacement for the corrugated horn antenna to offer a wideband compact feed.
There is an absence of simulated and measured results in the literature for such antenna. In this work the simulated results from SeltiQ_3D were compared to those from the finite element code HFSS (3)
An example of a small polyrod is shown in SELTIQ_3D mesh-form in Figure 4. The circular waveguide has an internal diameter of 12 mm, which flares to 14.5 mm at the throat of the waveguide. The dielectric rod has a permittivity of 2.2, fills the waveguide, and extends 40mm beyond the throat of the waveguide. In SeltiQ_3D a small dipole was placed 10.44 mm, which was ¼-wavelength at 11GHz, from the rear of the wavguide to excite the dominant TE11. The simulated guide wavelength was in good agreement with the analytic guide wavelength. Figure 5 shows a comparison of the E and H-plane radiation patterns at 10.5 GHz as computed by SeltiQ_3D and HFSS. Several different polyrods were simulated across a frequency range of 10 –12 GHz, the gains differed by at most 0.3 dB, but the patterns did show some minor differences, similar to those shown, around the sides and at the back where there was a consistent difference of some 2.5 dB.
To provide an additional test of the results from SeltiQ_3D, The radiation pattern in the H-plane was compare to that reported by Parkinson and Mehler (4) for and air filled open circular waveguide, which was excited with the dominate TE11 mode at 8.73 GHz, and this result is shown in Figure 6.
 Figure 4. The surface mesh of a polyrod antenna. |
 Figure 5. A comparison of the E- and H-plane radiation patterns computed by SeltiQ_3D and HFSS, for the polyrod in Figure 4. |
 Figure 6 Comparison between SeltiQ_3D and Parkinson and Mehler (4) of the radiation from an open-ended circular waveguide |
McCowen (5) previously reported the modelling of thin-dielectric substrates within SeltiQ_3D. In that work several coplanar stripline (CPS) tracks were modelled across the frequency range 1.5 – 12 GHz. At these frequencies, the dielectric substrate has a significant influence on the propagation characteristics of the CPS. Its parameters of thickness and permittivity affect both the characteristic impedance and the effective permittivity of the CPS. These parameters in turn have a direct influence on the wavelength of the electromagnetic propagation along the tracks, which in turn affects the electrical characteristics of microwave circuits. In (5), the tracks of the CPS were chosen to coincide with those fabricated and measured by Simons et al (6). The results of input impedance from SeltiQ_3D were shown to be in good agreement with those reported in (6).
The latter antenna is a good test problem as it has a complex geometry and is electrically quite large for a pc-based code and too large for HFSS (3). It also requires the modelling and excitation of circular waveguide, which needed validating in its own right as a separate problem.
REFERENCES
1. A McCowen and P R Foster, ‘The Application of SETIQ_3D to Dielectric-Loaded Antennas’, IEE Seminar on Software Validation, 29th March 2004
2. Lee, E., Hall, P.S. and Gardner, P., ‘Compact wideband monopole antenna’, Electronic Letters, Vol. 35, No. 25, 2157-8, 1999.
3. `Parametrics and Optimisation using ANSOFT HFSS’, Microwave Journal, November 1999
4. Parkinson, J.R. and Mehler, M.J., ‘Integral equation formulation of cylindrical feed-axisymmetric reflector coupling’, IEE Proceedings Vol 137,Pt. H, No.1,11-17, 1990
5. McCowen, A., ‘Efficient modelling of CPS and CPS-fed taperd slot antenna with a full-wave 3D moment-method analysis’, Int. Conf. on Antennas and Propagation (ICAP2003), 606-9.
6. Simons, R.N., Dib, N.I. and Katehi, L.B.P., ‘Modeling of Coplanar Stripline Discontinuities’, IEEE Trans. MTT-44 , 711-716, 1996.