This example illustrates how a probe-fed stacked annular ring antenna may be simulated in FEKO.  The structure has been analysed in [1], which provides easy validation for the FEKO results.  The basic model and dimensions are presented in Figure 1.

Figure 1:  Model of a microstrip patch in a cavity	Dimensions: (patch and cavity centred on origin)
	(a1, b1, d1) = (10, 29, 6.096) mm. εr1 = 2.2,  tan δ1 = 0.001. (a2, b2, d2) = (14, 31, 8) mm. εr2 = 1.07,  tan δ2 = 0.001. Feed location:  (xf, yf) = (21, 0) mm. Pin diameter:  0.325 mm.

Special Green's functions are ideally suited to modelling the dielectric layers as infinite planes.  The substrate layers were therefore set up as such, with an finite ground plane at the bottom of the stack.  FEKO allows the coupling of the feed pin to this infinite ground plane, which meant that the feed segment could be modelled as a standard wire segment with the required diemeter.  Figure 2 presents the simulated model, with dielectric layers and feed point visible.

Figure 2:  Special Green's function dielectric layers in the simulation model
(a) Bottom
(b) Side

The model shown in Figure 2 was used to compute the input input impedance of the antenna.  The computed result is presented in Figure 3 in comparison with the measurements and simulation data of [1].  The FEKO computation compares well with the published data, validating the FEKO model.

Figure 3:  Input impedance locus:  FEKO simulated data in comparison with published data [1]
(a) FEKO computation (b) Published measurement and computation