In this example XFdtd^® 6 is used to make calculations for an open-flanged waveguide with a specialized dielectric slab window material used to match the waveguide impedance to the impedance of a liquid located at the waveguide flange terminus.

The waveguide is of WR159 configuration with inside dimensions of 40mm by 20mm. The evaluation is for the frequency range (5 to 6) GHz. The length from the probe feed point to open flange end is 19cm. The waveguide is fed by a coaxial to waveguide transition with an input signal of 5.2GHz. The XFdtd Geometric Modeler was used to create all the waveguide components. The sides are created from four rectangular sections with an additional one for the reflective quarter-wave end plate. Two blocks define the dielectric slab and lossy dielectric used to model liquid material, and cylinders are used to construct the probe and feed line as shown in XFdtd in Figure 1. The geometry is then meshed in XFdtd and a feed port is specified. The feed port is the small bright green dot immersed in the green dielectric insulation at the base of the cylindrical coax-waveguide adapter shown in XFdtd mesh view in Figure 2.

After the calculation is complete the S11 scattering parameters are calculated and the UWB impulse response results are graphed in XFdtd in the frequency domain as viewed in Figure 3. XFdtd shows the expected result of a nearly 50 Ohm input impedance over the design frequency range. The field displays illustrate the physics of the TE10 mode excitation of field patterns in Figure 4. The EM fields can also be viewed as a full 3Dimension solid in Figure 5. The display of the lossy liquid material beyond the matching slab at the left is turned off to allow better viewing of the fields inside the liquid region. Steady state electric field X, Y, and Z component magnitudes in the principal XY plane of the waveguide are shown in Figures 6, 7, and 8. This WR159 waveguide analysis provided by Daniel Brooks of APREL Laboratories http://www.aprel.com.