2.2.3 Septum polariser
Let us consider the analysis of a septum polariser. Its typical shape is presented in Fig. 2.2.3-1. Since some of the readers may not be familiar with the principle of its operation we will first provide some brief explanation in this regard.
Fig. 2.2.3-1 A general view of a septum polariser.
On one end of the polariser we have two rectangular waveguides piled one above the other. To allow their separate connection with other circuits they are often bent to the right and left, respectively. In our case the lower (and bent to the right) waveguide is the input. The incident wave is vertically polarised. In the main body of the polariser the guides are separated by a septum, which later opens in a stepped way (see red part in Fig. 2.2.3-1) into a square waveguide output. Due to the presence of the septum and its shape about half of the energy is transformed to the horizontal polarisation. In a properly designed polariser the output phase shift between vertical and horizontal polarisations is close to 90°, and thus the output wave is circularly polarised. Depending on the choice of the input (lower or upper guide) we obtain right-hand or left-hand polarisation. Thus we have a structure, which is geometrically a three-port, but electrically a four-port since at the square waveguide output we need to consider separately the vertical and the horizontal polarisation. In our simulation of the polariser we will consider only its main part (without the bent input lines) as presented in Fig. 2.2.3-2. We can see there four ports with arrows indicating the polarisation assigned to it them. Port 1 marked in red is the input. Ports 2,3 and 4 are outputs, with ports 3 and 4 defined in the same square waveguide.
The polariser project has been stored in the file ..PassiveComponents\Septpol\septpol1.pro. It has been prepared using ..\examples\sept3.udo. This UDO allows changing various parameters of the polariser, for example six dimensions of the septum and a depth of the correction ridge as indicated in Fig. 2.2.3-2. Note that all the dimensions of this project are expressed in inches, as indicated by the Units dialogue.
Fig. 2.2.3-2 The structure of septum polariser considered in septpol.pro and the meaning of parameters used in sept3.udo. The numbers 1..4 indicate ports defined for S-parameter calculation and the arrows associated with them show their polarisations.
A look at the structure presented in 2D and 3D Windows after loading the project to QW-Editor as well as on the list of elements invoked by Select Element confirms that four ports have been defined. Ports 3 and 4 have the same size and are situated in the same place. They also have similar parameters defined in Edit Transmission Line Port dialogue, with one exception. In this dialogue window we can see in Template mode search details of port 3: Excitation with Ez field component and of port 4: Excitation with Ey field component. As a result the software will generate the vertically polarised mode template TE01 for port 3 and the horizontally polarised mode template TE10 for port 4. At the time of S-parameters extraction the software will calculate vector products of the fields present in the square waveguide output with each of the templates. This will allow assigning a proper part of the output energy to each of the considered virtual ports (3 and 4). The above discussion underlines that the correct mode template needs to be generated for each of the transmission line ports, regardless whether it has been declared as input or output. More discussion of the port templates will be given in the subsequent Sections of this Chapter.
Fig. 2.2.3-3 Results of analysis of S-parameters of septpol1.pro in 10-15 GHz band.
Please run the septpol1.pro with the post-processing set for extracting S-parameters from 10 GHz to 15 GHz. After about 3000-4000 of iterations open the Results window (
or 
buttons in Results tab). The results of S-parameters calculations stabilise in a form presented in Fig. 2.2.3-3. In a band around 12 GHz we can see that transmission to the ports 3 and 4 is at a similar level and that we have good matching of port 1 and good isolation between ports 1 and 2. Let us note that in the case of the polariser we are most interested in the ratio of |S41|/|S31| and in the phase difference arg(S31)-arg(S41). Both can be conveniently obtained using the calculator available in Config dialogue. For example, we highlight the characteristics of |S31| and |S41| and press “ / “ in the field Calculate new data. A new characteristic marked (|S41|/|S31|) appears on the list. It can be displayed as any other characteristics. Both (|S41|/|S31|) and (arg(S31)-arg(S41)) are shown in expanded scale in Fig. 2.2.3‑4 and they indicate that our polariser has very good properties in the band between 11.5 GHz and 12.5 GHz.
Note that when we display the combination of the original characteristics, the field needed for their description is much wider. Thus the cursor pane (i.e., the right margin of Results window ) may become too narrow and the value corresponding to the current cursor position may be cut from the display. To make it visible, try to modify the dimensions of the cursor pane and the chart pane. Using the mouse we can catch and move the boundary between the two panes.
Fig. 2.2.3-4 Results of calculation of |S41|/|S31| and arg(S31)-arg(S41).