10-09-2016, 11:53 AM
1454393043-antennahfsstut.doc (Size: 188.5 KB / Downloads: 5)
This exercise assumes that you already have some experience with Ansoft HFSS. Therefore, not every button click will be described in detail.
1. Introduction
In this exercise, we will concentrate on antenna post processing. Specifically, we will have a look at axial ratio and polarization ratio, and get familiar with the calculator. The very simple model we are going to build has two important advantages. First, due to its simplicity we will spend little time drawing and setting up the model. Further, once it’s solved, we will be able to create any linearly, circularly and elliptically polarized wave by playing with the amplitudes and phases of the two modes in the port, and we will have a good feeling for the kind of results we can expect.
The model we are going to build is presented in figure 1. It is a square open-ended waveguide radiating into air in the z direction. The port is at the bottom of the waveguide. We will solve for two modes, one with the electric field polarized in the x direction and one with the electric field polarized in the y direction.
2. Draw the model
Choose millimeters as the unit of length.
Create a box with its base point in (-10, -10, 0) and a size of (20, 20, 10). This will be the waveguide. Name it waveguide.
Create another box with its base point in (-18, -18, 3) and a size of (36, 36, 19). This will be the air into which the open-ended waveguide radiates. Name it airbox.
About the choices we made here: we intend to run this model at 10 GHz. At that frequency, the free-space wavelength is 30 mm. The outer faces of the airbox, where we will apply a radiation boundary, are all at least a quarter-wavelength away from the aperture of the waveguide. The top face of the airbox is even 40% of a wavelength away. The port is a third of a wavelength away from the aperture, which is usually a distance at which non-propagating higher-order modes that may be generated in the aperture will have died out. One doesn’t want those non-propagating higher-order modes to reach the port and decrease the accuracy. A good estimate of the safe-distance of the port involves the propagation constant of the higher-order modes. We’re not getting into that here.
At this point, we have two objects that are partially overlapping. To get rid of the overlap, the waveguide needs to be subtracted from the airbox. First, select the waveguide (Edit/Select or SEL icon). Then, Edit/Duplicate the waveguide in order to have a copy that can be used (and be lost) in the subtraction process. Third, choose Solid/Subtract. When prompted, select the airbox first, click OK, then select the copy of the waveguide and click OK. The resulting model looks like what you had before, but now the airbox has a hole in it that is exactly large enough for the waveguide to stick into.
This completes the drawing process. Exit from the Draw menu, Save, and return to the Executive Commands window.
3. Setup Materials
Assign air to both the waveguide and the airbox. Exit from the Setup Materials menu, Save, and return to the Executive Commands window.
4. Setup Boundaries and Sources
4.1 Port Definition
We start with the port. An illustration is provided in figure 2. Rotate the picture of the model (CTRL key in combination with the left mouse button) to view the model from the bottom. Select the bottom face of the waveguide. Make sure Source and Port are selected. Enter 2 for the number of modes. Don’t click the Assign button yet.
Let’s define an impedance line. An impedance line is used by HFSS to compute a voltage in the port, which is needed in two of the three impedance definitions. Impedance can be calculated from voltage and current, from voltage and power, and from power and current. Without an impedance line, only power and current are available.
With mode 1 highlighted, check the box Use Impedance Line. In the upper-left part of the window, below the coordinates, change the snap-to mode to other only (not vertex and not grid) and pick edge center from the options that pop up. OK to confirm.
Back at the impedance line, click Edit Line / Set. We want a line along the x axis from one edge center to the opposite edge center. Try to snap to the center of one appropriate edge. If you have to try it more than once, don’t click too fast. Keep an eye on the coordinates that are selected (upper left). When you get the correct ones, click the Enter button below Set Impedance Start. Then try to snap to the point on the opposite edge. When you see the correct vector, click Enter under Vector Length. You have now defined an impedance line.
Next, let’s define a calibration line. A calibration lines removes a potential 180-degree phase ambiguity by telling the software how the E field is directed at phase zero. Check the box in front of Use Calibration Line. Choose Edit Line / Copy Impedance.
Finally, check the box under Polarize E Field. This is an important step in this model. It makes sure that mode 1 is polarized the way we want it: along the lines we just defined.
Set polarization and impedance lines for mode 2 as well, this time along the y axis.
Click Assign to complete the definition of Port 1.
4.2 Boundary Conditions
We want to assign a radiation boundary to the outer surfaces of the airbox. Change “Graphical Pick” to “Object” (left part of window) and select the airbox by clicking it with the mouse. Make sure “Boundary” and “Radiation” are selected and give the new boundary a name, e.g. abc for absorbing boundary condition. Click Assign.
Next, we want to make the walls of the waveguide perfectly conducting. The quickest way to do that, with “Graphical Pick / Object” still selected, is to select the entire waveguide object with the mouse. Make sure “Boundary” and “Perfect_E” are selected. Give the new boundary a name, e.g. “walls”. Click Assign. A warning pops up because part of the waveguide is “port” already. That’s fine. A port won’t be overwritten.
At this moment, the aperture of the waveguide has a Perfect_E boundary condition as well. We will overwrite that with Perfect_H / Natural. For a change, select the appropriate face through the menu. Click Edit / Select / By Name. In the next window, make sure “Face” is checked and select the object “waveguide”. Find out what the correct face is, select it and click Done. Make sure “Boundary” and “Perfect_H / Natural” are selected and give the new boundary a name, e.g. aperture. Click Assign. A warning pops up. We’re okay here, since we are assigning boundaries in the correct order.
Choose Model / Boundary Display to check the port and the boundaries. When you’re satisfied, Close the Boundary Display and File / Exit the 3D Boundary / Source Manager. Save your work.
5. Setup Solution
We will first perform a quick “Ports Only” solution to check the modes in the port. In the Setup Solution Menu, select a frequency of 10 GHz. Check the Single Frequency box and uncheck the Adaptive box. Make sure the Sweep box is unchecked. The starting mesh will be the initial mesh. Under “Solve”, check “Ports Only”. Under “Port Solution”, make sure “All Modes” has been selected. The default ports field accuracy of 2% is adequate.
Click OK and hit the Solve button.
Once the solution process is complete, probably in less than a minute, click on Setup Executive Parameters / Port Impedances. The “3D Boundary / Source Manager” window comes up. In that window, zoom in on the port. Do this by using the Ctrl key, the Shift key and the left mouse button simultaneously. If necessary, use Shift+mouse to shift and Ctrl+mouse to rotate as well. Then, on the left-hand side, select Port1 by clicking on it in the list. Notice the arrow plot that shows up right away. It represents the E fields of mode 1. By selecting mode 2 from the port setup info below the plot, you get the arrow plot for mode 2. There are limited plot options under the button “Port Fields”.
The “Ports Only” solution is very useful. It allows you to check the modes in the port before doing the time-consuming 3D field solution. File / Exit from the Boundary / Source Manager. Click on Matrix (top of HFSS Executive Commands window). That allows you to inspect port impedances and propagation constants as well for the two modes. Note that the propagation constants are equal, except for a difference caused by discretization. This means that modes 1 and 2 are degenerate modes. If one doesn’t set a polarization line, one is likely to end up with linear combinations of these degenerate modes. It is important to be aware of that.
Now it’s time to setup the solution parameters for the full 3D field simulation.
In the Setup Solution menu, check “All” under “Solve”. “Ports Only” should not be checked. Check “Single Frequency” and check “Adaptive”. Select a frequency of 10 GHz. Request 3 adaptive passes, 20% tet refinement, and a Delta S of 0.002. Delta S needs to be small here since S11 itself will be small. Make sure “Sweep” is NOT checked. The starting mesh will be the initial mesh. However, we want to have extra mesh points on the faces of the airbox (the radiation boundary) with a spacing of one-sixth of a wavelength. Since the far field is computed from an integration over the radiation boundary, that will give us more far-field accuracy. In order to seed mesh points on the radiation surface, under Mesh Options select Initial Mesh once more. In the next window, click on Define Seed Operations. The Mesh3D window comes up. There, select the airbox and choose Seed / Object Face / By Length. In the next window that comes up, see the top of figure 3, enter a maximum element length of 5 mm and make the maximum number of elements to be added 10,000 (just some large number to make sure that our refinement value will be reached). Click OK and choose File / Exit and Save changes.
Back in the Initial Mesh Refinement window, see the bottom of figure 3, make sure that both “Lambda Refinement” and “Seed Based Refinement” are checked and click OK.
Accept the defaults in the rest of the window and click OK.