Tutorial for XFDTD Written by Cynthia Furse University of Utah

XFDTD 3D Finite Difference Time Domain code Written by Remcom, Inc.

To start XFDTD in the CADE Lab type 'xfdtd53' in a shell. * Suggestion: First open the folder where you want your XFDTD file in.

Geometry File XFDTD uses two file types for any simulation. The Geometry file contains information about grid spacing and size, material types, and the location of objects in the grid. Its extension is ".id". The other file type is the fdtd file. This file contains all simulation paramenters, including the location of sources, number of timesteps, and data to be saved during the simulation. To run a simulation, a Geometry file has to be created. After the Geometry file has been saved, an fdtd file needs to be created. The fdtd file automatically references the geometry file that was open when it is created. After the fdtd file is created and saved, calcfdtd can be run. The data will be stored in the directory that xfdtd was opened in. After the simulation run is complete, the fdtd file needs to be reloaded into xfdtd for data to be viewed and plotted.

This is what the XFDTD Window Looks Like

To Create an New Geometry:

Define the FDTD Grid Cell Size Total Space (model plus surrounding air) This means you will be defining the materials at the locations of the E field components

View the Grid.

This will show you the grid. So far it contains only air. Orientation. You are viewing the xy-plane. Zoom Factor Where in the plane.(layer#) Location of your cursor

To change the plane you are viewing: XY plane ZY plane ZX plane 3D view

Edit the Menu of Electrical Material Parameters

Add Electrical Materials Double select ADD Then, choose Next Available or Choose Color * Disabled “Num Lock” in your computer keyboard, if double click cannot process.

Edit Electrical Properties of the Materials to be in your model Material 0=air Material 1 = PEC (metal) Lookup the material properties for your material.  r  Only needed when computing SAR Enter your material

Edit the Color for Your Material If you click Choose Color instead of Next Available:

Display the Edit Panel to Draw or Edit your Model

Here are Several Model Elements Choose Material Types of Single- Cell Models Wire Plate Cube Build Layers above or below Blocks of Plate and Cube

Start to Define Your Geometry Choose Material

The Library has Other Elements

Let’s Start with a Single-Cell “Wire” made of Material 2, What I Named Substrate

Ex Ey Ez Each Cell has Ex, Ey, Ez components Electric Grid Lines Represent Each Ex,Ey,Ez Component… (The Grid has also been Zoomed) This means the Ex component is “in” material 2 (but the Ey,Ez components are in material 0=air) Ez is in Material 2 Ey is in Material 2

See How a Cube of Material is Defined in this layer: Ex Ey Ez

And in the Layer Above it Ex Ey No Ez’s * To erase or undo, use Free Space in Electrical Materials to cover.

Draw the Teflon Box for Patch Antenna on Slice 25 Alternatively we could have used rectangular box in the library. Hold Middle Mouse Button to Drag, Click Left Button to Build.

Add Another Layer Slice 26 * Or in Geometry Editing Tools, add number of layers in Additional Layers for above or below levels.

And Another Layer Slice27

The Top is Slice 28

But we want the top of the patch antenna to be metal, so choose the plate element PEC

Add a plate to the bottom, too. Slice 25 (We used the plate instead the cube to draw layers of PEC, therefore, it’s on the same layer with Taflon)

Look at this “sandwich” in the yz and xz planes If the plates and boxes aren’t lined up correctly, fix them

We want to add a feedpoint to the center of the patch. Use the cursor to find where this is. (27,23,28)

Look at the feed location View-XZplane (Make sure that’s your feedpoint when you put your cursor at your feed Location) Y=23

Use Single Cell Wire (PEC) from Edit Panel

Use these wires to connect top to bottom plate, leaving one cell gap for voltage feed source 2 single-cell wires in the z (vertical) direction gap

Find the Location of the Feed Point Gap with the Cursor (27,23,27) The location to put your source need to be one grid below the feedpoint because the source take one grid space. We put our cursor at the gap.

Save Your Geometry Now you have created your geometry file You have found where to put your source Save your geometry as ~.id file

Types of Simulations Steady State Steady state simulations use the sinusoid source found in the FDTD - Run Paramenters - Voltage Source menu. This is very useful to determine exact parameters of an object at the specified input frequency. This includes SAR values, efficiency, radiation patterns, and input impedance. Many of these values can be found by simply looking at the Steady State Option in the Plot menu. Transient Transient simulations are excellent for determining the general paramaters of an antenna over a range of frequencies. Transient simulations use a gaussian pulse, or one of its variations as a source. By running a transient simulation. One can easily determine the input impedance over a large frequency range. This allows you to find the resonant frequency of an antenna, and gives you the information needed to make changes to your design.

Transient Simulation: Open Voltage Sources Panel

Choose a Source and “Add Port to List”. You may have several sources. This is your feedpoint location They Can be edit in “Set Waveform” If you want to S-paramters results

A note on # of Time Steps For steady state simulations, enough time steps must be included to reach steady state conditions. Otherwise, the results will be invalid. To determine if convergence has been reached. Store near zone values. Look at the near zone values to determine if Steady State has been reached. For transient simulation, more time steps will allow larger FFT transforms. This will give plots more data points, giving smoother graphs.

Close the Source Window, and Reopen It to see the source location on your grid. The source location will only show when the source window is open.

Define what you want FDTD to save after calculation Other values of interest are available for sine wave source only These are available for pulsed FDTD

Save your geometry again

Save FDTD Parameters as a.fdtd file

Run FDTD

This is what is displayed Start simulation If you want to see EM fields, select yes, and you will be impressed

Simulation Starts

XY Plane of EM Field You can change the Plane and view fields in Different direction

Display Results When Simulation is Finished, Reopen the.fdtd file again, then display the result.

Simulation S-parameters Results First Click You can edit your plot parameters here Then Click

S11 Response You can see the resonance frequency of this antenna is around 1.1GHz

Impedance Measurements

Real Impedance Response

Steady State Simulation * First to find out what frequency your antenna operates at, then add the source.

Steady State Simulation Edit by clicking “Set Waveform”

Radiation Pattern (E-field) Set Up

Run FDTD

Display Steady State Data

Steady State Data

Radiation Pattern

Radiation Plot Setup

Display Plot

Plot Setup Change to Polar Plot Format

Radiation Pattern

End of Tutorial