Project Unit

Select an unit in the Unit option of Tool menu, and then generate a new GEMS project. Then the default unit will be the one you selected. Once you click on the new project button, the default unit cannot be modified any more. Of course, you can use different units to specify the dimensions of an object.

Drawing Guide

(1) Select an icon in the Tool Box, which you like to draw in       the picture region.

(2) Move the mouse icon to the picture region and select the       start point. The position of mouse icon is shown in the       text boxes below the picture area.

(3) Press the mouse left button one time to confirm the start       point. And then release the left button.

(4) Move the mouse icon to select the next dimension of the       object. If the object is out of picture region, press and       hold the right mouse button to move the display region.

(5) Press the mouse left button on time to confirm this       selection. And so on until finish the object drawing.

(6) The drswn object will appear in the "Object Browser" box       under the "Shape" entrance and its properties will be       listed in the "Properties" box.

Set Excitation Pulse

The shape of excitation is improtant and may significantly influce the simulation results, but its shpe requirement is so restrict if its spectrum can cover the frequency band of interest. GEMS provides several types of excitation pulse, namely, Gaussian pulse, differential Gaussian pulse, modulated Gaussian pulse and user defined pulse. The difference between the Gaussian pulse from differential and modulated Gaussian pulse is that Gaussian pulse includes large amount of DC conmponents, but in the differential and modulated Gaussian pulses.

     If a feed structure has a short circuit, all the excitation pulse will generate the same or similiar simulation results. Otherwise, you may see that the time domain signal is not convergent to zero.

Lumped Port Excitation

The most popular excitation for a port is lumped port, by which we can get the port information including return loss, port impedance, port voltage and current.

      To add a lumped port, you can draw a line between the ground and signal line at the feed point, and then select the normalized impedance, orientation, frequency range, delay for multiple excitation.

      The orientation is important, which may affect the result of return loss and transmission coefficient.

Lumped Port Output

The most useful output option for a port is lumped port, by which we can get the port information including transmission coefficient, port impedance, port voltage and current.

      To add a lumped port, you can draw a line between the ground and signal line at the observation point, and then select the normalized impedance, orientation and frequency range.

      The orientation is important, which may affect the result of transmission coefficient.

Wave Port Excitation

Wave port is used to exicte microstripline feed, coaxial cable, or parallel lines. Using the wave port, we can get the port information including return loss, port impedance, port voltage and current.

      To add a wave port, you can draw a line between the ground and signal line at the feed point, and then select the normalized impedance, orientation, frequency range, delay for multiple excitation.

      The orientation is important, which may affect the result of return loss and transmission coefficient.

Import and Export CAD Files

GEMS can import the most popular CAD files such as SAT, DXF, ProE, STEP, IGES and so on. GEMS will fix the minor problems automatically in a CAD file. GEMS will also allow you change the geometry unit and ratio.

     Compared to other EM software, GEMS can handle more complicated DXF files. For example, a complex chip model or a car model. In additon, GEMS can export a model in the SAT file format.

Import Voxel File

GEMS can import the human body or part of body from Voxel format to GEMS GUI directly. The dispersive material has been biult in the material library. You can use the downsample option to speed up the model display. GEMS allows you to change the resolution of the model, delete layers of tissues from the original design. You can also rotate the model in any orientation you like.

      You can select one or more regions to calculate the SAR and heat distribution. GEMS also allows you to place an antenna inside the human body, and apply palve wave excitation in the simulation.

Improve Improted CAD Model

Most popular commercial software today can export CAD files such as SAT, STEP, ProE and IGES. For some reason, the imported model may be not perfect, for instance, six surfaces of a cube may not be identical. In GEMS, the minor problems can be fixed or ignored during the GEMS simulation. GEMS also provides a way to improve or fix the imported model in a very efficient way.

      To do it, you can select a correct surface of the imported model, and copy and paste (Ctrl + c then Ctrl + v) to get a new surface, and then use shelling option in hte Operate menu to add a thickness to this surface.

Adaptive Mesh Generation

Mesh plays a critical role in the GEMS simulation, and not only affects the accuracy of simulation but also influnces the memory usage and simulation time. A poor mesh quality may cause the instability of simulation.

       GEMS provides a simple but very efficient mesh generation module to generate an accurate mesh for both simple and complicated structures. GEMS also allows you to check the mesh quality dynnmically.

       In addition, you can draw an option (object, source, or output) based on the current mesh distribution. It can overcome the approximation of cartisian mesh in part or all.

Parallel Processing Design

Unlike most other EM software, GEMS divides the computational domain into small sub-domains, and each processor/core will only simulate a sub-domain. Therefore, a computer cluster can be used efficently to simulate much larger problem than a single computer.

      GEMS provides a 3-D parallel processing module, which allows you to place the processors in a 3-D pattern to achieve a better job balance. If a subdomain is completely embedded inside a PEC structure or a space where we know the fields are zero all the time, you do not need to assign any real processors to this subdomain. Also, GEMS can detect the zero field region on the interface between subdomains then ignore this region during the information exchange in the parallel processing. For more detail, please click on the right icon to view the design procedure.

Parametric Variables

You can assign a fixed dimension to an object, for example, 5mmx6mmx7mm for a cube, now, you can set them be parameters like Lmm, Wmm and Hmm. Whenever you need to change the dimenssions, you only need to change values of L, W and H in the parameter list.

      The dimension can be also an expression like L=5*H and W=3*H, namely, you only need to control one parameter to change the dimenssion of cube.

Optimize Parametric Variables

GEMS allows you to optimize dimensionof structure, material type and different kinds of excitations to reach a better result.

      To optimize a dimension, you need to set this dimension as the parametric variable first. And then specify a range for this parametric variable.

     GEMS will run the cases one by one and generate the result for each case. You can find the better fit dimension for the desired result.

Advanced Mesh Design

Besides the adaptive mesh, GEMS also provides one advanced mesh options. Onlu in few applications, you need to involved in the advanced mesh design.

      Advanced mesh option allows you to design the local mesh detribution, for example, add key points and remove key points to change hte mesh quality.

     In the advanced mesh window, you can view the mesh with objects and move the mesh cross section to view the mesh variation.

Plane Wave Excitation
GEMS provides two types of plane wave excitation sources, namely, for finite structure and periodic structure. In finite structure case, the plane wave surface will divide the domain into two parts, namely, total field region (inner) and scattering field region (outter). If the measurement is located outside plane wave box, the measurement will be scattering field, otherwise it is total field.     

Periodic Structure Excitation

Plane wave excitation for periodic structure is located above or below hte periodic structure. No matter normal incidence or oblique incidence, the plane wave surface is always parallel to the x-y plane. The reflection coefficient is measured at the plane wave source side and the transmission coefficient is located at the opposite side of plane wave source.

Infinitely Thin PEC Structures

GEMS allows you to draw or import an infinitely thin structure, parallel to the coordinate plane or oriented in arbitrary direction. However, the infinitely thin structure has to be PEC material. No PEC material has a finite thickness.

       One thing is very important when you draw an infinitely thin PEC structure, that is, infinitely thin PEC structure has to be drawn after the objects that the PEC touches. Otherwise, the infinitely thin PEC object will be erased by the object it touches.

Domain Size

Computational domain can be equal to, or larger than or smaller than the the region occupied by objects. GEMS allows you to find the minimum domain size which encloses the objects exactly. Yo ucan change the domain size manually. Even you can cut part of the object as your simulation domain.

       In addition, GEMS allows you to control the number of cells between the object and absorbing boundary. This option is to keep the number of cells between object and absorbing boundary to be a reasonable number regardless of the cell size.

       The number of PML layers is invisible to the users although you can adjust the number of PML layers for the special cases.

Boundary Condition

The boundary conditions provided by GEMS can be used to simulate almost of the EM probelms. They are PML, PEC, PMC, Mur, and PBC. The PEC mirror and PMC mirror are used to truncate the symmetric pstructure and you need to output the far field pattern.

       Periodic structure can be only in the orrzontal directions (in x-y plane), namely, the PBC boundary can be only used in hte x- and y-directions.

       PML is the most important boundary condition, which can be used to truncate the open space problem. 6-layer PML is sufficient fo r most applications. The stability factor and low frequency factor are used to heal the instability problem and improve the performance of PML at the low frequencies.

Far Field Outputs

You can use GEMS to calculate the 2-D and 3-D far field pattern, directivity, gain, axial ratio, polarization sense, efficiency, at the different frequencies. You can control the angle resolution and number of cuts for both the 2-D and 3-D pattern. GEMS also allows you to output the current distributions on the surface of Huygens' box.

      In some special cases, you may only need to calculate the far field using not six Huygens' surfaces, namely, only one, or two. You can selec the surfaces to calculate the far field.

Microwave Components

You can design different types of microwave components in GEMS intergace. You can import a CAD model directly to GEMS interface for a practical design. Or you can design a microwave component using the tools provided by GEMS software.

Modeling Techniques

(1) Generate an array: you can easily generate an array       using the softclone or hardclone;

(2) Generate a complex structure using the sweep method;

(3) Generate a surface from a polygon;

(4) Generate a solidy object from the closed surfaces;

(5) Generate a hollow shell;

Useful Skills

(1) Hide an object or option;

(2) Generate desired local mesh;

(3) Smartly use PEC and copper material.

SAR and Heat Outputs

Improt whole body or part of human body, and specify the materal and mass density. Draw a cube in which yo ulike to calculate the SAR or heat inside. FOr heat calculation, the materilal must be lossy.

     Check the SAR ot heat option in the output window, and specify the freuquencies, where yo ulike ot output the SAR and heat.

     SAR and heat distrributios are 3-D data, and may time consuming in the simulation. Over output will degrade the simulation efficiency.

Surface Current Output

GEMS allows you to output the surface current distribution on the surface of conductor. Its value is calculated by using nxH, so the current will be on both sides of PEC surface.

      GEMS only outputs the surface current inside the specified region.

Generate GEMS Project Files

After you finish the excitation pulse setting, boundary and domain settings, object input, excitation setting, output specification, mesh design,and parallel processing design, you need to generate a GEMS project file, which can be simualted in the same computer, or submited to a computer cluster.

      In the project generation window, you have a chance to specify the convergent criteria, and decide where you will do the project processing.

Simulation Convergent Criteria

GEMS provides two ways to terminate a GEMS simulation, namely, terminate at the certain signal level, or, finish certain number time steps. You can terminate simulation at any time or you can extend the simulation to certain time steps during the simulation. In any cases, the frequency results will be meaningless if the time signal is not convergent.

Material Library

GEMS provides several material libraries including standard library and dispersive material library. You can add new material to the library for the current project or for current user. This library can be transfered from one user to another one.

       The dispersive material library is specially for US Air Force Brooks Segment model. You can modify it or calculate the material parameters for different frequencies.

      In additon, GEMS also provides the Debye, Lorentz and plasma dispersive models. GEMS can fit the dispersive curve from the discrete data you provided.

Adjust Object Order

GEMS follows the natural way that the object drawn later will erase the previous one if they have any overlap region. So the order is cretical in GEMS software. GEMS allows you to adjust the order of objects by using the order adjust option.

      IF you remove the overlap part first and then draw the next object, although the result may be also affected by the order since the material on the interface will be decided by the order, it is not much.

View Mesh Distribution

After you generate the mesh distribution, you can view the mesh distribution together with the objects inside the domain. You can view the mesh in three planes. In the mesh status, you can draw object or excitation or output options based on the designed mesh if the grid snap optioon is selected.

Mesh Background

There are types of background mesh in the GEMS designer, one is the uniform mesh and can be changed without affecting GEMS project setting; and another one is GEMS mesh which you designed for this project and may significantly influence your simulation results.

    There exist two different statuses: grid snap and non snap. In the grid snap status, the mouse icon can be moved only on the grids, otherwise, you can move the mouse icon to any place in the picture region.

Select Object Surface

You can select a surface of object and then generate a new object based on this surface. For example, you can select a surface of object and copy and paste this surface through Ctrl c and Ctrl v operations to generate a new surface. Then you can generate a new 3-D object which touches the object.

Relative Coordinate System

You can draw an object in the relative coordiante system. To do so, you need to generate a relative coordinate system first, and then draw an object in the relative coordinate system.

Analytic Formula Drawing

GEMS allows you to generate an special shape of object such as parabolic reflector antenna through its analytic expression based on a circular plate, which may have finite thickness.

Domain View

When a GEMS project includes Huygens' box or/and plane wave source, you can view the relative position of the simulation domain, palne wave and Huygens' box. It can help you graphically to check if the setting is correct.

Subgridding and ADI Design

GEMS provides a stable subgridding module to place fine mesh in a region where the fields may vary quickly than other regions. For more detail, please click on the right icon to view the design procedure.

Mode Extraction

The user may need to determine the mode excitation type in a port problem simulation. GEMS offers a mode extraction module, which allows the user to use different modes to excite a port or extract the power distribution in the different modes. For more detail, please click on the right icon to view the design procedure.