HFSS Training(technical part)

Ben-Gurion University. Course Antennas and Radiation.

Maksim Berezin

What is the HFSS?For most practical problems, the solution to Maxwells equations requires a rigorous matrix approach such as the Finite Element Method (FEM) which is used by Ansoft HFSS. The wave equation solved by Ansoft HFSS is derived from the differential form of Maxwells equations. For these expressions to be valid, it is assumed that the field vectors are: single-valued, bounded, and have a continuous distribution (along with their derivatives) Along boundaries of media or at sources, Field vectors are discontinuous Derivatives of the field vectors have no meaning

B t D H = J + t D= E= B=0

Boundary Conditions define the field behavior across discontinuous boundaries

Why do I Care?They Force the fields to align with the definition of the boundary condition As a user I should be asking What assumptions, about the fields, do the boundary conditions make? Are these assumptions appropriate for the structure being simulated?

Model ScopeTo reduce the infinite space of the real world to a finite volume, Ansoft HFSS automatically applies a boundary to the surface surrounding the geometric model Outer boundary Default Boundary: Perfect E

Model ComplexityTo reduce the complexity of a model, the boundary conditions can be used to improve the: Solution Time Computer Resources

Failure to understand boundary conditions may lead to inconsistent results

What are Common Ansoft HFSS Boundary Conditions?ExcitationsWave Ports (External) Lumped Ports (Internal)

Surface ApproximationsPerfect E or Perfect H Surface Finite Conductivity Surface Impedance Surface Symmetry Planes Radiation Surface

Largely the users responsibility

Material PropertiesBoundary between two dielectrics Finite Conductivity of a conductor

Transparent to the user

Surface ApproximationsPerfect E Forces the electric field perpendicular to the surface Outer Surface Default Boundary PEC/Perfect Conductor Material Property Model complexity: Reduced by eliminating conductor loss Perfect H Forces the electric field tangent to the surfacePerfect E Surface

Perfect H Surface

Finite Conductivity Lossy electric conductor. Forces the tangential electric field at the surface to: Zs(n x Htan). The surface impedance (Zs) is equal to, (1+j)/(), Model complexity: Reduced by eliminating conductor thickness Impedance Surface Represent surfaces of a known impedance Forces the tangential electric field at the surface to: Zs(n x Htan). The surface impedance (Zs) is equal to, Rs + jXs (Ohms/Square) Layered Impedance Models multiple thin layers in a structure as an Impedance Surface Lumped RLC Parallel combination

Surface Approximations (Continued)Symmetry Planes Enable you to model only part of a structure Perfect E or Perfect H Symmetry Planes Must be exposed to the outer surface Must be on a planar surface

Remember! Mechanical Symmetry does not Equal Electrical SymmetryModel complexity: Reduced by eliminating part of the solution volume

Full Model

Surface Approximations (Continued)Radiation Surface Allows waves to radiate infinitely far into space. The boundary absorbs wave at the radiation surface Can be placed on arbitrary surfaces Accuracy depends on The distance between the boundary and the radiating object The radiation boundary should be located at least one-quarter of a wavelength from a radiating structure. If you are simulating a structure that does not radiate, the boundary can be located less then one-quarter of a wave length (The validity of this assumption will require your engineering judgment). The incident angle The radiation boundary will reflect varying amounts of energy depending on the incidence angle. The best performance is achieved at normal incidence. Avoid angles greater then ~30degrees. In addition, the radiation boundary must remain convex relative to the wave. Perfectly Matched Layer (PML) Allows waves to radiate infinitely far into space. Not a Boundary Condition. Fictitious materials that fully absorb the electromagnetic fields impinging upon them. These materials are complex anisotropic. Types Free Space Termination or Reflection Free Termination Can only be placed on planar surface Model complexity: They do not suffer from the distance or incident angle problems of Radiation boundaries but should be place at least one-tenth of a wave length from strong radiators

Surface Approximations (Continued)Infinite Ground Planes Simulate the effects of an infinite ground plane Only affects the calculation of near- or far-field radiation during post processing Types: Perfect E, Finite Conductivity, or Impedance Surface Frequency Dependent Boundary Conditions The following boundary parameters can be assigned an expression that includes Freq: Finite Conductivity Impedance Lumped RLC Layered Impedance Supported Frequency Sweeps Single Frequency Discrete Sweep Interpolating Sweep

ExcitationsPorts are a unique type of boundary condition Allow energy to flow into and out of a structure. Defined on 2D planar surface Arbitrary port solver calculates the natural field patterns or modes Assumes semi-infinitely long waveguide Same cross-section and material properties as port surface 2D field patterns serve as boundary conditions for the full 3D problem Excitation Types Wave Port (Waveguide) External Recommended only for surfaces exposed to the background Supports multiple modes (Example: Coupled Lines) and deembedding Compute Generalized S-Parameters Port 4 Frequency dependent Characteristic Impedance (Zo) Measurements Constant Zo Perfectly matched at every frequency Lumped Port Internal Recommended only for surfaces internal to geometric model Single mode (TEM) and no deembedding Normalized to a constant user defined ZoPort 3 Port 2

Port 1

Excitations (Continued)Wave Equation The field pattern of a traveling wave inside a waveguide can be determined by solving Maxwells equations. The following equation that is solved by the 2D solver is derived directly from Maxwells equation.

1 2 ( ) E x , y k0 r E ( x , y ) = 0 r where: E(x,y) is a phasor representing an oscillating electric field. k0 is the free space wave number, r is the complex relative permeability. r is the complex relative permittivity. To solve this equation, the 2D solver obtains an excitation field pattern in the form of a phasor solution, E(x,y). These phasor solutions are independent of z and t; only after being multiplied by e-z do they become traveling waves. Also note that the excitation field pattern computed is valid only at a single frequency. A different excitation field pattern is computed for each frequency point of interest.

Excitations (Continued)Modes, Reflections, and Propagation It is also possible for a 3D field solution generated by an excitation signal of one specific mode to contain reflections of higher-order modes which arise due to discontinuities in a high frequency structure. If these higher-order modes are reflected back to the excitation port or transmitted onto another port, the S-parameters associated with these modes should be calculated. If the higher-order mode decays before reaching any porteither because of attenuation due to losses or because it is a non-propagating evanescent modethere is no need to obtain the S-parameters for that mode. Wave Ports Require a Length of Uniform Cross Section Ansoft HFSS assumes that each port you define is connected to a semi-infinitely long waveguide that has the same cross section as the Wave Portno uniform cross section at Wave Ports uniform cross section added for each Wave Port

Excitations (Continued)Wave Port Boundary Conditions

Perfect E or Finite ConductivityDefault: All outer edges are Perfect E boundary. Port is defined within a waveguide. Easy for enclosed transmission lines: Coax or Waveguide Challenging for unbalanced or non-enclosed lines: Microstrip, CPW, Slotline, etc.

Symmetry or ImpedanceRecognized at the port edges

RadiationDefault interface is a Perfect E boundary

Excitations (Continued)Lumped Port Boundary Conditions

Perfect E or Finite ConductivityAny port edge that interfaces with a conductor or another port edge

Perfect HAll remaining port edges

Perfect E

Perfect H Perfect H

Perfect E

Excitations (Continued)Excitation Calibration Ports must be calibrated to ensure consistent results. Determines: Direction and polarity of fields Voltage calculations. Solution Type: Driven Modal Expressed in terms of the incident and reflected powers of the waveguide modes. Definition not desirable for problems having several propagating quasi-TEM modes Coupled/Multi-Coupled Transmission Lines Always used by the solver Calibration: Integration Line Phase between Ports Modal voltage integration path: Zpi, Zpv, Zvi Solution Type: Driven Terminal Linear combination of nodal voltages and currents for the Wave Port. Equivalent transformation performed from Modal Solution Calibration: Terminal Line Polarity Nodal voltage integration path

Example Solution Types:Mode 1 (Even Mode)

Integration Line

Mode 2 (Odd Mode)

Integration Line

Port1

Modes to Nodes Transformation

Modal

Port2 2 Modes

2 Modes

T1

T2

SPICE Differential Pairs

T1 Port1 T2

T1

Terminal

Port2 T2

Example StructureCoax to Stripline

LAYER 1 (TOP SIDE) LAYER 2 (SIGNAL) LAYER 3 (BOTTOM SIDE)

Material PropertiesAll 3D (Solid) objects have material definitions To complete the model shown previously we must include the air that surrounds the structure.

air

Note: Substrate/Air boundary included in structure

Remember! Material Boundary conditions are transparent to the userThey are not visible in the Project Tree Example Material Boundary: Conductors Surface Approximations

Perfect Conductors Lossy Conductors

Perfect E Boundary (Boundary Name: smetal) Finite Conductivity Boundary

Forces E-Field perpendicular to surface Forces tangential E-Field to ((1+j)/())(n x Htan). Assumes one skin depth User must manually force Ansoft HFSS to solve inside lossy conductors that are a skin depthsmetal

Surface ApproximationsBackground or Outer Boundary Not visible in the Project Tree Any object surface that touches it Perfect E Boundary Default boundary applied to the region surrounding the geometric model Model is encased in a thin metal layer that no fields propagate through

outer Override with Radiation Boundary

What Port Type Should I Use?Example is easy decision Port touches background (External) Cross Section is Coax (Enclosed Transmission Line)

Wave PortSolution Type: Driven Terminal SPICE Output

Identify Port Type & Cross Section

Assign Excitation & Calibrate Port

Define Default Post Processing

Is it Really that Simple?Yes, but the geometric model was setup with several considerations 1. Only the face of the coax dielectric was selected for the port facePort Boundary conditions define outer conductor Material Definitions define inner conductor2.

Uniform port cross-sectionOnly supports a single mode Higher-order modes caused by reflections would attenuate before port Modes attenuate as a function of e-z, assuming propagation in the z-direction. Required distance (uniform port length) depends on modes propagation constant.

Uniform crosscross-section Rule of Thumb: 5x Critical Distance