HFSS EEP Array Antenna
The HFSS embedded element pattern (EEP) is a model which will excite an individual element in an array, while terminating all other elements, and then capture the field of the excited array element. This field will include the coupling effects from neighboring elements and as-installed near-field obstructions if such obstructions are defined in the HFSS project.
When elements of an array are closely spaced, neighboring elements and objects located within the nearfield can introduce mutual coupling between elements. This can cause changes in boundary conditions and introduce additional currents that will impact the radiation pattern. As the element spacing becomes smaller, the coupling effects become greater. Ansys’s HFSS software enables you to simulate and model these effects in their far-field antenna pattern. You can then export these far-field patterns from HFSS and import them into STK using the HFSS EEP Array antenna model.
The HFSS EEP Array Antenna configuration is broken down into following areas:
- General parameters - a set of parameters general to the antenna
- Element Configurations
- Beam Direction Providers
- Beamformers
Element Configurations, Beam Directions Providers, and Beamformers are subsections with specialized parameters.
General parameters
The following general antenna parameters appear at the top of the Antenna tab of the Basic - Definition page in the HFSS EEP Antenna's Properties Browser.
| Parameter | Description |
|---|---|
| Design Frequency |
This is the frequency of the antenna. The antenna design frequency is independent of the operational frequency of a transmitter, receiver, or radar. Changing the frequency of a transmitter, receiver, or radar does not update an embedded antenna's design frequency, nor vice versa. The design frequency is solely used at antenna configuration time to compute the antenna size from its max gain or beamwidth settings. A mismatch between signal frequency and antenna design frequency typically causes performance degradation. This will be one of the frequencies specified in the element FFD files. Only frequencies common to every element are allowed. See Defined Frequencies in the Element Configurations section for more information. STK will use this when showing the antenna’s 3D volume, but for analysis, STK will use the signal’s frequency to select the closest Defined Frequency in order to select the appropriate far-field data field. |
| Dimension |
Elements. Enter the number of enabled elements. Width. Enter the overall width of the array in meters. This is estimated using the following equation: w= |ymax - ymin| + Dymin. Height. Enter the overall height, in meters, of the antenna. This is estimated using the following equation: h= |xmax - xmin| + Dxmin |
Element Configurations
The Element Configuration tab enables you to define the physical aspects of the antenna elements. It consists of two parts: a Viewport and a Designer section. The Viewport section shows the layout of the currently configured elements. The Designer section enables you to change the physical layout of the elements, which changes the display in the Viewport. You can see these sections in the diagram below, with the Viewport on the left and Designer on the right.
Element Configuration Viewport
The Element Configuration Viewport provides a visualization of the positions of each element. From it you can easily understand the overall shape of the aperture.
Element Configuration Designer
The Designer section provides inputs and information about the physical layout of the elements. Selecting a file will populate the remaining input fields. The following parameters are available:
| Parameter | Description |
|---|---|
| Filename |
Enter or browse to the file that defines all the element characteristics. An N-element array will require a total of N+1 files. You must provide N FFD files that contain the frequency-dependent far-field data values, one FFD file for each element in the array. In addition to the N FFD files, you must provide one metadata file that will reference the N FFD files and include some additional information about each element. This metadata file can exist in two different formats: an older file format with a TXT file extension or a newer format that is an XML file. You only need to select the metadata file, and STK will read in the referenced FFD files automatically. Once the files are loaded into the HFSS EEP Array antenna model, STK will be able to steer the aperture in a particular direction. See Beam Direction Provider for additional steering information. |
| Defined Frequencies |
Lists all the frequencies that are common to all of the elements having far-field data field values defined. Each element can have two or more frequencies and thus two or more far-field data field values. You must set the antenna’s design frequency to one of these values. If you set the Design Frequency to a frequency other than what is shown in the Defined Frequency list, STK will set the Design Frequency to the closest Defined Frequency. For analysis, STK will use the signal’s frequency to select the closest Defined Frequency in order to select the appropriate far-field data field. |
| Gain Type |
For the XML-based metadata file, this will be available and enable you to switch between Realized Gain and Total Gain for the far-field gain pattern. Realized Gain will use the incident powers defined in the metadata file, whereas the Total Gain will use the accepted powers. The older TXT-based metadata files do not contain excitation information, so this field will not be available. In that case, STK will assume a 1W per element excitation value. |
| Defined Power Value |
Displays either the total incident power or accepted power of the array. The total defined power value is computed as:
where pi is the power of each element as determined by the Gain Type setting. |
| User Gain Offset | Use this field to add or remove an additional gain bias. It is often useful to adjust the overall gain when using the older metadata file, which doesn’t contain the excitation power values. |
Beam Direction Providers
The Beam Direction Provider tab enables you to select where the antenna points its beam. This information is delivered to the beamformer, which handles forming and steering the beam toward the specified direction(s). The direction providers use a spherical Az/El coordinate system.
The following sections describe the Type selections for Beam Direction Providers — Object, Auto Pointing, ASCII Data, and Script — including the parameters associated with the Type choice.
Object Beam Direction Provider
Selects the beam pointing directions based on the STK object(s) you select. The antenna's direction provider supports Basic, Temporal, Analysis Workbench, Angle, and STK plugin constraints on the selected object. However, STK supports all Communications and Radar constraints on their respective Communications and Radar objects. When you select Type = Object on the Beam Direction Provider tab, you can set the following fields:
| Parameter | Description |
|---|---|
| Enabled | When you select this check box, the antenna will perform steering; otherwise, it will not. |
| Azimuth Steering Limit A | Relative to the mechanical boresight, electronically steering in the azimuth direction is bounded between this value and the value defined by AzimuthSteeringLimitB. The steering will be held at this value when attempting to steer beyond this value. |
| Azimuth Steering Limit B | Relative to the mechanical boresight, electronically steering in the azimuth direction is bounded between this value and the value defined by AzimuthSteeringLimitA. The steering will be held at this value when attempting to steer beyond this value. |
| Elevation Steering Limit A | Relative to the mechanical boresight, electronically steering in the elevation direction is bounded between this value and the value defined by ElevationSteeringLimitB. The steering will be held at this value when attempting to steer beyond this value. |
| Elevation Steering Limit B | Relative to the mechanical boresight, electronically steering in the elevation direction is bounded between this value and the value defined by ElevationSteeringLimitA. The steering will be held at this value when attempting to steer beyond this value. |
| When steering limits exceeded |
Determines the behavior of the beam steering when the direction of interest is outside the steering limits. Options include the following:
|
| Selection Filter | Select the check boxes next to individual objects to filter for just that type of object for the available object list below. Click to see all objects. Click to clear all the individual check boxes selected. |
| Object Selection |
STK steers the antenna’s main lobe toward this object if the antenna has access to it. Click an object on the left to highlight it and then click A phased array antenna only has M-1 degrees of freedom, where M is the number of elements. If you also select a beam object, then the degrees of freedom are further reduced to account for the desired beam. Thus, STK considers only the first M-1 list items and ignores the remaining ones. |
| Use mechanical boresight when zero Objects are in Field of Regard |
Determines the behavior of the beam pattern when there are zero directions of interest within the Field of Regard. When you select this check box, and there are zero directions (no access intervals for antenna and objects), the antenna will point along the mechanical boresight. When you clear this check box and there are zero directions (no access intervals for antenna and objects), the antenna will have no gain pattern and thus a no-gain value (i.e., antenna is turned off). |
to move the object to the selected list on the right. You can select one or more objects. Remove an object from the selected list by highlighting it and selecting 
.Auto Pointing Direction Provider
The Auto Pointing Direction Provider is only available as a Beam Direction Provider. The Auto Pointing Direction Provider will steer the antenna’s beam toward the direction associated with the other object with an Access report. This option provides convenience when wanting to individually steer to different Access report objects because you don’t have to update the selected object within the Object Beam Direction Provider each time you run an Access Report.
The Auto Pointing option will disable 2D Graphics Contours and 3D Volume Graphics.
When you select Type = Auto Pointing on the Beam Direction Provider tab, you can set the following fields:
| Parameter | Description |
|---|---|
| Azimuth Steering Limit A | Relative to the mechanical boresight, electronically steering in the azimuth direction is bounded between this value and the value defined by AzimuthSteeringLimitB. The steering will be held at this value when attempting to steer beyond this value. |
| Azimuth Steering Limit B | Relative to the mechanical boresight, electronically steering in the azimuth direction is bounded between this value and the value defined by AzimuthSteeringLimitA. The steering will be held at this value when attempting to steer beyond this value. |
| Elevation Steering Limit A | Relative to the mechanical boresight, electronically steering in the elevation direction is bounded between this value and the value defined by ElevationSteeringLimitB. The steering will be held at this value when attempting to steer beyond this value |
| Elevation Steering Limit B | Relative to the mechanical boresight, electronically steering in the elevation direction is bounded between this value and the value defined by ElevationSteeringLimitA. The steering will be held at this value when attempting to steer beyond this value. |
| When steering limits exceeded |
Determines the behavior of the beam steering when the direction of interest is outside the steering limits. Options include:
|
Beam ASCII Data Direction Provider
See ASCII Data Direction Providers for information on this.
Beam Script Direction Provider
See Script Direction Provider for information on this.
Beamformers
The Beamformer tab enables you to select one of several beam/null forming methods, which in conjunction with the direction from the Beam/Null Direction Providers, computes the complex weights for each of the elements. STK can then use these weights to compute the antenna’s gain pattern.
There are two categories of beamformers: those that perform beam steering and those that perform beam steering and adaptive null steering. The beamformers that do not perform adaptive nulling will implement an amplitude tapering to reduce the side-lobe levels. Amplitude tapering only applies to Element Configurations that are linear arrays. However, the Script Beamformer and ASCII Data Beamformer file beam formers provide the ability to specify a custom complex weight value.
The following sections describe the possible beamformer types, depending on the array configuration.
Uniform beamformer
This algorithm produces an amplitude distribution of unity across the elements.
Dolph-Chebyshev beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It produces an amplitude distribution to achieve a minimum null-to-null beam width for a specified sidelobe level. The Dolph-Chebyshev beamformer will produce the optimal solution when spacing is >=wavelength/2. Setting the sidelobe level input to its most negative value (no sidelobes) will approach the binomial distribution. For more information, see the reference [“Antenna Theory Analysis and Design” Constatine A. Balanis, pp. 245 – 254]. This beamformer is only available for linear arrays.
| Parameter | Description |
|---|---|
| SidelobeLevel | Enter the desired sidelobe level on max gain. |
Cosine beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It provides a cosine amplitude distribution across the array to reduce the sidelobes at the cost of slightly increasing the width of the main lobe. This beamformer is only available for linear arrays.
The amplitude distribution is given by:
Cosine^x beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It uses an exponential form of a cosine function to reduce sidelobe levels at the cost of slightly increasing the width of the main lobe. This beamformer is only available for linear arrays.
The amplitude distribution is given by:
| Parameter | Description |
|---|---|
| x | Value of the cosine’s exponential. |
Hann beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It utilizes aspects of the uniform and the cosine-squared pattern to place a null near a sidelobe peak. This beamformer is only available for linear arrays.
The amplitude distribution is given by:
Hamming beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It utilizes aspects of the uniform and the cosine-squared pattern to place a null near a sidelobe peak. This beamformer is only available for linear arrays.
The amplitude distribution is given by:
Raised Cosine beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It combines aspects of the uniform and cosine weighting. The height of the first sidelobe will decrease by decreasing p. This beamformer is only available for linear arrays.
The amplitude distribution is given by:
| Parameter | Description |
|---|---|
| p | Input parameter characterizing the weighting family as shown in above equation. |
Raised Cosine-Squared beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It is like the Raised Cosine, except for the cosine being squared. A parameter, p, value of 0.08 is equal to the Hamming beam former. This beamformer is only available for linear arrays.
The amplitude distribution is given by:
| Parameter | Description |
|---|---|
| p | Input parameter characterizing the weighting family as shown in above equation. |
Blackman-Harris beamformer
This algorithm changes the phase and amplitude across the array elements to steer and shape the beam. It implements the well-known Blackman-Harris distribution across the elements. This beamformer is only available for linear arrays.
The amplitude distribution is given by:
Custom Taper File Beamformer
Use this to specify a time-dependent taper (element amplitudes) in a text file. Each weight specified in the file is matched to an element by means of the element ID. Thus, the file’s element ID must match the element ID as specified in the antenna’s element configuration.
This differs from the ASCII Data Beamformer in that the Custom Taper File will only change the amplitude. Thus, the beam will still be steered based on the Beam Direction Provider. In contrast, for the ASCII Data Beamformer, you specify both the amplitude and phase. This means the ASCII Data Beamformer will not steer unless steering is designed in the complex weights.
If the first character of a line is '#', the entire line is treated as a comment line. Uncommented file lines have the following meanings:
- The first uncommented line is the file format version (required), which tells STK the format of the remaining file.
- The second uncommented line is a sampling mode (required). You can only use "SampleAndHold".
- The third uncommented line defines the format of the element weight data.
- The fourth uncommented line is the number of elements (required).
The remaining uncommented lines are time-specified complex element weights (required) with the following format:
<time> <element 1 id> <element 1 complex weight> ... <element n id> <element n complex weight>
where element IDs MUST correspond to the ENABLED element ID specified in the Element Configuration and the complex weights are specified as a comma-separated real and imaginary values enclosed in parentheses.
Here is a sample file for the Custom Taper File Beamformer:
#My sample Custom Taper Beamformer file CustomTaperFileBeamformer v1 SampleAndHold 5 -1.0E300 0 (0.37464739463368862) 1 (0.79115929021973885) 2 (1.000) 3 (0.79115929021973885) 4 (0.37464739463368862) 10.0 0 (0.16785218258752427) 1 (0.68214781741247588) 2 (1.000) 3 (0.68214781741247588) 4 (0.16785218258752427) 1.0E300 0 (0.16785218258752427) 1 (0.68214781741247588) 2 (1.000) 3 (0.68214781741247588) 4 (0.16785218258752427)