Phased Array Antenna

The Phased Array Antenna model consists of many radiating elements. Each element is modeled as an isotropic pattern. By modifying the excitation (amplitude and phase) of each element differently, a phased array antenna can electronically steer its maximum gain toward a particular direction or main radiation axis. A phased array antenna not only can steer its maximum gain in a particular direction, but it can also steer nulls toward other directions to prevent radiation to and from other directions. The act of altering each element’s excitation is effectively accomplished through the assignment of weights to each element. This set of weights for a particular direction is called a steering vector and each weight is a complex number.

The Phased Array Antenna configuration is broken down into following areas:

General parameters - a set of parameters general to the antenna
Element Configurations
Beam Direction Providers
Null Direction Providers
Beamformers

Element Configurations, Beam and Null Directions Providers, and Beamformers are subsections with specialized parameters.

The Phased Array Antenna model uses angles that are measured in a spherical coordinate system. See Phased Array Antenna coordinate system for additional information.

General parameters

The following general antenna parameters appear at the top of the Antenna tab of the Basic - Definition page in the Phased Array 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.
Backlobe Suppression	Enter the amount of attenuation to apply to any angle in the back hemisphere. This enables you to model reduced gains due to a back plane or other type of physical construct that would hinder reception from the back side of the antenna.
Backlobe Suppression Type	Constant. Applies the same Backlobe Suppression value to all angles. Cosine. Applies the Backlobe Suppression value as a function of the cosine of the angle with complete suppression when the angle is 0 degrees and the Backlobe Suppression value is at a 90-degree angle.
Element Factor	Include Element Factor. Selecting this check box includes the individual element pattern. When selected, this will use a general raised cosine function. Clearing the check box can be helpful when comparing with other tools that only consider the array factor. When cleared, be aware that the maximum gain will not vary with the steering angle. Raised Cosine Exponent. Produces the cosine raised to the exponent power you specify.
Dimension	Elements. Enter the number of enabled elements. Width. Enter the length, in meters, of the antenna. Height. Enter the height, in meters, of the antenna.

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 consists of element grid points, and each grid point has a unique ID number. Not all Element Configuration Types provide grid points other than the original element positions. You can enable or disable grid points to represent failed elements, to represent eliminated elements due to physical obstructions, or to model Minimum Redundant Arrays (MRA).

Select the Show Grid check box to display extra elements that can be enabled or disabled.

Select the Show Labels check box to display each unique element ID. This ID is important if you want to use an external file to assign complex weights to each element. See ASCII Data Element Configuration File for more information.

To enable or disable an element, click the element's grid point.

When you use an ASCII Data Element Configuration file, the Viewport will show the element configuration but will not allow you to enable or disable the elements.

Element Configuration Designer

The Designer enables you change the physical layout of the elements. Element configuration choices include the following:

Polygon
Hexagon
Linear
Circular
ASCII Data

When changing the type of configuration, number of elements, or the lattice structure, all disabled elements will be lost.

Lattice Structure

Lattice Structure selections only appear when you select Hexagon or Polygon for Designer Type. The elements are placed on a grid called a lattice structure. The lattice structure provides options for the general arrangement of the element placement. There are several options for the Lattice Structure of the element configuration. The type of lattice structure can be either triangular or rectangular. Selecting the Equilateral check box will enforce equal distance between the nearest neighbors.

Parameter	Description
Triangular	Staggers the elements between rows and columns.
Rectangular	Aligns the elements between rows and columns.
Equilateral	For the triangular lattice structure, it will ensure the distance between all nearest neighbors are equal. For a rectangular lattice structure, it will ensure the distance in the X direction is equal to the distance in the Y direction.

Polygon element configuration

This represents a two-dimensional planar antenna array with N sides. You can arrange the elements on either a triangular or a rectangular Lattice Structure. Furthermore, the polygon shape does not necessarily have to be a regular polygon. Thus, the number of elements in the X direction can be different than the number of elements in the Y direction.

Parameter	Description
Number of Sides	Enter the number of sides of the antenna shape.
Lattice Structure	Provides options that determine the general arrangement of the element placement.
Type	Choices vary depending on the selected Element Configuration Type: Triangular. Elements are placed in a triangular structure. Rectangular. Elements are placed in a rectangular structure.
Equilateral	Each element is equal in distance to its structural nearest neighbors.
Number of Elements	X: Enter the number of elements in the X direction of the center row. Y: Enter the number of elements in the Y direction of the center column.
Unit Type	Unit Type defines the X and Y element spacing in either Wavelength Ratio or Distance units. The Wavelength Ratio unit type spaces the elements relative to the wavelength (e.g., Design Frequency). The Distance unit type spaces the elements at an absolute distance.
Spacing	X: Enter the distance, in the unit type, between the elements in the X direction. Y: Enter the distance, in the unit type, between the elements in the Y direction.
Max Look Angle	See the Max Look Angle section below.

Hexagon element configuration

This is a six-sided specialization of Polygon. It is included primarily for your convenience. It represents a two-dimensional planar antenna array with an aperture shape consisting of six sides. Like Polygon, you can arrange the elements on either a triangular or a rectangular Lattice Structure. Likewise, the hexagon shape does not necessarily have to be a regular hexagon. Thus, the number of elements in the X direction can be different than the number of elements in the Y direction. However, for Hexagon, the number of elements in X and Y must be odd.

Parameter	Description
Lattice Structure	Provides options that determine the general arrangement of the element placement.
Type	Choices vary depending on the selected Element Configuration Type: Triangular. Elements are placed in a triangular structure. Rectangular. Elements are placed in a rectangular structure.
Equilateral	Each element is equal in distance to its structural nearest neighbors.
Number of Elements	X: Enter the number of elements in the X direction of the center row (must be odd). Y: Enter the number of elements in the Y direction of the center column (must be odd).
Unit Type	Unit Type defines the X and Y element spacing in either Wavelength Ratio or Distance units. The Wavelength Ratio unit type spaces the elements relative to the wavelength (e.g., Design Frequency). The Distance unit type spaces the elements at an absolute distance.
Spacing	X: Enter the distance, in the unit type, between the elements in the X direction. Y: Enter the distance, in the unit type, between the elements in the Y direction.
Max Look Angle	See the Max Look Angle section below.

Linear element configuration

Represents an antenna array in one dimension.

Parameter	Description
Num Elements	Enter the number of elements in the array.
Unit Type	Unit Type defines the element spacing as either Wavelength Ratio or Distance units. The Wavelength Ratio unit type spaces the elements relative to the wavelength (e.g., Design Frequency). The Distance unit type spaces the elements at an absolute distance.
Spacing	Spacing is the distance, in the unit type, between adjacent elements.
Tilt Angle	Enter the angle at which the array deviates from the X axis, with positive tilt being toward the positive Y axis (i.e., counterclockwise).
Max Look Angle	See the Max Look Angle section below.

Circular element configuration

Represents a planar antenna array with elements arranged in a circular pattern.

Parameter	Description
Num Elements	Enter the number of elements around the circle.
Unit Type	Unit Type defines the element spacing as either Wavelength Ratio or Distance units. The Wavelength Ratio unit type spaces the elements relative to the wavelength (e.g., Design Frequency). The Distance unit type spaces the elements at an absolute distance.
Spacing	Spacing is the distance, in the unit type, between adjacent elements.

ASCII Data Element Configuration

An ASCII Data Element Configuration file enables you to define the number of elements and their placement through an external file. See the ASCII Data Element Configuration File topic for details and file format.

When you choose Type = Ascii Data and the number of elements in your file is 20,000 or greater, STK will display the message "Click here to show aperture elements" before attempting to redraw the elements in the viewport, since the redraw could take a while to process. If you click anywhere in the viewport, STK will redraw the element configuration.

Max Look Angle

The Max Look Angle fields contain informational parameters that identify the maximum steering angle that can be attained before grating lobes enter the antenna's view and cause ambiguities.

The maximum look angle does not restrict or constrain the simulation in any way and is not associated with any parameters associated with its parent.

When a transmitting antenna is steered beyond its maximum angle, then the antenna will produce maximum gain (and maximum radiation) in two different locations, one in the direction of the desired beam steering direction and the other in the direction of the grating lobe. Likewise, with a receiving antenna, it will have maximum reception gain in two different directions.

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: Clamp-to-Limit: Causes the antenna to clamp to the steering limit(s) when the object is outside the Steering Limits. This is the default. Ignore Object: Causes the antenna to ignore the object when the object is outside the steering limits.
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 Select All Filters to see all objects. Click Clear All Filters 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 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 . 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).

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: Clamp-to-Limit: Causes the antenna to clamp to the steering limit(s) when the object is outside the Steering Limits. Ignore Object: Causes the antenna to ignore the object when the object is outside the steering limits.

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.

Null Direction Providers

The Null Direction Provider tab enables you to select where the antenna points its null(s). This information is delivered to the beam former, which handles forming and steering the beam toward the specified direction(s). The direction providers use a spherical azimuth and elevation coordinate system. See spherical Az/El coordinate system for additional information. The Type choices for Null Direction Provider are the following: Object, ASCII Data, and Script.

Object Null Direction Provider

Selects the pointing directions for nulls based on your selection for STK Object(s). The antenna’s direction provider supports basic, temporal, vector, special, and STK plugin constraints on the selected Object. STK also supports all Communications and Radar constraints on their respective Communications and Radar objects. When you select Type = Object on the Null Direction Provider tab, you can set the following fields:

Parameter	Description
Enabled	Select this check box to have the antenna perform null steering.
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 null steering when the direction of interest is outside the steering limits. Choose one of the following: Clamp-to-Limit: Causes the antenna to clamp to the steering limit(s) when the object is outside the Steering Limits. Ignore Object: Causes the antenna to ignore the object when the object is outside the steering limits.
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 Select All Filters to see all objects. Click Clear All Filters to clear all the individual check boxes selected.
Object Selection	STK steers the antenna’s null toward this object if the antenna has access to it. Click an object on the left to highlight it and then click 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 . 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.

Null ASCII Data Direction Provider

See ASCII Data Direction Providers for information on this.

Null 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.