When you create antenna arrays such as a uniform linear array (ULA), you can use antennas that are built into Phased Array System Toolbox™. Alternatively, you can use Antenna Toolbox™ antennas. Antenna Toolbox antennas provide realistic models of physical antennas. They are designed using method of moments. Phased array antennas represent more idealized antennas that are useful for radar performance analysis and higher level modelling. Some phased array antennas cannot be physically realized, such as the isotropic antenna but are still conceptually useful. You can build and analyze systems using both types of antennas in an identical manner. This example shows how to construct a phased array with either Phased Array System Toolbox or Antenna Toolbox™ antennas.
When you use an Antenna Toolbox™ antenna in a Phased Array System Toolbox™ System Object™, the antenna response will be normalized by the maximum value of the antenna output over all directions. The maximum value is obtained by finding the maximum of the antenna pattern sampled every five degrees in azimuth and elevation.
Start by creating a uniform linear array (ULA) of crossed-dipole antennas from Phased Array System Toolbox. Crossed-dipole antennas are used to produce circularly-polarized signals. In this case, set the operating frequency to 2 GHZ and draw the power pattern. Use the
pattern method of the
phased.CrossedDipoleAntennaElement System object™.
fc = 2.0e9; crosseddipoleantenna = phased.CrossedDipoleAntennaElement('FrequencyRange',[500,2500]*1e6); pattern(crosseddipoleantenna,fc,[-180:180],0,... 'Type','powerdb')
The main axis of this antenna points along the x-axis.
Then, create an 11-element ULA array of crossed-dipole antennas. Specify the element spacing to be 0.4 wavelengths. Taper the array using a Taylor window. Then, draw the array pattern as a function of azimuth at 0 degrees elevation. Use the
pattern method of the
phased.ULA System object.
c = physconst('LightSpeed'); elemspacing = 0.4*c/fc; nElements = 11; array1 = phased.ULA('Element',crosseddipoleantenna,'NumElements',nElements,... 'ElementSpacing',elemspacing,'Taper',taylorwin(nElements)'); pattern(array1,fc,[-180:180],0,'PropagationSpeed',c,... 'Type','powerdb')
Next, create a uniform linear array (ULA) using the helix antenna from Antenna Toolbox. Helix antennas also produce circularly polarized radiation. Helix antennas are created using the
First, specify a 4-turn helix antenna having a 28.0 mm radius and 1.2 mm width. The
Tilt properties let you orient the antenna with respect to the local coordinate system. In this example, orient the main response axis (MRA) along the x -axis to coincide with the MRA of the cross-dipole main axis. By default, the MRA of the antenna points in the z -direction. Rotate the MRA around the y -axis by 90 degrees.
radius = 0.028; width = 1.2e-3; nturns = 4; helixantenna = helix('Radius',radius,'Width',width,'Turns',nturns,... 'TiltAxis',[0,1,0],'Tilt',90);
You can view the shape of the helix antenna use the
show function from Antenna Toolbox.
Then, draw the azimuth antenna pattern at 0 degrees elevation at the operating frequency of 2 GHz. Use the
pattern function from Antenna Toolbox.
Next, construct an 11-element tapered uniform linear array of helix antennas with elements spaced at 0.4 wavelengths. Taper the array with a Taylor window. You can use the same
phased.ULA System object from Phased Array System Toolbox to create this array.
array2 = phased.ULA('Element',helixantenna,'NumElements',nElements,... 'ElementSpacing',elemspacing,'Taper',taylorwin(nElements)');
Plot the array pattern as a function of azimuth using the ULA pattern
method which has the same syntax as the Antenna Toolbox
Comparing the two array patterns shows that they are similar along the mainlobe. The backlobe of the helix antenna array pattern is almost 15 dB smaller than that of the crossed-dipole array. This is due to the presence of the ground plane of the helix antenna which reduces backlobe transmission.