Design and Analyze Cassegrain Antenna

This example shows how to create a cassegrain antenna. A typical parabolic antenna consists of a parabolic reflector with a small feed antenna at its focus. Parabolic reflectors used in dish antennas have a large curvature and short focal length and the focal point is located near the mouth of the dish, to reduce the length of the supports required to hold the feed structure. In more complex designs, such as the cassegrain antenna, a sub reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point. Cassegrain provides an option to increase focal length, reducing side lobes. Such type of antennas can be used in satellite communications as well as Astronomy and other emerging modes of communications

Define Parameters

Parameters given below helps designing of the cassegrain antenna

Rp = Diameter of main reflector

fp = Focal Length of main reflector

Rsub = sub reflector diameter

fhyp = focal length of hyperbola

Rp=0.3175;
fp=0.2536;
Rsub=0.033;
fhyp=0.1416;

Model Cassegrain Antenna

Cassegrain antenna typically has three structures. First is the main reflector which is parabolic, second is the subreflector which is hyperbolic, third is the exciter element. Focus of the main reflector and the near focus of the hyperbolic subreflector coincides. The energy is directed from the subreflector towards main reflector. The parabolic reflector converts a spherical wavefront into a plane wavefront as the energy directed towards it appears to be coming from focus.

ant = cassegrain('Radius',[Rp Rsub],'FocalLength',[fp fhyp])
show(ant);
ant = 

  cassegrain with properties:

        Exciter: [1×1 hornConical]
         Radius: [0.3175 0.0330]
    FocalLength: [0.2536 0.1416]
           Tilt: 0
       TiltAxis: [1 0 0]
           Load: [1×1 lumpedElement]

Define Exciter Element

Conical horn is used as the default exciter for cassegrain. It is oriented towards the sub reflector. It is designed at a frequency which gives the desired performance. The aperture diameter is chosen analytically to give the desired co-planar pattern beam width.

Exciter=design(hornConical,17.7e9);
Exciter.FeedWidth=3.4e-3;
Exciter.Tilt=270;
Exciter.TiltAxis=[0 1 0];
show(Exciter);

Solve Structure

Default cassegrain antenna uses PO solver for main reflector and MoM solver for sub reflector and exciter. Diameter of main reflector is 39 and that of sub reflector is 4 . Solvers are used according to the size of main reflector and subreflector. Typically, if the diameter sub reflector is greater than 5 , then it is recommended to use PO solver for it. Normally the expected diameter of subreflector is to be less than 20% of main reflector to minimize the blockage by the subreflector.

Plot Radiation Pattern and Mesh Antenna

Plot the radiation pattern of cassegrain antenna at 18GHz.The expected pattern need to generate a pencil like beam with less spill over and minor lobe radiation. Default Cassegrain antenna generates more number of triangles, so it can be meshed manually for different mesh edge lengths for the required numbers of triangular generation. This solves the structure quickly, but the gain decreases compared to default in such cases. Mesh can be controlled for exciters and reflectors separately with different mesh edge lengths.

ant=cassegrain;
pattern(ant,18e9);
figure;
mesh(ant);
figure;
mesh(ant,'maxEdgeLength',30e-3);

Comparing with parabolic reflector which doesn't have subreflector shows that cassegrain configuration increases directivity

ant=reflectorParabolic;
ant.Exciter=hornConical;
ant.Exciter.Tilt=90;
figure;pattern(ant,18e9);

S-parameters and Impedance Plot

Plot the s-parameters and impedance of cassegrain antenna over a frequency range of 18 GHz to 18.8 GHz. It provides a bandwidth of about 100MHz and the structure resonates at 18.51 GHz.

ant=cassegrain;
s=sparameters(ant,linspace(18e9,18.8e9,25));
figure;rfplot(s);
figure;
impedance(ant,linspace(18e9,18.8e9,25));

Current Distribution

Analyze the current distribution of cassegrain at a specified frequency

current(ant,18e9,'scale','log10');

Use Different Exciters with Tilts

Several other elements are supported as exciters like horn, waveguides and so on. Exciter can be tilted in the required direction based on the need.

Exciter=design(horn,16.2e9);
Exciter.Tilt=270;
Exciter.TiltAxis=[0 1 0];
ant.Exciter=Exciter;
show(ant);
figure;
pattern(ant,18e9);

Conclusion

Using the Cassegrain design increases antenna performance as the feed antenna is directed forward rather than towards a dish in front fed antenna. This orientation reduces side lobes and the use of dual reflectors helps in tailoring radiation pattern for maximum antenna performance

References

Obulesu Dandu, "Optimized Design of Axially Symmetric Cassegrain Reflector antenna using Iterative Local Search Algorithm".