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Introduction
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WIPL-D Pro is a Method of Moments
(MoM) based code which enables very accurate EM simulation of
arbitrary 3D structures. Among them are antennas in various
technologies: wire antennas, horn and aperture antennas,
reflector antennas, microstrip antennas, phased array antennas,
helical antennas etc. For parabolic reflector antennas, a
special type of aperture antennas, MoM based simulation gives
more accurate results than approximate techniques based on
physical optics PO and/or uniform/geometrical theory of
diffraction (UTD/GTD) which are widely used for simulation of
dishes.
A model of the reflector is built
in WIPL-D Pro using predefined Reflector object editor, shown to
the right. Automatic geometry and mesh generation enable
creation of parabolic reflector of circular, elliptic or
rectangular shape (with or without rounded corners), with
central or offset feed. Two types of meshing are available:
Classic and Advanced. Advanced meshing is customized for
reflectors of circular and elliptical shape and simulation of
such reflector requires less unknowns than with classic meshing.
This way the simulation time for large reflectors is decreased.
Alternatively, a model of the
reflector can be imported from a CAD file, or custom-defined by
the user, through a script file. |
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Example: Cassegrain Antenna
Cassegrain antennas are a subcategory of
reflector antennas. Reflector antennas have been used from discovery of
electromagnetic wave propagation onwards. The most important
applications of reflector antennas are in radar, space communications,
radio astronomy and wireless communications.
Cassegrain antenna consists of two reflectors (primary and secondary)
and a feeder. The main characteristics of Cassegrain antennas is their
high directivity. The bigger diameter of antenna reflector is used, the
better gain is achieved.
WIPL-D Simulation
A model of Cassegrain antenna created in WIPL-D Pro is shown in Fig. 1.
We will assume that given antenna is used for satellite communications
in Ka band. A close-up of the feeder and the primary reflector is shown
in Fig. 2. Note that the primary reflector is curved, unlike in
splash-plate reflector antennas.
In reflector antenna systems, horn antennas are often used as feeders
(Fig. 3). In this project, feeder is specially designed in order to
suppress back radiation. That was done by adding a choke to horn
aperture edge. Length of choke is equal to quarter of free-space
wavelength (parameter Lam/4 on Fig. 3). Axial two-level design enables
dual mode electromagnetic wave propagation.
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Figure 1. Cassegrain antenna |
Figure 2. Cassegrain primary
reflector, half antenna model |
In WIPL-D Pro, reflectors and feeders can
be efficiently designed using built-in parameterized objects (BoR,
Reflector, Circle, ...). One can use (Anti-) Symmetry feature to reduce
the computational burden of simulation, so in this problem only quarter
of the antenna is modeled (Fig. 1). All the antenna parts are considered
to be perfectly conducting.
Operating frequency is 26.5 GHz (Ka-band). For model parameters given in
Tab. 1, we will calculate antenna gain. In this case, the dish radius is
equal to 100 wavelengths, which makes this model challenging due to its
vast electrical size.
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Figure 3. Feeder, half antenna
model |
Table 1. Dish
radius
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Radiation pattern in 2D (phi-cut), where
phi=0, is shown in Fig. 4. Please note that the theta angle is measured
with respect to the xOy plane.
Number of unknowns, memory requirements, and simulation time of analysis
are given in Tab. 2. Computer used for these calculations is Intel® Core2 Quad CPU at 2.83 GHz, 8 GB RAM.
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Figure 4. Radiation pattern,
phi-cut |
Table 2. Simulation data
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Model |
No. of unknowns (memory [GB]) |
Time @ 26.5 GHz [min] |
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quarter |
38525 (11.9) |
117.2 |
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Conclusion
WIPL-D Pro offers specialized geometrical objects to be used as building
blocks for complex antenna system models. Reflector shapes can also be
imported from a CAD file, or they can be customized according to a
user-defined shape (script file). Hence, practically any type of a
reflector antenna can be easily modeled.
Proper usage of WIPL-D Pro features (for example, Symmetry), enables
simulation using only quarter of structure, which is very important for
analysis of electrically big structures, since we reduce memory used and
simulation time dramatically.
Cassegrain antenna of this size is a challenging simulation task and
this sort of antenna is usually analyzed using geometrical optics
methods. However, WIPL-D Pro successfully simulates this antenna using
very accurate higher order MoM.
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