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The EM simulation of complex antenna
systems presents a challenge for modern computational software. The goal
of a circuit-EM co-simulation is to alleviate the computational
complexity decomposing the complete system into parts that need to be
simulated using full-wave approach, and parts that would be modeled by
predefined library components. Encapsulation of proven parameterized
antenna models and their reuse within the same or another project is
essential in rapid development of complex systems.
Co-Simulation in WIPL-D Microwave
The microwave (MW) circuit is composed of models of lumped elements,
transmission lines and discontinuities which are characterized by
relatively simple analytical equations based on physical and material
properties of components. The circuit solver uses these models to
calculate S-parameters of components, and finally S parameters of the
circuit (in reference to its ports). Main benefit of using circuit
analysis is the speed in which results are obtained. The circuit
analysis is much faster than full-wave EM analysis, even if done over a
wide frequency range because of simple component models. Simulation time
does not depend on the size of the circuit (in wavelengths).
However, the validity range of analytical models is often small which
significantly limits their applications. Besides, such a circuit model
takes into account conductor and substrate losses in a typical circuit,
but not radiation mechanisms. It is also difficult to include enclosure
effects, since there may be box resonances or waveguide modes in our
physical implementation. Furthermore, parasitic coupling between various
circuit components is not accounted for.
The most significant drawback of the approach is that there is no
possibility to model a device that can’t be described by standard
library analytical models. Therefore, there is a clear benefit from
inclusion of EM simulation capabilities into the circuit simulation. EM
models imported into the circuit can be parameterized and treated as any
other circuit components in an optimization cycle. EM models allow
investigation of wanted or unwanted radiation and coupling mechanisms.
Parts of the system whose radiation or EM coupling to other parts of the
system are not crucial can be modeled analytically. Moreover, measured
data over a frequency range can be imported as a component. Finally, a
model of any antenna or antenna component of interest can be included
and electromagnetically simulated, on-the-fly, at the circuit simulation
runtime. Hence, antennas can be excited in ways that are closer to how
the structures are used in practice.
WIPL-D Microwave offers all the possibilities mentioned above, as well
as some additional features:
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Radiation pattern produced by all the
radiating components in the circuit (Fig. 1),
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A WIPL-D designed EM model of circuit
components in the library (see Fig. 2) with de embedded S parameters
calculation,
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Mode-matching models of the most
important rectangular waveguide discontinuities,
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Library EM models of basic antennas
and most often encountered implementation technology transitions
(see Fig. 3),
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An impedance calculator built into
each component specification window (Fig. 3).
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Figure 1. Microwave circuit
schematic |
Figure 2. Full-wave microstrip
patch antenna model |
Simulation Example
MW circuit shown in Fig. 1 is analyzed using WIPL-D Microwave. The
circuit consists of two microstrip patch antennas and a feeding network:
microstrip lines, microstrip T junction and bends. All of the circuit
parameters are optimized in order to achieve matching at 10 GHz. Patch
antennas are physically separated by a distance which is also forwarded
from the circuit, as a parameter. We specified this distance to be 25 mm
along x-axis. We specified that the only radiating components in the
circuit are patch antennas, although there was a possibility to account
radiation from other components as well.
Full-wave model of the library patch
antenna is shown in Fig. 2. Its definition dialog box is shown in Fig.
3.
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Figure 3. Library microstrip
patch antenna dialog box |
Figure 4. S11 parameter |
The circuit is analyzed in the frequency
band from 9 to 11 GHz in nine uniformly distributed points. Parameter
S11 is calculated in order to analyze matching, and shown in Fig. 4.
Gain and near field from both antennas are calculated at 10 GHz, and
shown in Figs. 5 and 6. Each EM model was simulated separately so gain
and near fields were obtained by superposition of these results. The
simulation speed-up is significant since two simulations of individual
antennas are faster then solving the whole problem at once. The more
antennas in an array we have, the speed-up is larger.
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Figure 5. Gain in 3D |
Figure 6. Near field |
Conclusion
EM and circuit co-simulation offers a very effective way of designing
antennas. Including antenna EM models into the circuit design
environment provides greater flexibility in modeling of accompanying
circuitry without decreasing the overall simulation speed.
Parameterization and reuse of antenna models facilitates complex
designs. Automation and optimization tools offer further increase of
design productivity. The overall result is better system performance
achieved in less time, at a lower cost.
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