FASTANT

 

Overview

Fastant is an advanced antenna simulation program developed under the di­rection of Professor Weng Cho Chew at the Center for Computational Elec­tromagnetics and Electromagnetics Laboratory at the University of Illinois at Urbana ­Champaign. The volume­-surface integral equation is used to model various antennas in complex environments. Using the Multilevel Fast Multi­pole Algorithm (MLFMA),1,2 Fastant can simulate very large problems, such as antennas mounted on a car.

Fastant Examples

To illustrate the use of Fastant, six examples are shown here. 

Circular Microstrip Antenna

Figure 1 shows a top ­down view of a circular microstrip antenna, with surface currents computed by Fastant. The corresponding reflection coefficient and input impedance are shown in Figure 2. The geometry and configuration of this simulation are stored in the files microstrip.facet, microstrip.vol and microstrip.input.

Figure 1: Current distribution of a circular microstrip antenna.

(a) Reflection coefficient

(b) Input impedance

Figure 2: Reflection coefficient and input impedance of a circular microstrip antenna.

Dipole Antenna

Figure 3(a) shows the current distribution and geometry of a dipole antenna of length 0.375 m at an excitation frequency of 300 MHz. The gain distribution associated with this antenna is shown in Figure 3(b).

With a hexagonal cross section, the dipole was excited by six delta ­gap sources, one at the center of each face. The potential of each source was fixed at 1 V. The excitation frequency of these sources ranged between 300 and 450 MHz, at 10 MHz intervals. Figures 4 and 5 demonstrate the variation of the reflection coefficient and input impedance as a function of excitation frequency.

Observations of the radiated field were made at polar angles θ = 30◦, θ = 60◦ and θ = 90◦. Because Fastant expects the elevation angle θe rather than the polar angle θ, one must remember that θe = 90◦ - θ. The azimuthal angle θa =-f  ranged from 0◦ to 360◦ at intervals of 1◦.

There are 942 unknown current elements in this simulation. Since this is a relatively small problem, Fastant employed L­U decomposition to invert the system matrix, rather than a fast algorithm.

Being perfectly conducting, the antenna is completely described by only the dipole.facet and dipole.input files. The example archive contains both of these files.

(a) Surface current

(b) Radiation gain

Figure 3: Surface current and radiation gain of a dipole antenna at 300 MHz.

(a) Smith chart

(b) Rectangular axes

Figure 4: Reflection coefficient of a dipole antenna at 300 MHz, on a Smith chart and on rectangular axes.

Figure 5: Input impedance of the dipole antenna at 300 MHz.

Coated Dipole Antenna

The coated dipole antenna consists of the dipole antenna described in the pre­vious section, coated with a 2 mm­ thick layer of er =3.2 dielectric material. This material is modeled by 1,880 dielectric tetrahedra.

The metallic portions of the antenna are described in coated.facet, which is identical to dipole.facet from the previous section. The dielectric portions of the antenna are specified in coated.vol. The last file associated with this simulation, coated.input, is substantially similar to dipole.input from the previous example. However, Fastant must be told that there is one material which is not perfectly conducting. In addition, the ICOAT value must be set to some positive integer (which identifies the material in the .vol file), the IBOUNDARY flag for the material must be 3 (to tell Fastant that this is a dielectric material), and the dielectric constant must be set to (3.2, 0.0).

The addition of a dielectric coating has pushed the number of unknown current elements in this example to 5,318. A simulation of this size is still not excessively large, so L­U decomposition was once again chosen for inversion of the system matrix. Figures 6, 7 and 8 show the results of this Fastant simulation.

(a) Surface current

(b) Radiation gain

Figure 6: Surface current and radiation gain of a coated dipole antenna at 300 MHz.

(a) Smith chart

(b) Rectangular axes

Figure 7: Reflection coefficient of a coated dipole antenna at 300 MHz, on a Smith chart and on rectangular axes.

Figure 8: Input impedance of the coated dipole antenna at 300 MHz.

Grounded Monopole Antenna

In this example, a 0.075 m monopole antenna is mounted on a 1 m ground plane. As in the simple dipole example, this example is composed only of perfect conductors, and requires only monopole.input and monopole.facet.

Simulations were conducted at 11 frequencies equally spaced between 800 MHz and 1.3 GHz, inclusive. Rather than rely on L­U decomposition, the system matrix was inverted with the GMRES iterative solver, accelerated by MLFMA. The GMRES restart parameter was set to 100, and the method was limited to 500 iterations. GMRES will terminate if the residual norm falls below 10−4 .

This problem is described by 2, 765 unknown current elements. When using MLFMA, Fastant divides this problem into three levels at the lower frequencies, and four levels at higher frequencies. Figures 9, 10 and 11 show the results of the simulations.

(a) Surface current

(b) Radiation gain

Figure 9: Surface current and radiation gain of a monopole antenna at 300 MHz, over a ground plane.

(a) Smith chart

(b) Rectangular axes

Transmission Lines: Port Currents

With the use of a .load input file, Fastant can compute the currents passing through ports specified on a simulated structure. To demonstrate this, consider the basic transmission line shown in Figure 12. This line is used in three distinct simulations: a standalone line, two parallel lines, and perpendicular sets of two parallel lines. Fastant was used to calculate the currents on the lines and ground planes, in addition to the port currents (specified at the ends of each line).

Figures 13, 14 and 15 show the surface and port currents associated with one, two and four transmission lines, respectively. These correspond to example files with base names of oneline, twolines and fourlines. Each simulation requires .input, .facet, .vol, .wire and .load files to produce the appropriate output.

Other parameters, such as input impedance and reflection coefficient, are also calculated for each of these examples. However, figures displaying these pa­rameters have been omitted. They may be reproduced using the example files distributed with this manual.

(a) Surface currents

�.(b) Port currents

�.(a) Surface currents

�.(b) Port currents

�.(a) Surface currents

�.(b) Port currents

 

Isolated XM Antenna

In later examples, an XM antenna is simulated in the presence of more compli­cated objects. It is thus useful to examine the isolated antenna in detail. The bare XM antenna is shown with its surface current distribution in Figure 16.

Fastant is also capable of simulating the XM antenna when covered with a radome. Figure 17(a) shows the current distributed excited on the radome. In Figure 17(b), the radome was removed from the plot, showing the current distribution on the antenna in the presence of the radome.

Figure 16: Current distribution of an isolated XM antenna.

(a) Radome

(b) Antenna (a) Reflection coefficient

(b) Input impedance

XM Antenna on a Camaro Roof

The first (and simplest) of the complicated XM simulations places an XM an­tenna at the rear center of a metallic Chevrolet Camaro roof. Because the structure has been extended significantly, the number of unknown current ele­ments has increased to 29,708, making this a substantial problem. Figure 19 shows a view of the simulation geometry (with induced surface currents) and the corresponding radiation patterns of the antenna­ roof structure. The an­tenna structure shown in Figure 16 is located within the yellow ring on the roof. The reflection coefficient of the antenna is plotted in Figure 20, while Figure 21 shows the antenna’s input impedance on this structure. The basic Fastant in­put configuration and problem geometry are described in the files roof.input, roof.facet and roof.vol.

This example makes use of the self box interaction (SBI) method to directly invert the portion of the system matrix corresponding to the antenna. This results in increased accuracy at the center of activity of the structure. The self box is described in the file roof.sbi.

(a) Surface current

                        (b) Radiation gain

                        (a) Smith chart

�.(b) Rectangular axes

 

XM Antenna on a Camaro Body without Chassis

Extending the previous structure further, this example details the simulation of an XM antenna on the roof of the entire Camaro body. This structure contains neither the car’s chassis nor its wheels. The number of unknown current elements in this problem has jumped to 205,211. The induced currents and corresponding radiation patterns are displayed in Figure 22. The reflection coefficient and input impedance are shown in Figures 23 and 24, respectively.

The simulation files for this example are camaro­nc.input, camaro­nc.facet, camaro­nc.vol and camaro­nc.sbi. Running this simulation will take several hours on a fast workstation.

(a) Surface current

                        (b) Radiation gain

                        (a) Smith chart

�.(b) Rectangular axes

 

XM Antenna on a Complete Camaro

Finally, the Camaro is completed by adding a chassis and wheels. The XM an­tenna retains its position at the rear of the roof. At a frequency of 2.345 GHz, this problem requires 508, 089 unknown current elements for each simulation. The memory required to simulate the problem was in excess of 3.5 GB. Be­cause of the size and complexity of this simulation, it was only run for one frequency. Consequently, frequency ­dependent plots of reflection coefficient and input impedance are unavailable. Figure 25 shows the induced currents and radiation patterns for this simulation.

The files camaro.input, camaro.facet, camaro.vol and camaro.sbi are re­quired by this example. This simulation may be expected to take twice as long as the previous example. While the memory requirements may be prohibitive on a typical desktop computer, this example is not a demanding task for a supercomputer.

(a) Real surface current

(b) Radiation gain

 

REFERENCES:

 

  1. Chao, HY; Chew, WC; Lu, CC: A fast volumesurface integral equation solver for radiation and scattering from wire antennas, impedance surfaces and inhomogeneous dielectric objects. IEEE Antennas and Propagation Society International Symposium (2002) 16–21.
  2. Chew, WC; Jin, JM; Michielssen, E; Song JM, Fast and Efficient Algorithms in Computational Electromagnetics, Artech House, MA, 2001.