23rd Annual Review of Progress in Applied Computational Electromagnetics

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23rd Annual Review of Progress in Applied Computational Electromagnetics
March 19-23, 2007 - Verona, Italy ©2007 ACES
Interfacing FEKO and MATLAB for Microstrip Antenna Design
Randy L. Haupt
The Pennsylvania State University
Applied Research Laboratory
P. O. Box 30
State College, PA 16804-0030
[email protected]
Abstract: This paper provides details on how to use MATLAB as an effective interface to FEKO. Code
to implement this interface and an example are presented.
Keywords: MATLAB, FEKO, optimization, microstrip antenna
1. Introduction
Combining FEKO's [1] electromagnetic modeling power with MATLAB's [2] signal processing and
graphics capability, results in a powerful computational electromagnetics tool. These software tools can
be run using command line files. Some situations that are advantageous to combine MATLAB and FEKO
include antenna design optimization, adaptive antennas, antenna placement on a vehicle, and radar cross
section modification. The next section describes how to combine MATLAB and FEKO and some specific
commands that make it all happen. These commands are used in a demonstration example to design a
microstrip patch antenna.
2. Calling FEKO from MATLAB
MATLAB and FEKO make use of the ASCII files shown in Table 1. All the files have the same name, so
they are easy to associate with the same project. The extensions differentiate their use. The MATLAB
commands are in mpatch.m. Data written from MATLAB to be used by FEKO is stored in the mpatch.txt
file. FEKO commands are in the mpatch.pre file. Data written by FEKO for use by MATLAB is written
in the mpatch.ffe and mpatch.out files. Figure 1 displays a flowchart of the MATLAB-FEKO software
performance. The flowchart boxes contain the operation to be performed, while the software package that
performs the operation is listed to the right of the box.
Table 1. Relevant computer files.
File name
contents
mpatch.m MATLAB m file
mpatch.txt ASCII file with variable values
mpatch.pre PREFEKO file
mpatch.out FEKO output file
mpatch.ffe FEKO file with angles, electric field, gain
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23rd Annual Review of Progress in Applied Computational Electromagnetics
March 19-23, 2007 - Verona, Italy ©2007 ACES
Figure 1. MATLAB-FEKO flowchart.
Assume that MATLAB has created an antenna design in a way that all the (x,y,z) coordinates of
the design are known. These coordinates are written to the text file, mpatch.txt using
fid=fopen('mpatch.txt','w');
N=length(xa);
fprintf(fid,'%6.2f\n',N);
for iq=1:N
fprintf(fid,'3%6.2f\n',xa(iq),ya(iq),za(iq));
end
fclose(fid);
The file, mpatch.txt, has N+1 lines. The first line contains the number of points. The following N lines
contain the coordinates of the points.
Next, MATLAB draws a picture of the antenna design and labels the points. In this example, the
antenna is a microstrip antenna (Figure 2).
figure(1);plot(xa,ya,'-o');axis equal
for ii=1:length(xa)
text(xa(ii),ya(ii)+za(ii),num2str(ii))
end
This part of the code is not necessary, but is quite helpful in getting the geometry correct in FEKO.
The following commands run PREFEKO and FEKO from MATLAB:
!prefeko mpatch > output.txt
!runfeko mpatch > output.txt
The "> output.txt" part of the commands places output generated in the running of PREFEKO and
FEKO into a file rather than displaying them on the computer screen.
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23rd Annual Review of Progress in Applied Computational Electromagnetics
March 19-23, 2007 - Verona, Italy ©2007 ACES
The lines in PREFEKO that read from the text file and create the points defining the outline of the
antenna are given by
#N= fileread("mpatch.txt",1,1)
!!for #i = 1 to #N
#ax[#i]= fileread("mpatch.txt",1+#i,1)
#ay[#i]= fileread("mpatch.txt",1+#i,2)
#az[#i]= fileread("mpatch.txt",1+#i,3)
DP
p#i
#ax[#i]
!!next
#ay[#i]
#az[#i]
The defining points on the antenna are labeled p1 to pN. After these points are formed in FEKO, then the
structure is built out of triangles, polygons, wires, etc. FEKO performs the calculations and writes the
output to mpatch.out and possibly to other files, such as mpatch.ffe. MATLAB can easily read
the ASCII file where the far field information is written using the textread command
[t,p,rEt,iEt,rEp,iEp,gt,gp,g]
=textread('mpatch.ffe','2%f (%f,%f) (%f,%f) 3%f');
Reading from mpatch.ffe is more difficult but possible using m-files downloaded from the MATLAB
website [2].
3. Microstrip Antenna Design Example
As an example, a rectangular microstrip patch antenna is designed using FEKO-MATLAB
interface. The goal is to minimize s11 by varying
Lx
Patch length in x-direction
Ly
Patch width in y-direction
w
g
Microstrip feed line width
Length of inset gap in x-direction
The first run minimized s11 at 2 GHz. The resulting patch had the dimensions (in mm):
[Lx Ly cp gp]= 51.1612
34.2412
1.4555
18.5485
The second run minimized s11 from 1.99 to 2.01 GHz. The resulting patch had the dimensions:
[Lx Ly cp gp]= 51.5962
36.8475
1.6520
17.2615
A plot of s11 over a 1% bandwidth is shown in Figure 3. A genetic algorithm combined with
FMINSEARCH in MATLAB performed the optimization. As expected, attempting to get 1% bandwidth
from this structure is very difficult. The steps used to perform this simple optimization apply to much
more complex antenna designs.
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23rd Annual Review of Progress in Applied Computational Electromagnetics
March 19-23, 2007 - Verona, Italy ©2007 ACES
Figure 2. Figure drawn by MATLAB before passing point to FEKO.
Figure 3. s11 when patch optimize at 2 GHz (dashed line) and from 1.99 to 2.0 GHz (solid line).
4. Conclusions
Combining FEKO and MATLAB has many advantages. MATLAB has signal processing and
graphics capabilities that are useful while FEKO has the electromagnetic computational engine needed for
antenna design. This paper showed a simple example of using MATLAB-FEKO to design a rectangular
microstrip patch antenna.
References
[1] FEKO Suite 5.1, EM Software and Systems (www.feko.info), 2005.
[2] MATLAB Version 7.3.0.267, The (www.mathworks.com), Aug 3, 2006.
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