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[antennaware] NEC2 Visualization

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Subject: [antennaware] NEC2 Visualization
From: (Kenneth Earl Harker)
Date: Thu, 30 Apr 1998 10:33:16 -0500 (CDT)
     I am working on a term project in my visualization (computer graphics)
class this semester involving the visualization of certain aspects of the 
radiation pattern of antennas modeled with NEC2.

     At the moment, we have created a small application with a main window
in which the graphics happen and a few buttons and slider bars and whatnot 
below it.  The program loads in a .nec file from disk, draws the antenna
in the main window from the data in the GW cards, and puts in boom, antenna
mast and groundplane objects in the case of a yagi (I haven't tried this with 
other types of antennas just yet.)  You click on a button, wait about three
minutes for the NEC2 code to chug away and output a 16MB data file and for our
program to parse it into memory, and sooner or later a cloud of small cubes
shows up around the antenna - each little cube represents a point x that is 
c * dBi away from the center of the antenna where dBi is the value NEC2
computes for that particular az and el angle from the center and c is constant.
In other words, the cloud of points shows the antenna's pattern in 3D.
Our application lets the user fly around and view the antenna and its 
pattern from any angle in 3D.  (We also allow the user to adjust the shape and 
position of wires in the antenna's geometry with mouse movement, but that's
sort of unrelated to this post.)  The next step is to stitch together these 
control points into a single surface and render it smooth shaded, which will 
make it easier to see the shape.

    But there's more that we can visualize from the output that NEC2 gives 
us.  One thing I think we can do very easily is color each point in our pattern
one of three colors depending on the polarization sense at that az/el angle.
I've noticed in one output file we've gotten from a 6-element yagi for 50MHz
that we ran at a one-degree resolution in both azimuth and elevation (yes,
this produces one _big_ output file) that while the vast majority of the 
output lines were LINEAR sense, there are occasional lines of RIGHT and LEFT
circular sense.  I think it would be interesting to see if there's a 
pattern there - maybe certain minor lobes of this particular antenna's pattern 
are circularly polarized?

    Now, to my questions; I'm not especially knowledgable about antenna
modelling, having learned most of what I know for this particular
project.  Given output like what I'm attaching below, what would you
like to see graphically visualized?  Gain, of course, we're already doing,
and polarization sense is probably easy.  Would visualizing POLARIZATION
TILT be useful?  PHASE DEGREES maybe?  Any other ideas?

                                                - - - RADIATION PATTERNS - - -

  - - ANGLES - -           - POWER GAINS -       - - - POLARIZATION - - -    - 
- - E(THETA) - - -    - - - E(PHI) - - -
  THETA     PHI        VERT.   HOR.    TOTAL      AXIAL     TILT   SENSE     
 DEGREES  DEGREES       DB      DB      DB        RATIO     DEG.              
   90.00      .00    -999.99   18.86   18.86     .00000   -90.00  LINEAR    
0.00000E+00      .00    7.20823E+00    98.52

Also, is there an easy way to determine in a .nec input file where the 
electrical center of the antenna is?  I know the output data, specified in
az/el angles is centered on the electrical center of the antenna, NOT the 
physical center (which is easy to determine.)  If I can find the electrical
center in the same coordinate space as the geometry of the wires, that would
be very useful to know.

Finally, the application we're working on is currently only running on Silicon
Graphics IRIX boxes; it should be possible to recompile it without 
modification for Linux boxes, and in principle there's no reason we couldn't 
get it run on a Windows box eventually.  As we get closer to the end of the 
semester, I'll get a web page together with some screen shots.

Kenneth E. Harker      "Vox Clamantis in Deserto"
University of Texas at Austin                  Amateur Radio Callsign: KM5FA
Department of the Computer Sciences         President, UT Amateur Radio Club
Taylor Hall TAY 2.124               Maintainer of the Linux Laptop Home Page
Austin, TX 78712-1188 USA  

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