Hacker Chronicles 2/HACKER2.BIN/349.MN.DOC

Text File | 1988-02-08 | 14KB | 266 lines

              MININEC Antenna Analysis

    MN is an attempt to make the MININEC antenna analysis program easier to
use, especially by amateur radio operators.  No changes have been made to the
modeling algorithm contained in MININEC version 10, as released by the Naval
Ocean Systems Center (except to fix a bug involving S-parameter loads), but
substantial changes have been made in the way the program interacts with the
user.  This note is just intended to help you with some of the new features.
The easiest way to learn to use the program in the first place is by trying it
out on the sample antenna files provided.

                New Features

    The most important new feature is a provision for antenna files.  This
allows the antenna characteristics to be captured from a text file instead of
from the keyboard.  This saves you the considerable trouble of reentering a
complicated antenna geometry by hand every time you start up the program.  It
allows you to interrupt your work, save it away, and come back to it later when
it is convenient, without having to start from scratch. It also allows you to
build up your own library of designs, from which you can easily retrieve and
modify antennas for new applications.  You can use any text editor to create
the antenna file, as long as it comes out in ordinary ASCII characters.
Commands are available for listing, loading, viewing, and editing antenna
files.

    If you have a Hercules Graphics Card, or compatible monochrome graphics
card, you can plot the azimuth and elevation directive patterns on your screen
in polar form using the optional Plotting Program.  The standard ARRL log dB
plotting scale is used, so that your patterns can be directly related to those
in amateur antenna publications.  The plots are saved automatically and can be
viewed at a later time without redoing the analysis.  A comparison mode allows
you to switch screens instantly between the plots of two antennas, permitting
very revealing comparisons.

    A new command has been added to compute gain, front-to-back ratio,
maximum sidelobe level, and beamwidth.  You no longer have to extract these
antenna parameters manually from the directive pattern data.

    You may specify the antenna dimensions in feet, inches, meters,
centimeters, or millimeters.  Elevation angle (the angle with respect to the
horizon) is used instead of zenith angle (the angle with respect to overhead).
A file containing a record of the data generated during the antenna analysis is
automatically created and given the same name as the antenna file, with the
extension .RUN.

    The all-capital-letters printing style has been done away with, and
upper or lower case is accepted from the keyboard.  Defaults are provided for
most keyboard inputs and are indicated by [brackets].  Entering just a carriage
return selects the default.

    MN will run either with or without an 8087 math coprocessor chip.  If
it takes more than one minute to fill and factor the mutual impedance matrix,
the program will beep when it is done to alert you that results are ready.

                Antenna Files

    An antenna file must have the extension .ANT.  The antenna file can be
specified as a command line parameter when starting the program; otherwise you
will be prompted for it.  In either case you need not enter the .ANT extension,
it will be supplied by the program.  The format is illustrated below by a
sample file for a 3 element beam.  No comments are actually permitted within a
file, but they may be added freely at the end.


3 element Yagi        {1 line title for the antenna}
free space    {Can be any other character string for antennas over ground}
24.94 MHz        { "Hz", "KHz", or "GHz" may also be used}
3 wires, inches      { "feet", "meters", "centimeters", or "millimeters"}
10  0,-111.189,0  0,111.189,0  1    {For each wire: # segments, XYZ coords
10  -69,-117.666,0  -69,117.666,0  1     of each end, radius}
10  73,-108.144,0  73,108.144,0  1
1 source        {Number of sources (feedpoints)}
5,100,0            {Segment number, voltage, phase for each source}
0 loads            {Number of loads}


    The numbers can be separated by any combination of spaces and commas,
but not by tabs.  The words "wires", "source", and "loads" actually may consist
of any character strings, but must be present.  Everything is case insensitive.

    X and Y are in the horizontal plane and +Z is up.  0 deg azimuth angle
is in the +X direction, and 90 deg azimuth is in the +Y direction.  The horizon
is at 0 deg elevation angle and +90 deg is overhead.  Unidirectional antennas
are assumed to be aimed in the +X direction by the subroutine that computes
gain, F/B, max sidelobe level, and beamwidth.  The Z coordinate can be set to 0
if only free space modeling is performed.

                Segments and Wires

    MN allows you to specify how many segments each wire is divided into
for analysis purposes.  The more segments used the higher the accuracy, but the
longer the analysis takes.  The number required depends on the complexity of
the antenna and the accuracy of the results required.  Generally 6-10 segments
per half wave are adequate for most purposes.  However, for each antenna you
should verify that the number of segments chosen is adequate by trying a larger
number to see if the results change significantly.  The number of wires is
limited to 50, and the total number of segments to 100 (twice the number of
segments as the original MININEC).

    Segments include internally generated logical jumpers between the ends
of individual wires, as well as wire sections having finite length and carrying
current.  Segments are also called pulses.  Two wires having their ends at the
same XYZ coordinates will be considered to be connected by MN, and current will
be allowed to flow between the two wires as if they were soldered together.
For example, each loop of a cubical Quad antenna consists of 4 wires whose
endpoints share some common coordinates.

                Sources

    Feedpoints are called sources and are permitted at any pulse.  You must
have the feed symmetry in mind when you divide the driven wires into segments.
In the Yagi example above, the driven element uses an even number of segments
(10).  One is consumed internally by MN, leaving an odd number for antenna
currents (9), and thus a single central pulse at the feed point (pulse #5).
The first time an antenna file is run the .RUN output file generated by MN
should be examined to verify that the pulses have been distributed and numbered
the way you intended.  In fact, the easiest way to figure out which pulse
number to specify as the feed point is by making an initial guess, loading the
antenna file, quitting, and then examining the antenna geometry section of the
.RUN file to see how the segments really were allocated.

                Loads

    You may specify up to 50 loads for one antenna.  This number is large
to allow modeling of CCD antennas having many capacitors.  Here is an example
of an antenna using one resistive load:


ZL1ACW's big rhombic
free space
24.94 MHz
4 wires, feet
25   -182.7 0 35      0 81.35 35   .00337    {.00337 is the radius of
25   0 81.35 35       182.7 0 35   .00337     #12 wire in feet}
25   182.7 0 35       0 -81.35 35  .00337
25   0 -81.35 35      -182.7 0 35  .00337
1 source
100,100,0
1 load
resistor    {This may be anything other than "s-parameter" for RL/RC loads}
50,740,0    {Pulse #, load resistance, load reactance}


    S-parameter loads may be specified also.  This allows complex frequency
dependent loads to be modeled, unlike RL/RC loads whose reactance values do not
vary with frequency.  Here is an example of one:


W3DZZ trap dipole for 80 through 10 meters
free space
14.150 MHz
4 wires, feet
10  0 -54 0   0 -32 0  .00337
10  0 -32 0   0   0 0  .00337
10  0   0 0   0  32 0  .00337
10  0  32 0   0  54 0  .00337
1 source
20,100,0
2 loads
s-parameter
10,2        {Pulse #, order of s-parameter function}
0 1        {Numerator, denominator coefficents of s^0}
8.2 0        {    "           "          "       "  s^1}
0 4.92E-4    {    "           "          "       "  s^2}
30,2        {Second load ... }
0 1
8.2 0
0 4.92E-4    {Scientific notation allowed}

    The traps for this antenna are 8.2uH in parallel with 60 pF.  The
S-parameter representation for a parallel LC circuit is:

    Ls/(1+LCs^2)

    L should be in uH and C in uF, since the program works in MHz
internally.

                Real Ground

    For antennas over ground the program will ask a series of questions to
establish the ground characteristics.  You can specify up to seven different
ground "media", each having its own dielectric constant and conductivity,
distance from the antenna, and height.  In addition, MN can accomodate ground
screens consisting of radials.  To model radials, enter 2 for number of media,
circular for boundary type, and the length of the radial wires for the X
coordinate of the next media interface.  Enter earth dielectric constant and
conductivity for both media.  The effective impedance of the ground screen is
then added in parallel to that of the first ground media.

    Usually the most suitable reference for an antenna modeled over real
ground is not a dipole in free space, but a dipole or monopole having the same
polarization over the same kind of earth, at the same height, and operating at
the same frequency.  Earth reflection coefficients vary with polarization and
frequency, as well as with dielectric constant and conductivity.  Expect to see
negative gains in dBd terms at very low elevation angles, using realistic earth
characteristics, for antennas that exhibit gain in free space.  (Remember that
dBd refers to a dipole in free space, not to a dipole substituted for the test
antenna.)  In particular, keep in mind that vertical antennas have very little
response at elevation angles near 0 deg over real ground (even over salt
water).  The same is true for horizontal antennas over all types of ground
unless they are very high.  The file EARTH.DOC is included as a guide for
choosing the conductivity and dielectric constant of your particular earth when
modeling antennas over real ground.

                Gain & F/B

    The routine which finds beamwidth and maximum sidelobe level, and which
produces azimuth data for the plot, only searches the radiation pattern from 0
to 180 degrees.  To save time, it does not search the 0 to -180 degree half of
the pattern, which is assumed to be symmetric.  For antennas which do not
possess mirror symmetry, you can make a second run with the Y coordinates of
all wires negated to analyze the antenna in the missing half plane.  The
pattern search is done in 2 degree steps, and for an elevation angle of 0
degrees for free space models.  For antennas over ground, it prompts you for
the elevation angle to use while searching.  If the maximum lobe is found at 0
degrees azimuth angle (the normal case for unidirectional antennas aimed in the
+X direction) then the 3 dB beamwidth is displayed.  If a sidelobe is found
which is larger than the rear lobe then its level and angle are displayed.  If
the main lobe is found at an angle other than 0 its level and angle are shown.

    The radiation pattern used in all cases is the total pattern, which is
the RMS sum of horizontal and vertical components.  This is a realistic model
for evaluating HF antenna performance on randomly polarized incoming skywave
signals.  It also facilitates the analysis of imperfect antennas.  For example,
quads show only about 25-30 dB front-to-side ratio due to incidental vertically
polarized radiation from the out-of-phase sides of the quad loops.  In
addition, the performance of Beverage antennas can be accurately modeled, since
this antenna consists of a long horizontal wire which responds principally to
vertically polarized signals, where the pickup of horizontal fields may reduce
directivity.  Note that for linearly polarized antennas the magnitude computed
for orthogonal polarization terms is typically down more than 130 dB in MN
analysis, so that the use of the total horizontal and vertical field will not
lead to inaccurate results where only the response at one polarization is
desired.

                Aborting Calculations

    Because some of the calculations can be quite time consuming, a
provision has been made for aborting from various program loops by pressing the
<Esc> key.  Aborting a calculation will not disturb any calculations already
completed, and the order of calculation has been arranged so that the most
interesting results are available first.  This allows you to use the abort
feature to get faster results, as well as to escape from command mistakes.  For
example, if you are only interested in obtaining input impedance, gain, or F/B,
you may hit <Esc> after they are displayed to terminate the azimuth pattern
search.  This search is done to find the beamwidth and maximum sidelobe level,
and also generates data for the azimuth plot.

                Editing Antenna Files

    Your favorite text editor or word processor may be invoked from within
MN to edit the current antenna file.  The name of your editor should be placed
in a file called MN.CFG.  MN will append the current antenna filename to the
editor name and execute this string as a DOS command whenever the E command is
entered.  After you exit the editor MN will reread the antenna file.  All
antenna parameters will be reset to those in the newly edited file, so any
temporary changes made using the Change commands will be lost.  The .RUN file
is also overwritten each time the antenna file is read back in.



                    Brian Beezley, K6STI
                        507½ Taylor St.
                    Vista, CA 92084