QRZ! Ham Radio 8/QRZ Ham Radio Callsign Database - Volume 8.iso/mac/files/ant_nec/minihelp.txt

Text File | 1996-06-25 | 10KB | 198 lines

From Jeff Jones  AB6MB
[e-mail]
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Here is some help for new Mininec users who don't have the manual...

How Mininec works is by taking the antenna elements that you specify
and divides them into segments. It uses the individual segments to
calculate the total currents and voltages for your antenna. so the
more segments the better the accuracy but the longer it takes for
Mininec to calculate. Mininec will ask you for the number of wires your
antenna has. A dipole has 1 wire and a 3 element beam would have 3
wires, one each for the reflector, driven and director elements. Each
one of those wires will be broken up into the number of segments that
you specify. 10 is generally a good number to use.

All individual wires consist of X,Y,Z coordinates. Think of yourself
facing broadside to a dipole or where you are now standing the wire
being parallel to you. The X coordinate comes out of the dipole at
a 90 angle right at you. The Y coordinate goes from left to right or
corresponds to the length of the dipole. The Z coordinate is the height of
the dipole above ground. All measurements are in meters. (multiply feet
by .3048 to convert to meters).

As an example, take the following wire dipole;

Beginning of wire         End of wire
X    Y    Z        X    Y    Z
0    0    10        0    10.19    10

This would be a 20 meter dipole at 10 meters high. Easy right? Sure you say,
how did he do that? OK here is how you do it.

Let's start with the beginning of the antenna, in this case a dipole;

First what is X? Remember X is coming out at a 90 degree angle right at you.
A dipole has nothing that comes out at a 90 angle out at you so X is 0.

Second what is Y? Remember Y corresponds to the length of a dipole. So the
starting point of the length of a dipole is zero or Y = 0.

Thirdly what is Z? Remember Z corresponds to the height of a dipole. As this
end of the dipole is at 10 meters above ground, Z = 10.

Now for the end of the antenna. X is 0 as there is nothing coming out at us
at 90 degrees. Y is now the endpoint of the dipole and as this antenna is a
20 meters dipole Y = 10.19 (meters). Z is the height of the dipole at the
endpoint and as it is the same height above ground as the beginning of the
dipole Z = 10 (meters).

This is how the dipole looks to Mininec;

                          Length
                Y = 0                       Y = 10.19
        -----------------------------
                 /            |  H
                            /            |  e
                  X = 0    /                |  i  Z = 10
                          /            |  g
                         /                  |  h
                        /                   |  t

Remember X is coming straight out of the side of the dipole at a 90 angle
right at you.

Now that you have your wire setup, the next step is the concept of
Segments. This is how Mininec breaks the wires up for it's computations.
10 segments is a good number to use to start with. Mininec computes
the currents and voltages in each segment and uses those results to
calculate the total efficiency and radiation patterns of your antenna.
The more segments the better the accuracy and the slower it's computations.

Source Pulses are where you feed your antenna at. If you have a dipole
divided up into 10 segments then to feed it in the middle you would
use segment 5. Basically a source pulse is nothing more then what segment
you feed your antenna at.

Let's use Mininec now to create a antenna model;

FREQUENCY (MHZ)?  Let's put 14 here.

ENVIRONMENT (+1 FOR FREE SPACE, -1 FOR GROUND PLANE)? Put -1 for real ground.

NUMBER OF MEDIA? Put 1 here

RELATIVE DIELECTRIC CONSTANT, CONDUCTIVITY? 13,.005 works well as this is about
average ground.

NO. OF WIRES? Put 1 here as we are modeling a dipole.

NO. OF SEGMENTS? Put 10 here as this a good number for modeling.

END ONE COORDINATES (X,Y,Z)? 0,0,10  (This is beginning end of the dipole
                       and all numbers are in meters)

END TWO COORDINATES (X,Y,Z)? 0,10.19,10   This is the other end of the dipole
                                          and 10.19 is the length and 10 is
                                          height above ground. Both in meters.

                     RADIUS? .001  This the diameter of the wire in meters.

CHANGE WIRE NO. 1 (Y/N)?  If OK put N

  CHANGE GEOMETRY (Y/N)?  If OK put N

NO. OF SOURCES? Put 1 here. This tells the program how many sources of power
                            are being applied to your antenna. Ie.. coax

PULSE NO., VOLTAGE MAGNITUDE, PHASE (DEGREES)? 5,1,0  This tells Mininec where
to put the coax at. Remember the 10 pulses? Pulse number 5 is in the middle of
your antenna. Voltage is 1 volt and forget about the phase just put zero there.

NUMBER OF LOADS? 0  This is for traps, resistors, etc...

You will see a menu with various options. Choose P - COMPUTE FAR FIELD PATTERNS

It will now say BEGIN MATRIX FILL

Nex it will now print out something like this;

PULSE 5        VOLTAGE = ( 1, 0J )
        CURRENT = ( 9.728E-03, -48.54408 J )
          IMPEDANCE = ( 68.27696, -48.54408 J  )
          POWER = 4.864229E-03 WATTS

CALCULATE PATTERN IN DBI OR VOLTS/METER (D/V)? Put D here for DBI. DBI
usually stands for DBs over a Isotropic Dipole. Substract 2-3 DBs to
get what it would be in the real world as a Isotropic dipole doesn't exist.
Dipoles are considered to have 2-3 DB gain over a Isotropic Dipolei.

Now Mininec will ask you for ZENITH angles. The Zenith goes from 0 degrees
straight up to the sky swings directly out to the horizon and then to 180
degrees below the antenna all at right angles to the antenna. This corresponds
to broadside radiation. Still not clear? Think like this, face the horizon,
put your hand straight up, this is 0 degrees straight up. Okay, now point your
hand at the horizon, this is a zenith angle of 90 degrees. Point your hand
at the ground, this is a zenith angle of 180 degrees. Now here is how Mininec
likes to see it. Put in 0 (starting angle),10 (how many degrees to increment),
10, (how many times to increment). This works out to start out at 0 degrees
pointing straight up and increment 10 degrees towards the horizon 10 times.
Or goes from 0 degrees and steps 10 degrees at a time until it reaches 90 deg.
At 0 and each 10 degrees Mininec will calculate the strength of the radiation
pattern in DB's. Also 90,-10,10 will also work. This starts at 90 degrees
straight off the side and works back up to 0 degrees. This is exactly the
same as 0,10,10 only it does it backwards. Also you can use even smaller
increments like 0,5,20. This will calculate at every 5 degrees from 0 to
90 degrees. What ZENITH really does is calculate the radiation angles
off the side of your antenna. So when Mininec asks the following;

ZENITH ANGLE : INITIAL, INCREMENT, NUMBER? Put in 0,10,10.

Now Mininec will ask you for the Azimuth angles. 0 degrees corresponds to
pointing straight out towards the horizon at 90 degrees from the center of the
dipole. Imagine you are standing facing the horizon. If you point your hand
straight out towards the horizon this is Azimuth angle of 0 degrees. Now keep
it out and move it sideways 10 degrees to your left. This a Azimuth angle of
10 degrees. If you had moved it to your right this would have been a Azimuth
angle of -10 degrees. What the Azimuth angle is really good for is seeing
if there is cloverleaf pattern off the broadside of your dipole or for seeing
the amount of front, side and back radiation on Yagi antennas. As we are
not interested in this at this time do the following;

AZIMUTH ANGLE : INITIAL, INCREMENT, NUMBER? Put in 0,0,0

FILE PATTERN (Y/N)? Put N or this will save the results into a file and you
                    will have to view the file to see the results.

Mininec will now calculate the Far Field pattern for you. You will see
5 columns named respectively; ZENITH ANGLE, AZIMUTH ANGLE, VERTICAL PATTERN (DB)
HORIZONTAL PATTERN (DB), TOTAL PATTERN (DB). Under the columns with the
(DB) after their names is the calculated amount of power in DB's your antenna
is putting out. The TOTAL PATTERN is what you want to look at. This is the
power output in DB's that your antenna is calculated to be radiating and has
the most useful figures. Due to the way the Mininec calculates using the
ZENITH ANGLE and numbers of 0,10,10 it will be backwards compared to
published antenna radiation pattern charts. 90 degrees is actually 0 degrees
and 80 is 10, 70 is 20, etc... on antenna radiation pattern charts. You can
either just convert it in your head or do put 90,-10,10 instead of  0,10,10.
Then is will correspond to antenna radiation charts.

For some more good information and help get a copy of the 1991 February QST
article called "MININEC: The Other Edge of The Sword".
Also get Volume 3 of "The ARRL Antenna Compendium" as there is a great
article called "Modeling HF Antennas with MININEC" that is really helpful.
These 2 resources will really deepen your understanding of Mininec!

Here's some good figures for RELATIVE DIELECTRIC CONSTANT and CONDUCTIVITY
for ground;

    20,.03    Very good ground    Rich soil
    14,.01    Good ground        Rich soil
    13, .005      Average ground        Heavy clay soil
 12-14, .002    Poor Ground        Cities, Industrial Areas
     5, .001    Very poor ground    Cities, Industrial Areas, High building.