Part 1: Setting Up the Model


Geometry

The first step is to set up the geometry cards. The values are in meters. The Z values on the GW (Wire specification) cards could be used to set height above ground, but use a GM (Coordinate Transformation) card instead. That will make it easier to try different elevations. Also note that the antenna can not touch the ground (no Z=0.00 values, after transformed) even when a ground plane is not included in the calculation. At least one comment (CM) card and a comment end (CE) card are required.

CM yagi4elem_2m.nec - Geometry Control Cards for 4-element Yagi
CM Tag NS X1        Y1        Z1       X2        Y2        Z2        RAD
CE
GW 1 11 -0.504E+00 0.140E+00 0.00E+00 0.504E+00 0.140E+00 0.000E+00 0.005E+00
GW 2 11 -0.481E+00 0.520E+00 0.00E+00 0.481E+00 0.520E+00 0.000E+00 0.005E+00
GW 3 11 -0.453E+00 0.711E+00 0.00E+00 0.453E+00 0.711E+00 0.000E+00 0.005E+00
GW 4 11 -0.457E+00 1.091E+00 0.00E+00 0.457E+00 1.091E+00 0.000E+00 0.005E+00
GW 5 11  0.000E+00 0.000E+00 0.00E+00 0.000E+00 1.118E+00 0.000E+00 0.022E+00
GM 0  0  0.000E+00 0.000E+00 0.00E+00 0.000E+00 0.000E+00 1.000E+00 0.000E+00
GE 1

Check that it looks correct using xnecview:
$ xnecview yagi4elem_2m.nec


Program Control Cards

We'll start by looking at the antenna in free space and sweep the frequency over a range that should include the resonate frequency. Adding these cards to the end of the geometry cards will do the job:

EX  0    2    6   00  1.000E+00 0.000E+00
FR  0    9    0    0  1.400E+02 1.000E+00
RP  0   37   36 1000  0.000E+00 0.000E+00 5.000E+00 1.000E+01 0.000E+00 0.000E+00
EN  0    0    0    0


EX sets a voltage source at the middle segment of wire #2.
FR specifies that values should be calculated at 9 frequencies, starting at 140MHz, in steps of 1MHz.
RP specifies the radiation sampling pattern. In this case there are 37 steps, 5 degrees each, around the Y-axis (0-180 deg) and 36 steps of 10 degrees around the X axis. The fifth field is actually 4 separate values, each 1 or 0, that control the output format. Details are on the RP page of the NEC2 User's Manual.

Run NEC2 and display the results. I like to use a script, attached below, to automate the process a bit:

$ ./nec_run.sh yagi4elem_2m.nec
XNECVIEW 1.35

#  freq.       Zr       Zi      SWR     gain      f/b      phi    theta

     140   25.019  -16.559  2.27933     9.03     9.91       90       90
     141   27.935  -8.3439   1.8611     9.24    10.58       90       90
     142   31.928  0.21067  1.56607     9.39    10.89       90       90
     143   37.616   9.0631  1.42196     9.49    10.85       90       90
     144   46.062   17.879  1.46112     9.55    10.56       90       90
     145   59.053   25.391  1.63417     9.56    10.12       90       90
     146   78.873   27.494  1.86763     9.54     9.61       90       90
     147    103.7   13.494  2.11946     9.47     9.09       90       90
     148   111.37   -25.63  2.37289     9.38     8.63       90       90
$

 
 


Previous - 2m Yagi Antenna Analysis
ċ
nec_run.sh
(0k)
JERRY DUNMIRE,
Apr 20, 2012, 10:17 PM
ċ
yagi4elem_2m.nec
(2k)
JERRY DUNMIRE,
Apr 20, 2012, 10:15 PM
Comments