Antenna Site -
(16 July 2015)
Example 1 - 4NEC2 Analysis
On page 1, I created and saved
the NEC file "LP200500". After you start 4NEC2, go to the
menu to find and open the LP200500.nec file. This is what you
should see after opening the file for the first time.
"Calculate" from the 4NEC2 menu bar, then select "NEC Output Data".
You should see something similar to the screenshot below.
Select "Frequency Sweep", "Hor.", "0:Power Gain", Start 200,
500, Step 2, Theta Forward 90, Theta Backward -90, then press
you press "Generate", 4NEC2 opens the input file and begins the
calculations. Your screen should look similar to the one
After the calculations are
done, you should see this display:
the VSWR sweep, you can see 2 very sharp "spikes", one near 320 MHz and
another near 420 MHz. These are anomolous resonances that
from the impedance of the rear short transformed through 1/2 wavelength
of the feeder to the approximate location of the active elements at
Below, you can view the Gain and F/B sweeps of this antenna.
can notice that these "spikes" also occur in the gain and F/B at
roughly the same frequencies. The pattern shown on the left
the increased backlobe at 324 MHz, near one of the "spikes".
most Ham uses, these "spikes" can be moved to frequencies where they
don't cause any problems. To move the spikes, move the
of the rear termination - closer to the rear element moves the "spikes"
higher in frequency - farther away from the rear element moves them
lower. If your application requires no "spikes" or very
change in gain & F/B, then you should redesign the LP by using
Tau of 0.93 or higher.
Here is the Smith Chart display for this antenna. Notice that the
impedance nicely circles around 50 ohms!
Roger: email to
Roger Cox W8IO - Spring