The W8IO Antenna Site - LPCAD software

(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 "File" menu to find and open the LP200500.nec file.  This is what you should see after opening the file for the first time.

LP200500 in 4NEC2

Select "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, Stop 500, Step 2, Theta Forward 90, Theta Backward -90, then press "Generate".

LP200500 in 4NEC2

After you press "Generate", 4NEC2 opens the input file and begins the calculations.  Your screen should look similar to the one below until done.

4NEC2 calculations

After the calculations are done, you should see this display:

4NEC2 results

In the VSWR sweep, you can see 2 very sharp "spikes", one near 320 MHz and another near 420 MHz.  These are anomolous resonances that come from the impedance of the rear short transformed through 1/2 wavelength of the feeder to the approximate location of the active elements at these frequencies.

Below, you can view the Gain and F/B sweeps of this antenna.

4NEC2 curves

You can notice that these "spikes" also occur in the gain and F/B at roughly the same frequencies.  The pattern shown on the left shows the increased backlobe at 324 MHz, near one of the "spikes".  For most Ham uses, these "spikes" can be moved to frequencies where they don't cause any problems.  To move the spikes, move the location 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 minimal change in gain & F/B, then you should redesign the LP by using a Tau of 0.93 or higher.

Here is the Smith Chart display for this antenna. Notice that the impedance nicely circles around 50 ohms!

Smith Chart

For information on StressCAD, click here.

Comments are welcome!

contact Roger: email to 

rgcox2 (at) gmail.com

Roger Cox W8IO - Spring Lake, MI