Part 1: Setting Up the Model
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.
Check that it looks correct using xnecview:
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 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: