xnec2c is a GTK+ graphical interactive version of nec2c. It
incorporates the nec2c core which it uses for reading input files
and calculating output data, but it does not need and indeed does
not produce an output file by default. Since xnec2c incorporates
the nec2c core, it has access to all internal buffers, including
structure data, frequency-related data (structure currents, input
impedance, gain etc) and radiation pattern data (for the far field
and near field). It therefore has the ability to graphically display
user-requested data directly, as the frequency loop progresses or
after input from the user. Graphs of frequency-related data and
the current or charge distribution evolve as the frequency loop
progresses, and radiation patterns (far and near field) are
sequentially drawn for each frequency step. A new frequency can be
entered by the user from spin buttons in the main or radiation
pattern windows or by clicking on the graphs of frequency-related
data. New output data are then computed and displayed in text and
graphical form.

Homepage:
https://www.qsl.net/5b4az/pages/nec2.html
