A lot of the questions in the forum can be answered by playing with antenna design. This venerable program, originally written in Fortran, used to cost thousands of dollars and was classed as a munition - making it illegal to export (without permission). Here it is now free but still valid.
References & History
Antennae are fascinating and easier to understand if you can play with them. In the real-world you have to be 10 wavelengths away from any unit under test - and there are too many rainy days here. Using this venerable tool you can enter elements and see graphical and 3D plots of the fields - valid for receive and transmit.
There are a lot of dead ends with this package and plenty or derivitive works.
4nec2 is a completely free Nec2, Nec4 and windows based tool for creating, viewing, optimizing and checking 2D and 3D style antenna geometry structures and generate, display and/or compare near/far-field radiation patterns for both the starting and experienced antenna modeler.
When running frequency sweeps, linear or logarithmic style SWR, Gain, F/B-ratio and impedance line-charts are produced. With the included Optimizer and Sweeper one is able to optimize antenna- and/or other environment-variables for Gain, resonance, SWR, efficiency and/or F/B, F/R-ratio. With the sweeper one is able to graphically display the effect of changing one or more of these variables for a specified range of values/frequencies.
For the starting modeler a graphically based 3D geometry-editor is included which requires no additional NEC knowledge while still enabling you to create and visualize and compare current-distribution, far/near-field patterns and Gain/SWR charts. More experienced modelers can use the gradient style and/or the genetic algorithm based optimizers to improve their designs.
To list some of its features or see some screenshots, click the appropriate links below on the left. To download and try the software click upmost two links below on the right (setup.exe) or the third link for a single zip containing all separate files.