For the standard methods you can use any of the three text books.
- Taflove
- Sullivan
- Kunz&Luebbers
(searching google with the names above and FDTD should give you links to the books)
For a more accurate method look at
**broken link removed**
and download the paper:
# D. J. Robinson and J. B. Schneider, "On the Use of the Geometric Mean in FDTD Near-to-Far-Field Transformations," IEEE Trans. Antennas Propagat. vol. 55, no. 11, pp. 3204--3211, 2007. (336 kB PDF file.)
PS. Don't trust the code in Sullivan's book. There are a number of errors (some parts even don't compile). Furthermore it is not easy to modify his code. Writing your own code from scratch is
probably faster.