I checked all threads on the subject. General view is that Sullivan's code (when implemented without typos) shows a similar spectrum to the real thing (experiment, HFSS, original fdtd result by Sheen, published in IEEE, 1991), but the frequencies are incorrect. For instance the main mode at 7.5 GHz (correct value) comes at ~ 6.2. This is quite dissapointing as the book is actually quite good (although full of mistakes), and I could not so far find an alternative code (even tried to contact Mr. Sheen, but have not recieved yet any reply).
If you know an alternative book with a similar code, I would be really interested to look at it.
best regards
feodor