I've got to disagree with WIRELESS's statement as a general one. I've modeled several antennas that work very much like the model.
In this particular case, though, I am going to have to agree with him and say cut the conduit piece long. Maybe you can trim the bottom of it.
You could probably work the model difficulties out (and I do think you're having model difficulties) but I don't know if you can with the demo version.
If you want to work on the model for modeling reasons, look at the antenna modeling tips at www.cebik.com
, to get an idea of the limitations of the program.
If you're more interested in the final antenna, cut and try is going to be easier and faster in this case.