  Narrow antiresonances going to zero transmission are found to occur for
general (2n,0)(n,n)(2n,0) carbon nanotube heterostructures, whereas the
complementary configuration, (n,n)(2n,0)(n,n), displays simple resonant
tunneling behaviour. We compute examples for different cases, and give a simple
explanation for the appearance of antiresonances in one case but not in the
other. Conditions and ranges for the occurence of these different behaviors are
stated. The phenomenon of anti-resonant tunneling, which has passed unnoticed
in previous studies of nanotube heterostructures, adds up to the rich set of
behaviors available to nanotube based quantum effect devices.
