  The Heisenberg antiferromagnetic spin chain with both dimerization and
frustration is studied. The classical ground state has three phases (a Neel
phase, a spiral phase and a colinear phase), around which a planar spin-wave
analysis is performed. In each phase, we discuss a non-linear sigma model field
theory describing the low energy excitations. A renormalization group analysis
of the SO(3) matrix-valued field theory of the spiral phase leads to the
conclusion that the theory becomes $SO(3) \times SO(3)$ and Lorentz invariant
at long distances. This theory is analytically known to have a massive spin-1/2
excitation. We also show that $Z_2 ~$ solitons in the field theory lead to a
double degeneracy in the spectrum for half-integer spins.
