  We discuss spin-$\frac12$ two-dimensional (2D) Heisenberg antiferromagnet
(AF) on a square lattice at T=0 in strong magnetic field H near its saturation
value $H_c$. A perturbation approach is proposed to obtain spectrum of magnons
with momenta not very close to AF vector in the leading order in small
parameter $(H_c-H)/H_c$. We find that magnons are well-defined quasi-particles
at $H>0.9H_c$ although the damping is quite large near the zone boundary. A
characteristic rotonlike local minimum in the spectrum is observed at ${\bf
k}=(\pi,0)$ accompanied by decrease of the damping near $(\pi,0)$. The
suggested approach can be used in discussion of short-wavelength excitations in
other 2D Bose gases of particles or quasi-particles.
