G general theory is presented for studying the influence of reflections on
mode competition in gyrotrons, Strong effect is found in the illustrative e
xample: the coaxial cavity TE28,16 gyrotron operating close to the optimal
point in the general parameter space. In some cases, the fixed-field approx
imation and the self-consistent approach lead to qualitatively different pr
edictions, which emphasizes the need for self-consistent calculations in th
e final stage of design of a gyrotron.