Large fully steerable reflector-type antennas have gravitational deformations of main mirror surface and supports of subreflector. These deformations are variable depending on elevation angle and may reduce antenna gain on high frequencies significantly. To prevent the gain fall the homologous principle is employed at large reflectors design, according to which a main mirror has different focal length and position at various elevation angles. Accordingly, a subbreflector should be moved to proper location at each antenna angle position. This paper presents algorithms of experimental determination of optimal subreflector position at elevation angles range providing maximum gain for given antenna. Results of radioholography measurements and the data of mirror system geometry are used as basis.