A calibration network system for an array antenna having a plurality of antenna elements and a plurality of Transmit/Receive (T/R) modules coupled to the antenna elements through a plurality of corresponding coaxial transmission lines, the calibration network system characterized by: a waveguide arrangement (14; 314) comprising at least one pair of rectangular waveguides (141 and 142; 21 and 22; 31 and 32) having an inside height of side-walls smaller than an inside width of top and bottom walls; said at least one pair of rectangular waveguides being stacked in parallel relation to each other to form a lower rectangular waveguide (142; 22; 32) and an upper rectangular waveguide (141; 21; 31) spaced apart from the lower rectangular waveguide (142; 22; 32) at a distance of about a quarter of a desired operating wavelength of the array antenna; said at least one pair of rectangular waveguides (21 and 22) including a plurality of circular through-holes (23; 33) extending through at least one side-wall (24; 34) of the stacked rectangular waveguides from a bottom wall (221; 321) of the lower rectangular waveguide (22; 32) to a top wall (212; 312) of the upper rectangular waveguide (21; 31) and having a diameter sufficient to accommodate coaxial transmission lines passing through said circular through-holes (23; 33); said at least one side-wall having openings (27; 37) between said circular through-holes (23; 33) and an interior regions (26 and 25; 36b and 36a) of said lower (22; 32) and upper (21; 31) rectangular waveguides, correspondingly, in order to provide coupling of the coaxial transmission lines into said at least one pair of rectangular waveguides (21 and 22; 31 and 32); at least one 90 degree phase shifter having two phase shifter terminals (161 and 162) and coupled at one phase shifter terminal to one end of the upper rectangular waveguide; and at least one RF power divider/combiner having an input port configured for coupling to a reference Transmit/Receive (T/R) Module and two output ports (151 and 152) being coupled, correspondingly, to said at least one 90 degree phase shifter at another phase shifter terminal, and to one end of the lower rectangular waveguide. a waveguide arrangement (14; 314) comprising at least one pair of rectangular waveguides (141 and 142; 21 and 22; 31 and 32) having an inside height of side-walls smaller than an inside width of top and bottom walls; said at least one pair of rectangular waveguides being stacked in parallel relation to each other to form a lower rectangular waveguide (142; 22; 32) and an upper rectangular waveguide (141; 21; 31) spaced apart from the lower rectangular waveguide (142; 22; 32) at a distance of about a quarter of a desired operating wavelength of the array antenna; said at least one pair of rectangular waveguides (21 and 22) including a plurality of circular through-holes (23; 33) extending through at least one side-wall (24; 34) of the stacked rectangular waveguides from a bottom wall (221; 321) of the lower rectangular waveguide (22; 32) to a top wall (212; 312) of the upper rectangular waveguide (21; 31) and having a diameter sufficient to accommodate coaxial transmission lines passing through said circular through-holes (23; 33); said at least one side-wall having openings (27; 37) between said circular through-holes (23; 33) and an interior regions (26 and 25; 36b and 36a) of said lower (22; 32) and upper (21; 31) rectangular waveguides, correspondingly, in order to provide coupling of the coaxial transmission lines into said at least one pair of rectangular waveguides (21 and 22; 31 and 32); at least one 90 degree phase shifter having two phase shifter terminals (161 and 162) and coupled at one phase shifter terminal to one end of the upper rectangular waveguide; and at least one RF power divider/combiner having an input port configured for coupling to a reference Transmit/Receive (T/R) Module and two output ports (151 and 152) being coupled, correspondingly, to said at least one 90 degree phase shifter at another phase shifter terminal, and to one end of the lower rectangular waveguide.