the electron cyclotron resonance heating (ecrh) system will provide microwave energy at 28, 35, and 56 ghz to heat electrons in the thermal barrier and maximum potential regions in the end-plugs of the mirror fusion test facility (mftf-b) tandem mirror machine to design an effective quasi-optical antenna system for ecrh which will convert the te/sub 0n/ output of an overmoded circular waveguide to a linearly polarized collimated microwave beam requires an accurate determination of the modal content of microwave energy present in the waveguide. for this purpose, mode-selective directional couplers have been designed and fabricated for the three ecrh operating frequencies. the couplers are designed to minimize mode conversions in the circular primary waveguide and to excite only the te/sub 10/ mode in the overmoded rectangular secondary waveguide. the secondary waveguide is tapered in the large dimension down to a fundamental te/sub 10/ rectangular waveguide so conventional microwave components can be adapted to the directional couplers. these tapers are carefully designed to minimize reflection of the te/sub 10/ mode and mode conversion, and subsequent reflection of the te/sub 30/ mode as the taper passes through the te/sub 30/ cutoff. fabrication processes that provide extremely close tolerances and excellent surface finishes are described