A low profile, low cost receiver/transmitter system includes a planar waveguide-to-microstrip adapter, a dielectric loaded waveguide fed slot array antenna, and a downconverter all formed on a single dielectric substrate made of polytetrafluoroethylene. The waveguide antenna includes a set of slots disposed in a surface thereof to receive/transmit a microwave/millimeter wave signal. The waveguide-to-stripline adapter is connected to the waveguide antenna and includes a taper section attached to a microstrip line. The taper section, which may be linear or may follow some other more complex taper function such as a Chebyshev function, adapts a signal propagating within the waveguide antenna to the microstrip line or vise versa.