Broad-band printed-circuit balun employing coupled-strip all pass filters

Claim 1

A TEM mode microwave balun constructed in a medium selected from media including stripline, microstrip and airstrip, comprising: a dielectric substrate; a pattern of conductive traces along the surface of said substrate, said pattern including a trace corresponding to an unbalanced transmission line, a pair of branches extending in generally opposite directions along said surface of said substrate from the trace of said unbalanced line; a first close-coupled-strip-transmission line filter comprising a first pair of close-coupled, generally parallel, conductive traces connected together at a first end thereof and being open at a second end, said first filter having a 360° insertion phase; a second close-coupled-strip-transmission line filter comprising a second pair of close-coupled, generally parallel, conductive traces connected together at a first end thereof and being open at a second end, said second filter having a 180° insertion phase; a spaced pair of generally parallel conductive traces along the surface of said substrate constituting a balanced transmission line, one trace of said spaced pair being connected to a trace of said first coupled-strip filter at said open end thereof and the other trace of said spaced pair being connected to a trace of said second coupled-strip filter at said open end thereof, each of the remaining traces of said coupled-strip filters being connected to a corresponding one of said branches.