US Patent 7250828 Compact balun

A distributed backwards-wave balun comprising first and second pairs of coupled transmission line sections having one line section of the first coupled pair connected in series with one line section of the second coupled pair. A differential port is connected across the outer ends of the series-connected line sections and a single-ended port is connected to the inner end of the other line section of one of the coupled pairs. The balun includes an inductive load, connected in parallel with the differential port, in which the electrical length of the coupled line sections is less than one quarter of the wavelength of the centre frequency of the operating band of the balun.