Wideband impedance transformer with out-of-band suppression characteristics

This article presents the design of a wideband impedance transformer with out-of-band suppression characteristics. The out-of-band suppression characteristics are obtained by loading several transmission zeros at both the lower and upper stop-bands. For experimental validation, a 50-to-25 Ω transformer has been implemented at a center frequency (f₀) of 2.6 GHz. The measured results were in good agreement with simulations, showing a return loss better than 20 dB over 0.92 GHz (2.1-3.02 GHz) and an out-of-band suppression better than 18 dB over DC to 1.42 and 3.8 to 6.65 GHz.