Tunable Impedance Transformer Based on Split Strip Lines

In this work we represent a tunable three-section impedance transformer made on the basis of cascade-connected sections of strip lines. The second and third sections are made on coupled three-wire strip lines. These lines are formed by split strip lines and by control strips overlapping the parts of split strip lines on an additional substrate. Transformer parameter control is performed by regulating the capacitance of lumped elements that are inserted between the control strips and the ground plane. The use of split strip lines makes it possible to obtain a more uniform distribution of current density over the cross section of the strips. A change in the regulation capacitances leads to a variation in the characteristic impedance of the second and third stages of the transformer up to 50% of the average value. As a result, the controlled transformer was tuned to a pre-determined return loss level of -20dB with an input load of 50 Ohm and output loads from 2.3 Ohm to 3.98 Ohm in the ±15% frequency band.