Dual-Band Impedance Matching Transformer with Optimized Total Structure Lengths

To reduce design size and realize the structure miniaturization, this article presents a dual-band (DB) impedance matching transformer with optimized total structure lengths. The proposed transformer is composed of two parts in series. The first part is a L-type structure with one loaded stub and one transmission line, responsible for realizing 0.02 conductance value at lower frequency
$f_{1}$
. When certain type and length of stub is selected, the L-type structure can own the shortest line length. The other part in series is a H-shaped form, separately completing electric susceptance matching at
$f_{1}$
and the admittance matching at the second frequency i.e.
$f_{2}$
. The H-shaped structure has four parameters, one related to fixed
$\pi/2$
electrical length at
$f_{1}$
is used to match required susceptance about
$f_{2}$
, one adopted to obtain 0.02 conductance at
$f_{2}$
, the others are applied to realizing the match at
$f_{1}$
and obtaining the smallest size of H-shaped structure. In design, the proposed transformer adopts a cascade matching mode as a whole, that is, according to the sequence, the matching at
$f_{1}$
is completed first, and then the load matching at
$f_{2}$
is achieved. To make design simpler and convenient, only two line widths are used with characteristic impedance 50 Ohms and 120 Ohms. Hereinto, 50 Ohms is arranged to the main transmission line connecting the load and matching source, and 120 Ohms to loaded stubs. The load with resistances and capacitance is given to gain different complex numbers at two arbitrary independent frequencies i.e.
$f_{1}$
and
$f_{2}$
. An example based on the proposed transformer is designed to verify the ideas and related calculation equations. Simulated and calculated results are in good agreement.