A Study on Design of Microstrip Linear Tapered Line Impedance Transformer Using FFT

A series of processes from design to measurement of a microstrip linear tapered line impedance transformer is studied. The exponential taper is well known as a MSL impedance transformer, and the design theory is relatively easy, but it requires the use of staircase approximation for manufacturing, and has a problem that it takes time to manufacture. On the other hand, the linear taper is easy to manufacture, but has a problem that the calculation is troublesome because the theoretical calculation required for the design is given in the integral form. So, in this study, FFT is applied as a design method of the linear taper, and confirmed the effectiveness of presented method. It is considered that the linear taper is easy to manufacture, but it is confirmed by experiments that the measurement value would be affected if the linear line is not accurate. This method can be applied to fast calculation of the reflection coefficient of a tapered line having an arbitrary function as a characteristic impedance.