To Optimally Design Microstrip Nonuniform Transmission Lines as Lowpass Filters

A method is proposed to optimally design the Microstrip Nonuniform Transmission Line (MNTLs) as lowpass filters. Some electrical and physical restrictions are used to design MNTLs. To optimally design the MNTLs, their strip width is expanded as truncated Fourier series, firstly. Then, the optimum values of the coefficients of the series are obtained through an optimization approach. The performance of the proposed structure is studied by design and fabrication of two lowpass filters of cutoff frequency 2.0 GHz.

💡 Research Summary

The paper introduces a systematic methodology for designing microstrip non‑uniform transmission lines (MNTLs) as low‑pass filters (LPFs). Conventional microstrip filters rely on lumped inductors and capacitors placed alongside uniform transmission lines, which adds size, insertion loss, and design complexity. In contrast, an MNTL’s characteristic impedance and propagation constant vary continuously along its length as a function of the strip width w(z), allowing the line itself to realize the desired frequency response.

The authors first parameterize the width profile using a truncated Fourier series:

 w(z) = a₀ + Σ_{n=1}^{N}