MM-wave CRLH antenna gain improvement by matching with reactive stubs

The paper describes a method of return loss (RL) reducing for a CoPlanar Waveguide (CPW) Composite Right/Left-Handed (CRLH) zeroth-order resonance (ZOR) mm-wave antenna by matching the radiating structure impedance Z_S = R_S + jxX_S to the characteristic circuit impedance Z₀ = 50 Ω. The working frequency is f = 40 GHz and the initial RL value was S₁₁ = -16.21 dB, meaning a VSWR = 1.37. Using appropriate formulas, the resistive R_S and reactive X_S parts of Z_S were found. The second step was to find the length l of a short-ended parallel reactive stub and the suitable point d for stub implantation in order to match the antenna to the characteristic impedance Z₀. With these data, the layout of the antenna feeding line was modified by adding the appropriate matching stub. After that, the return loss at the feeding line input was S₁₁ = -43.50 dB. The return loss was drastically improved, demonstrating effectiveness of the matching process.