This paper introduces a novel highly sensitive wideband differential microwave sensor based on substrate-integrated waveguide (SIW) technology for dielectric testing on microwave substrates. The sensor is a single-layer 3-port device and consists of a wideband mode converter that transforms the quasi-TEM mode of a microstrip line into the SIW’s dominant TE₂₀ mode, and two independent SIW cavities operating with TE₁₀₁ mode.

One of these cavities is the test cavity and the other is the reference cavity. The performance of the proposed differential sensor in detecting the relative permittivity of microwave substrates was evaluated. The average and Bode sensitivities were computed and showed an outstanding sensitivity to negative frequency shifts. The sensor was designed and fabricated on an RT5880 Duroid Substrate, and standard microwave substrates were tested. The sensor achieves an extremely low decoupling factor, indicating effective noise or interference rejection; high sensitivity, and a wideband operation. The sensor outperforms existing SIW-based differential sensors, especially in terms of sensitivity and decoupling.