This research investigates the characterisation of liquids using primarily substrate integrated waveguides and extending this to other interesting conventional transmission lines. Focus is drawn to liquid mixture quantification, which is significant in the distinction of the quantity of one biological or chemical liquid from another. This work identified and confirmed that microwave resonance methods are best suited to perform mixture quantification due to their high sensing accuracy and inherent single point detection. The tracking of the resonant frequency change with either the corresponding return loss or insertion loss (depending on the type of resonant structure) gives a good solution in this regard. On the other hand, it was affirmed that transmission line methods are best suited for general broadband characterisation of a particular liquid. Three major outputs were achieved in this research work, namely: (i) In-SIW millimetre wave sensor; (ii) SIW slot antenna microlitre sensor and (iii) Sub-terahertz CSRR sensor for solid dielectric characterisation. Using the SIW slot antenna sensor, microlitre liquid volumes of 7 μl were characterised and binary mixtures quantified with an overall accuracy of better than 3 % when compared with results from a commercial sensor. The In-SIW millimetre wave sensor showed proof of concept through simulation results of the characterisation of 15 μl liquid volume results when compared to 100 ml liquid volume measurement done using the Keysight dielectric probe. The sub-terahertz CSRR sensor was used to characterise solid dielectrics using its multifuctionality capability of performing both resonant measurements and transmission line measurements
