In this thesis I describe both modelling and experimental measurements of superconducting mi- crowave resonators. These devices have many scientifically important applications, including as

photon detectors, filter-banks, parametric amplifiers and increasingly as part of quantum circuits for qubit readout. The next generation of astronomy will require resonators that are very low in loss to create highly sensitive kinetic inductance detectors (KIDs). The multiplexing of hundreds of

KIDs into arrays has already been successful in imaging red-shifted galaxies and CO lines. How- ever, to create arrays for lower frequency (sub-100 GHz) and lower intensity signals of interest,

resonator losses and detection limits need to be improved. I begin by exploring the interplay be- tween resonator design and loss, and end with the development of an innovative new KID concept

for the detection of lower frequency signals. I develop a numerical model to simulate dependence on resonator quality factor with readout power when both two-level systems and quasiparticle heating effects cause dissipation. Using scattering parameter theory and adapting the Rothwarf-Taylor expressions, I solve for quasiparticle population and energy stored in a resonator simultaneously. I present a range of simulation results describing the variety of large-signal regimes which can result from considering both mechanisms. I then use this model to analyse the behaviour of coplanar waveguide (CPW) resonators. Using connection matrix theory, I design a half-wavelength resonator capacitively coupled a readout line, and outline the design of the test system and data analysis software used to analyse them. I present results on the differences in resonator behaviour for niobium, aluminium and titanium on silicon, high resistivity silicon and sapphire. I show how the choice of thin-film and dielectric changes the maximum quality factors achievable for a given geometry, and that the variation in quality factor with readout power is substrate dependent. Next, silicon dioxide is patterned in regions of low and high electric field strength on the CPW devices and the consequences on their dissipative responses modelled and compared. Oxide on

resonators is typically avoided to reduce associated low power two-level systems loss but is an arguably unavoidable material needed to realise full detectors. The effect of oxide is shown to depend on location, metal, and substrate – and I find it significantly changes both the low and high readout power behaviour.

Finally, I propose the ‘Yagi-KID’, which incorporates a scheme for the CPW resonators de- signed previously to be DC biased. DC current has been used to tune resonator frequency and

is predicted to suppress the superconducting gap and reduce pair-breaking frequency. I integrate this with a planar, double dipole Yagi antenna fabricated on membranes. I design a bespoke test system to multiplex Yagi-KIDs, and present initial dark tests. These results show promise that with continued development, these devices could measure the effect of current on gap suppression directly, and once illuminated measure frequencies below the fundamental pair-breaking limit of the superconducting aluminium.