  Parasitic electromagnetic fields are shown to strongly suppress the quality
(Q)-factor of superconducting coplanar waveguide resonators via non-local
dissipation in the macroscopic environment. Numerical simulation and low
temperature measurements demonstrate how this parasitic loss can be reduced,
establishing a Lorentzian lineshape in the resonator frequency response and
yielding a loaded Q-factor of 2.4 x 10^5 for niobium devices on sapphire
substrates. In addition, we report the dependence of the Q and resonance
frequency shift Delta f_0 with input power and temperature in the limit where
loss from two-level systems in the dielectric dominate.
