Loss measurements have been carried out on a cylindrical pillbox cavit
y whose metallic flat end walls have been replaced by an ordered seque
nce of high purity sapphire and air dielectric layers. The loss of the
TE01 mode at 18.99 GHz was substantially lowered. An improvement in c
avity Q for this mode from 8.30 X 10(3) to 531 X 10(30 was observed. T
hese experimental results closely reproduce two independent theoretica
l simulations. All measurements were taken at 30-degrees-C.