  Dielectric resonators are key components for many microwave and millimetre
wave applications, including high-Q filters and frequency-determining elements
for precision frequency synthesis. These often depend on the quality of the
dielectric material. The commonly used material for building the best cryogenic
microwave oscillators is sapphire. However sapphire is becoming a limiting
factor for higher frequencies design. It is then important to find new
candidates that can fulfil the requirements for millimetre wave low noise
oscillators at room and cryogenic temperatures. These clocks are used as a
reference in many fields, like modern telecommunication systems, radio
astronomy (VLBI), and precision measurements at the quantum limit.
High-resolution measurements were made of the temperature-dependence of the
electromagnetic properties of a polycrystalline diamond disk at temperatures
between 35 K and 330 K at microwave to sub-millimetre wave frequencies. The
cryogenic measurements were made using a TE01{\delta} dielectric mode resonator
placed inside a vacuum chamber connected to a single-stage pulse-tube
cryocooler. The high frequency characterization was performed at room
temperature using a combination of a quasi-optical two-lens transmission setup,
a Fabry-Perot cavity and a whispering gallery mode resonator excited with
waveguides. Our CVD diamond sample exhibits a decreasing loss tangent with
increasing frequencies. We compare the results with well known crystals. This
comparison makes clear that polycrystalline diamond could be an important
material to generate stable frequencies at millimetre waves.
