Low temperature microwave characterisation of lithium fluoride at different frequencies

Precise knowledge of dielectric properties of materials is required to implement the material in high frequency devices and circuits. At microwave frequencies complex permittivity (dielectric constant and loss tangent) are the two mandatory parameters prior to any design. We have identified Lithium Fluoride as a potential candidate, which can be used in conjunction with superconducting and non-superconducting parts of several microwave communication devices. Even though dielectric constant of LiF is known at room temperature there only limited data presented at cryogenic temperatures. We have used a dielectric post resonator for the microwave characterisation of the rod shaped LiF crystal. In this paper, we have reported the dielectric constant (perpendicular component of the real part of complex permittivity) and loss tangent of two LiF crystals as a function of temperature (15-290 K) at frequencies of 8 and 16.5 GHz. We have also studied and reported the temperature coefficient of frequency and permittivity. The concept of using temperature coefficient of frequency as a standard is proved to be wrong in this paper. Microwave properties of other Fluorides are also compared with the LiF crystal.