To be published in IEEE Transactions on Microwave Theory and Techniques 2001 Jacob et al. 1 MICROWAVE PROPERTIES OF LOW LOSS POLYMERS AT CRYOGENIC TEMPERATURES Mohan V. Jacob a , Janina Mazierska a , Kenneth Leong a and Jerzy Krupka b a Electrical and Computer Engineering, School of Engineering, James Cook University, Townsville, QLD 4811, AUSTRALIA b Instytut Mikroelektroniki i Optoelektroniki Politechniki Warszawskiej, Koszykowa 75, 00-662 Warszawa, POLAND ABSTRACT Dielectric loss tangent and permittivity of Polytetrafuoroethylene (Teflon), high density Polyethylene and cross-linked Polystyrene (Rexolite) were measured at temperature range from 28 K to 84 K and frequency of approximately 18 GHz. The material properties were determined by measurements of the resonant frequency and the Q-factor of a TE 011 mode cylindrical superconducting cavity containing a sample under test. It has been demonstrated that these materials exhibit very low losses at cryogenic temperatures (2×10 -6 for Teflon, 5×10 -5 for HD Polyethylene and 11×10 -5 for Rexolite). Due to low losses these materials can be useful in construction of various high Q-factor microwave devices for operation at cryogenic temperatures. I. INTRODUCTION Cryogenic electronics is a fast growing branch of modern electronics especially since the discovery of high temperature superconductors that allowed for significant reduction of losses and noise figures in filters and microwave oscillators. Successful design and manufacturing of low loss devices at cryogenic temperatures require careful choice of dielectric materials for their construction. To ensure high Q-factor of an electronic system all the components should exhibit small losses. Single crystal dielectrics like sapphire, YAG, SrLaAlO 4 or quartz [1] are the lowest loss materials available. However they are hard to machine into complicated shapes. Hence various plastic materials, easier to machine, have been typically used as supports or other less demanding elements of a cryogenic microwave device or a cryogenic system. Teflon, Polyethylene and Rexolite have been on the market for many years, are inexpensive and easy to fabricate and machine into a desired size and shape. Information on microwave properties of Teflon, Polyethylene and Rexolite exists for wide range of frequencies but at room and higher temperatures only [2,3]. There is no data on complex permittivity of these materials available for cryogenic temperatures except for one temperature (77 K) for Teflon and Rexolite [4]. In this paper we present results of accurate measurements of the permittivity and loss tangent of three low loss polymers in the cryogenic temperature range from 25 K to 80 K using the dielectric resonator technique with superconducting thin films. We have been used an accurate multi-frequency measurement and data processing technique (TMQF) [5,6]. Our measurement method accounted for noise, delay due to transmission lines and its frequency dependence, and cross-talk in measurement data. We also took into consideration variations of samples’ dimensions with temperature to ensure high accuracy.