Manuscript Number: 605

1 Abstract: The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity ε = ε ′- jε″, and the loss tangent, tan δ = ε″ / ε′. This paper describes two convenient laboratory based methods to obtain ε′, ε ″ and hence tan δ of fibre-reinforced thermoplastic (FRTP) composites. One method employs a microwave network analyser in conjunction with a waveguide transmission technique, chosen because it provides the widest possible frequency range with high accuracy. The values of the dielectric constant and dielectric loss of glass fibre reinforced (33%) low density polyethylene, LDPE/GF (33%), polystyrene, PS/GF (33%), and Nylon 66/GF (33%), were obtained. Results are compared with those obtained by another method using a high-temperature dielectric probe