Development of a wideband microwave reactor with a coaxial cable structure

A wideband microwave reactor with an output that includes the 915 MHz and 2.45 GHz ISM (Industrial, Scientific and Medical) bands was designed and fabricated. The reactor structure incorporated a coaxial cable, and a liquid sample was placed in the space between the inner and outer conductors. Insertion of a truncated cone-shaped polytetrafluoroethylene (PTFE) device was used as a method to reduce microwave reflection over a wide frequency range. The reactor had a volume of 360 ml and was designed employing a 3D electromagnetic simulation. Ultrapure water and a 0.1 M NaOH solution were selected as the liquid samples and experimentally measured permittivity data for these liquids were employed during the reactor simulations. The measured reflection ratio exhibited the same trend as the simulation results between 800 MHz and 2.7 GHz. The reflection ratio was especially low in the case of the NaOH solution (less than 2%), although this value increased to more than 40% upon removal of the PTFE insert. Microwave heating tests demonstrated that this reactor was able to heat liquid samples at 915 MHz, 1.7 GHz and 2.45 GHz, with estimated microwave absorption efficiencies varying between 28% and 66% depending on the frequency, sample type and heating duration. The reflection ratio and heating data demonstrated that this reactor functioned over a wide frequency range between 800 MHz and 2.7 GHz. A non-uniform temperature distribution in the sample remained a challenge that must be addressed in future work.