Ida, Nathan: 'Introduction to microwaves' (Control, Robotics and Sensors, 2018), 'Open Resonator Microwave Sensor Systems for Industrial Gauging: A practical design approach', Chap. 1, pp. 1-12, DOI: 10.1049/PBCE103E_ch1
IET Digital Library, https://digital-library.theiet.org/;jsessionid=12feavroo9qif.x-iet-live-01content/books/10.1049/pbce103e_ch1