Parker, E.A.; Hamdy, S.M.A.: 'Rings as elements for frequency selective surfaces', Electronics Letters, 1981, 17, (17), p. 612-614, DOI: 10.1049/el:19810430
IET Digital Library, https://digital-library.theiet.org/;jsessionid=1pq5spnmmkddc.x-iet-live-01content/journals/10.1049/el_19810430