Постульга, А. С., and С. Е. Мартынюк. “Influence of the Directional coupler’s Parameters on the Characteristics of Butler Matrix 4×4”. Visnyk NTUU KPI Seriia - Radiotekhnika Radioaparatobuduvannia, no. 57, June 2014, pp. 43-50, doi:10.20535/RADAP.2014.57.43-50.