In-situ characterization of spurious modes in HE11 transmission lines with a 5-port couplerB. Plaum, W. Kasparek, C. Lechte, J. Ruiz, D. Tretiak and H. IdeiEPJ Web of Conferences, 32 (2012) 04010DOI: https://doi.org/10.1051/epjconf/20123204010