Wu, G. Aderhold, S. Checchin, M. Martinello, M. Ozelis, J.P. JACoW Geneva, Switzerland 978-3-95450-178-6 10.18429/JACoW-SRF2015-TUPB098 English 840-843 TUPB098 cavity coupling SRF LLRF radio-frequency Contribution to a conference proceedings 2015 2015-12 http://dx.doi.org/10.18429/JACoW-SRF2015-TUPB098 http://srf2015.vrws.de/papers/tupb098.pdf An accurate cavity test involves the accurate power measurement and decay time measurement. The directional coupler in a typical cavity test llrf system usually has low directivity due to broadband requirement and fabrication errors. The imperfection of the directional coupler brings unexpected systematic errors for cavity power measurement in both forward and reflect power. An error analysis will be giving and new specification of directional coupler is proposed.