The radiation losses of various E-plane bends are determined within the Ka band through resonator measurements using a standard reflectometer set up for determining the unloaded Q-factor Q sub u. To avoid errors arising from the additional radiation loss of the coupling iris, the measurements are carried out with two different resonator lengths. To separate the pure radiation losses from the unknown losses encountered in the metallic walls, the same procedure is used to determine the wall losses of nonradiating straight groove-guide sections assumed to be approximately the same, as in the case of the bends. It is found that two types of waves exist that are separated by the cutoff frequency. The first type comprises stable waves guided at any groove depth and curvature radius; the second comprises the unstable waves whose radiation is shown in the measurement results. It is noted that an analysis of the radial field decay of the z-directed electric and magnetic field components can be readily made using convenient approximations for the cylinder functions of real argument and complex order (Sommerfeld, 1962). The severe increase in the radiation loss with the frequency is seen as a problem for practical use if shallow grooves and small radii are required.