Transmission characteristics of a rectangular waveguide with a perpendicularly inserted mercury‐vapor discharge tube, have been measured. The waveguide was excited in the TE01mode and the plasma column was either perpendicular or parallel to the electric vector of the waveguide mode. In the first case the transmission spectrum contains the well‐known Tonks‐Dattner resonances and some additional unexplained small peaks. In the second case the transmission spectrum has only small peaks superimposed on a smooth curve. Noise emission spectra were also measured for both polarizations and showed the same characteristics as the absorption spectrum.To explain these results, dispersion curves were measured for the symmetric, dipole, and quadrupole modes propagating on the plasma cylinder. In addition the dispersion curve for a new symmetric mode was measured. This mode is of the same nature as the fast dipole modes responsible for the subsidiary Tonks‐Dattner resonances. By combining these experimental results it has proved possible to gain an understanding of how the various peaks in the transmission spectrum are caused by coupling to plasma waves. In particular, the small previously unexplained peaks are due to coupling to slow plasma waves as also has been proved by direct measurement of the slow waves leaking out through the holes in the waveguide.