A new slot antenna concept allows the generation of large volume 2.45 GHz discharges. The microwave power is coupled from an annular waveguide (ring cavity) through ten equidistantly positioned resonant coupling slots into the quartz plasma chamber. Langmuir double probe axial distribution measurements of the electron density and temperature are performed for argon, helium and nitrogen discharges in a power range from 120 to 1200 W and a pressure range from 10 to 150 Pa. Characteristic differences in the discharge behavior for the various gases are presented. With argon a surface wave at the plasma-quartz interface can be excited, already at 200 W. The resulting plasma column can extend up to 450 mm in length along the tube with 160 mm diameter. The surface wave mode is identified as a m=5 mode by visual inspection of the plasma formation and by recording the azimuthal electric field distribution. In contrast to argon, for helium and nitrogen no surface wave generation for power levels up to 1200 W are observed. Within the investigated power range both gases show a diffusion controlled variation of the electron density. This is verified by fitting the axial density data to the time-independent diffusion equation for helium and nitrogen and to a simplified surface wave model for argon.