The ICP in closed quartz chamber is an effective solution to local stealth of aircraft. The attenuation of the electromagnetic wave is strongly affected by distribution of electron density in incidence direction. In this paper, an experiment that the planar ICP discharged in all-quartz chamber (30.8 cm×30.8 cm×5.8 cm) was conducted. The E-H mode transition of Ar/Air ICP was observed, and the width and the proportion of ICP area versus the RF power were measured. Such physical phenomena were explained rationally by ICP-source theoretical model. A new microwave diagnostics is presented based on the Hβ spectrum broadening and the characteristics of planar-coil ICP. It is used to diagnose the spatial electron density distribution of Ar/Air in incidence direction. The parameters of electron density distribution function in the incident direction are separately solved by the fitting results of the Hβ stack broadening and microwave interferometer. And the curves of electron density distribution versus RF power are obtained. We obtain a stable ring plasma source with electron density varying from 0.5×1011～3.2×1011 cm-3. The experiment demonstrates that electron density is prominently affected by the RF power and gas species. The peak value of electron density is positioned near the center of ICP. It is shown that the electron density of Ar ICP is higher than the air, but the area of air ICP is larger through comparing the curves of Ar/Air electron density.