In a previous study, the characteristics of plasma generated by fast electrons behind a grid anode with short glow discharge were studied using numerical simulation. The source of the post-anode plasma electrons is considered to be the direct current glow discharge itself in the gap between a cathode and a grid anode. However, the electron attenuation of the microwave radiation in the post-anode space measured in experiments does not correspond to the numerical predictions. In this paper, the current-voltage characteristics of the short glow discharge with a grid anode and the spectral characteristics of the discharge in both the electrode gap and the space behind the grid anode are studied; the effective thickness of the plasma in the post-anode space is estimated using a spectral method.