  The total solar fluxes at 1, 2, 3.75, and 9.4 GHz were observed continuously
from 1957 to 1994 at Toyokawa, and from 1994 until now at Nobeyama, Japan with
the current Nobeyama Radio Polarimeters. We examined the multi-frequency and
long-term datasets, and found that not only the microwave solar flux but also
its monthly standard deviation well indicates the long-term variation of solar
activity. Furthermore, we found that the microwave spectra at the solar minima
of Cycle 20~24 agree with each other. These results show that the average
atmospheric structure above the upper chromosphere in the quiet Sun has not
varied for half a century, and suggest that the energy input for atmospheric
heating from the sub-photosphere to the corona has not changed in the quiet Sun
despite significantly differing strengths of magnetic activity in the last five
solar cycles.
