  To better understand the influence of the activity cycle on the solar
atmosphere, we report the time variation of the radius observed at 37 GHz
($\lambda$=8.1 mm) obtained by the Mets\"ahovi Radio Observatory (MRO) through
Solar Cycles 22 to 24 (1989-2015). Almost 5800 maps were analyzed, however, due
to instrumental setups changes the data set showed four distinct behaviors,
which requested a normalisation process to allow the whole interval analysis.
When the whole period was considered, the results showed a positive correlation
index of 0.17 between the monthly means of the solar radius at 37 GHz and solar
flux obtained at 10.7 cm (F10.7). This correlation index increased to 0.44,
when only the data obtained during the last period without instrumental changes
were considered (1999-2015). The solar radius correlation with the solar cycle
agrees with the previous results obtained at mm/cm wavelengths (17 and 48 GHz),
nevertheless, this result is the opposite of that reported at submillimetre
wavelengths (212 and 405 GHz).
