  We present the integrated polarization properties of the four compact radio
sources 3C48, 3C138, 3C147 and 3C286, from 1 to 50 GHz, over a 30-year time
frame spanning 1982 to 2012. These four sources are commonly used as flux
density and polarization calibrators for cm-wave interferometers. Using the
polarized emission of Mars, we have determined that the true position angle of
the linearly polarized emission of 3C286 rises from 33 degrees at 8 GHz to 36
degrees at 45 GHz. There is no evidence for a change in the position angle over
time. Using these values, the position angles of the intergrated polarized
emission from the other sources are determined as a function of frequency and
time. The fractional polarization of 3C286 is found to be slowly rising, at all
frequencies, at a rate of ~0.015%/year. The fractional polarizations of 3C48,
3C138, and 3C147 are all slowly variable, with the variations clearly
correlated with changes in the total flux densities of these sources.

Instrumentation and Methods for Astrophysics