Integrated Polarization Properties of 3C48, 3C138, 3C147, and 3C286

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.