1 Introduction

As part of a continuing study of the polarization and rotation measure properties of extragalactic radio sources, we present linear polarization measurements of 154 radio galaxies at 4.75 GHz. This database represents a subset of measurements planned at several wavelengths between U-band and L-band. The MPIfR 100-m radiotelescope [along with a polarization matrix calibration procedure (cf. Sect. 2)] was the instrument of choice because of its high sensitivity and low instrumental polarization. The single-dish observations also have the advantage of sampling the flux at large spatial scales not possible with a synthesis telescope. The relative contributions to the observed Faraday rotation of the galaxy, the intergalactic medium, and the source itself are still uncertain. Augmenting the existing data is important in improving the reliability of tests that can define the sites and nature of Faraday rotation, and magnetic fields in the environments of radio sources and in the intergalactic medium.

The sample of radio galaxies was selected from a catalogue of polarization data of radio sources compiled by Kronberg and his collaborators (Simard-Normandin et al. 1981). Although most of the sources had polarization data at one or more wavelengths, more accurate values at additional wavelengths were required to either estimate, or confirm a Faraday rotation measure. The new data reflect the improvement in quality due to the high sensitivity and accuracy of the 100-m telescope's receivers and polarimeter.