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The astrophysical content of this thesis is based on radio observations of ten extended radio sources. Each source was observed at several different frequencies. Since this type of study had not been widely attempted in the past, the first chapters give the preliminary instrumental and technical developments needed. After an introductory chapter, the improvements made to the geometry of the One-Mile telescope are given. The best method of making polarization maps with the same telescope is described in Chapter 3. Since much of the project involved comparing maps at different resolutions, methods were derived to convolve maps to a common resolution (Chapter 4). Chapters 5-11 form the astrophysical section of the thesis. Maps of the radio galaxy 3C 452 at four frequencies are given in Chapter 5. The spectral distribution across the source was found to be consistent with that expected from a simple model of synchrotron losses in the radiating electrons. Extensive mapping of the large radio galaxy 3C 31 is presented in Chapter 6, together with the spectral and polarization properties of the source. Remarkable jets were found and the directions of the magnetic fields within them were determined. Several models of jets could be ruled out by these results. Chapters 7-11 contain results from a systematic study of six powerful radio galaxies and one quasar (3C 47, 79, 219, 234, 300, 382, 430). Total intensity and polarization maps were made of each source at two frequencies and the compact components were mapped with a much higher resolution. The magnetic field structure in the extended regions of the sources was modelled using a Monte Carlo procedure and the existence of large scale components to the magnetic fields was inferred. The thermal electron densities within the sources were also derived and maps of the magnetic field direction are given for five of the sources. The spectral distributions across each source were also obtained and evidence for re-acceleration of relativistic electrons in the extended source components was found. The implications of the results from the seven sources for the various source models are considered.