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Due to their steep spectra, low-frequency observations of FR II radiogalaxies potentially provide key insights in to the morphology, energetics andspectrum of these powerful radio sources. However, limitations imposed by theprevious generation of radio interferometers at metre wavelengths has meantthat this region of parameter space remains largely unexplored. In this paper,the first in a series examining FR IIs at low frequencies, we use LOFARobservations between 50 and 160 MHz, along with complementary archival radioand X-ray data, to explore the properties of two FR II sources, 3C452 and3C223. We find that the morphology of 3C452 is that of a standard FR II ratherthan of a double-double radio galaxy as had previously been suggested, with noremnant emission being observed beyond the active lobes. We find that thelow-frequency integrated spectra of both sources are much steeper than expectedbased on traditional assumptions and, using synchrotron/inverse-Compton modelfitting, show that the total energy content of the lobes is greater thanprevious estimates by a factor of around 5 for 3C452 and 2 for 3C223. We go onto discuss possible causes of these steeper than expected spectra and providerevised estimates of the internal pressures and magnetic field strengths forthe intrinsically steep case. We find that the ratio between the equipartitionmagnetic field strengths and those derived through synchrotron/inverse-Comptonmodel fitting remains consistent with previous findings and show that theobserved departure from equipartition may in some cases provide a solution tothe spectral versus dynamical age disparity.

FR II radio galaxies at low frequencies I: morphology, magnetic field strength and energetics