  Low-frequency radio continuum observations of edge-on galaxies are ideal to
study cosmic-ray electrons (CREs) in halos via radio synchrotron emission and
to measure magnetic field strengths. We obtained new observations of the
edge-on spiral galaxy NGC 891 at 129-163 MHz with the LOw Frequency ARray
(LOFAR) and at 13-18 GHz with the Arcminute Microkelvin Imager (AMI) and
combine them with recent high-resolution Very Large Array (VLA) observations at
1-2 GHz, enabling us to study the radio continuum emission over two orders of
magnitude in frequency. The spectrum of the integrated nonthermal flux density
can be fitted by a power law with a spectral steepening towards higher
frequencies or by a curved polynomial. Spectral flattening at low frequencies
due to free-free absorption is detected in star-forming regions of the disk.
The mean magnetic field strength in the halo is 7 +- 2 $\mu$G. The scale
heights of the nonthermal halo emission at 146 MHz are larger than those at 1.5
GHz everywhere, with a mean ratio of 1.7 +- 0.3, indicating that spectral
ageing of CREs is important and that diffusive propagation dominates. The halo
scale heights at 146 MHz decrease with increasing magnetic field strengths
which is a signature of dominating synchrotron losses of CREs. On the other
hand, the spectral index between 146 MHz and 1.5 GHz linearly steepens from the
disk to the halo, indicating that advection rather than diffusion is the
dominating CRE transport process. This issue calls for refined modelling of CRE
propagation.
