  While repeating fast radio bursts (FRBs) remain scarce in number, they
provide a unique opportunity for follow-up observations that enhance our
knowledge of their sources and potentially of the FRB population as a whole.
Attaining more burst spectra could lead to a better understanding of the origin
of these bright, millisecond-duration radio pulses. We therefore performed
$\sim$20 hr of simultaneous observations on FRB 121102 with the Effelsberg
100-m radio telescope and the Low Frequency Array (LOFAR) to constrain the
spectral behaviour of bursts from FRB 121102 at 1.4 GHz and 150 MHz. This
campaign resulted in the detection of nine new bursts at 1.4 GHz but no
simultaneous detections with LOFAR. Assuming that the ratio of the fluence at
two frequencies scales as a power law, we placed a lower limit of $\alpha$ >
-1.2 $\pm$ 0.4 on the spectral index for the fluence of the instantaneous broad
band emission of FRB 121102. For the derivation of this limit, a realistic
fluence detection threshold for LOFAR was determined empirically assuming a
burst would be scattered as predicted by the NE2001 model. A significant
variation was observed in the burst repeat rate R at L-band. During
observations in September 2016, nine bursts were detected, giving R = 1.1 $\pm$
0.4 hr$^{-1}$, while in November no bursts were detected, yielding R < 0.3
hr$^{-1}$ (95% confidence limit). This variation is consistent with earlier
seen episodic emission of FRB 121102. In a blind and targeted search, no bursts
were found with LOFAR at 150 MHz, resulting in a repeat rate limit of R < 0.16
hr$^{-1}$ (95% confidence limit). Burst repeat rate ratios of FRB 121102 at 3,
2, 1.4, and 0.15 GHz are consistent within the uncertainties with a flattening
of its spectrum below 1 GHz.
