  We present a study of the impact of a bright quasar on the redshifted 21cm
signal during the Epoch of Reionization (EoR). Using three different
cosmological radiative transfer simulations, we investigate if quasars are
capable of substantially changing the size and morphology of the H II regions
they are born in. We choose stellar and quasar luminosities in a way that is
favourable to seeing such an effect. We find that even the most luminous of our
quasar models is not able to increase the size of its native H II region
substantially beyond those of large H II regions produced by clustered stellar
sources alone. However, the quasar H II region is found to be more spherical.
We next investigate the prospects of detecting such H II regions in the
redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matched
filter technique. We find that H II regions with radii ~ 25 comoving Mpc or
larger should have a sufficiently high detection probability for 1200 hours of
integration time. Although the matched filter can in principle distinguish
between more and less spherical regions, we find that when including realistic
system noise this distinction can no longer be made. The strong foregrounds are
found not to pose a problem for the matched filter technique. We also
demonstrate that when the quasar position is known, the redshifted 21cm data
can still be used to set upper limits on the ionizing photon rate of the
quasar. If both the quasar position and its luminosity are known, the
redshifted 21 cm data can set new constrains on quasar lifetimes.
