  We assess the impact of inhomogeneous reionization on detection of HII
regions surrounding luminous high redshift quasars using planned low frequency
radio telescopes. Our approach is to implement a semi-numerical scheme to
calculate the 3-dimensional structure of ionized regions surrounding a massive
halo at high redshift, including the ionizing influence of a luminous quasar.
As part of our analysis we briefly contrast our scheme with published
semi-numerical models. We calculate mock 21cm spectra along the line of sight
towards high redshift quasars, and estimate the ability of the planned
Murchison Widefield Array to detect the presence of HII regions. The
signal-to-noise for detection will drop as the characteristic bubble size grows
during reionization because the quasar's influence becomes less prominent.
However, quasars will imprint a detectable signature on observed 21cm spectra
that is distinct from a region of typical IGM. At epochs where the mean
hydrogen neutral fraction is ~30% or greater we find that neutral gas in the
IGM surrounding a single quasar will be detectable (at a significance of 5
sigma) within 100 hour integrations in more than 50% of cases. 1000 hour
integrations will be required to detect a smaller neutral fraction of 15% in
more than 50% of cases. A highly significant detection will be possible in only
100 hours for a stack of 10 smaller 3 proper Mpc HII regions. The accurate
measurement of the global average neutral fraction (<x_HI>) will be limited by
systematic fluctuations between lines of sight for single HII regions. We
estimate the accuracy with which the global neutral fraction could be measured
from a single HII region to be 50%, 30% and 20% for <x_HI> ~ 0.15, 0.3 and 0.5
respectively.
