  We report on systematic experimental mapping of the transmission properties
of two-dimensional silicon-on-insulator photonic crystal waveguides for a broad
range of hole radii, slab thicknesses and waveguide lengths for both TE and TM
polarizations. Detailed analysis of numerous spectral features allows a direct
comparison of experimental data with 3D plane wave and finite-difference
time-domain calculations. We find, counter-intuitively, that the bandwidth for
low-loss propagation completely vanishes for structural parameters where the
photonic band gap is maximized. Our results demonstrate that, in order to
maximize the bandwidth of low-loss waveguiding, the hole radius must be
significantly reduced. While the photonic band gap considerably narrows, the
bandwidth of low-loss propagation in PhC waveguides is increased up to 125nm
with losses as low as 8$\pm$2dB/cm.
