  Modern radio interferometers sensitive to low frequencies will make use of
wide-band detectors. For such wide bandwidths, dispersive atmospheric effects
introduce variations in the fringe delay which change through the band of the
receivers. These undesired dispersive effects must be estimated and calibrated
with the highest precision. We studied the achievable precision in the estimate
of the ionospheric dispersion and the dynamic range of the correlated fringes
for different distributions of sub-bands in low-frequency and wide-band
interferometric observations. Our study is focused on the case of sub-bands
with a bandwidth much narrower than that of the total covered spectrum (case of
LOFAR). We computed the uncertainty of the ionospheric delay, the delay
ambiguity, and the dynamic range of the fringes using four different kinds of
sub-band distributions: constant spacing between sub-bands, random spacings,
spacings based on a power-law distribution, and spacings based on Golomb rulers
(sets of integers whose sets of differences have non-repeated elements). For a
large number of sub-bands ($> 20$, depending on the delay window) spacings
based on Golomb rulers give the most precise estimates of dispersive effects
and the highest fringe dynamic ranges. Spacings based on the power-law
distribution give similar results, although better than those with the Golomb
rulers for smaller number of sub-bands. Random distributions result in large
fringe dynamic ranges, but the estimate of dispersive effects is worse. A
constant spacing of sub-bands results in very bad fringe dynamic ranges, but
good estimates of ionospheric dispersion. Combining all the results, the
power-law distribution gives the best compromise between homogeneity in the
bandwidth sampling, precision in the estimate of ionospheric effects, dynamic
range of the correlated fringes, and group-delay ambiguity.
